DISTRIBUTION STATEMENT A: Approved for public release; distribution is unlimited.
DISTRIBUTION STATEMENT A: Approved for public release; distribution is unlimited.
Although the words “he,” “him,” and “his” are used sparingly in this course to enhance communication, they are not intended to be gender driven or to affront or discriminate against anyone.
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PREFACE By enrolling in this self-study course, you have demonstrated a desire to improve yourself and the Navy. Remember, however, this self-study course is only one part of the total Navy training program. Practical experience, schools, selected reading, and your desire to succeed are also necessary to successfully round out a fully meaningful training program.
COURSE OVERVIEW: To introduce the student to the subject of Test Equipment who needs such a background in accomplishing daily work and/or in preparing for further study.
THE COURSE: This self-study course is organized into subject matter areas, each containing learning objectives to help you determine what you should learn along with text and illustrations to help you understand the information. The subject matter reflects day-to-day requirements and experiences of personnel in the rating or skill area. It also reflects guidance provided by Enlisted Community Managers (ECMs) and other senior personnel, technical references, instructions, etc., and either the occupational or naval standards, which are listed in the Manual of Navy Enlisted Manpower Personnel Classifications and Occupational Standards, NAVPERS 18068.
THE QUESTIONS: The questions that appear in this course are designed to help you understand the material in the text.
VALUE: In completing this course, you will improve your military and professional knowledge. Importantly, it can also help you study for the Navy-wide advancement in rate examination. If you are studying and discover a reference in the text to another publication for further information, look it up.
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“I am a United States Sailor.
I will support and defend the Constitution of the United States of America and I will obey the orders of those appointed over me.
I represent the fighting spirit of the Navy and those who have gone before me to defend freedom and democracy around the world.
I proudly serve my country’s Navy combat team with honor, courage and commitment.
I am committed to excellence and the fair treatment of all.”
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1. Test Equipment Administration and Use.................................................................. 1-1
2. Miscellaneous Measurements................................................................................... 2-1
3. Basic Meters ............................................................................................................. 3-1
4. Common Test Equipment......................................................................................... 4-1
5. Special-Application Test Equipment........................................................................ 5-1
6. The Oscilloscope and Spectrum Analyzer................................................................ 6-1
I. Glossary.................................................................................................................. AI-1
II. References Used to Develop This TRAMAN ........................................................ AII-1
INDEX ......................................................................................................................... INDEX-1
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The figures listed below and included in this edition of NEETS, Module 16, Introduction to Test Equipment, were provided by Huntron Instruments, Inc. Permission to use these illustrations is gratefully acknowledged. Permission to reproduce these illustrations must be obtained from the source.
Huntron Instruments, Inc. 5-22, 5-23, 5-24, 5-25, 5-26, 5-27, 5-28 A&B, 5-29, 5-30, 5-31, 5-32, 5-33, 5-34, 5-35, 5-36, 5-37, 5-38
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The Navy Electricity and Electronics Training Series (NEETS) was developed for use by personnel in many electrical- and electronic-related Navy ratings. Written by, and with the advice of, senior technicians in these ratings, this series provides beginners with fundamental electrical and electronic concepts through self-study. The presentation of this series is not oriented to any specific rating structure, but is divided into modules containing related information organized into traditional paths of instruction.
The series is designed to give small amounts of information that can be easily digested before advancing further into the more complex material. For a student just becoming acquainted with electricity or electronics, it is highly recommended that the modules be studied in their suggested sequence. While there is a listing of NEETS by module title, the following brief descriptions give a quick overview of how the individual modules flow together.
Module 1, Introduction to Matter, Energy, and Direct Current, introduces the course with a short history of electricity and electronics and proceeds into the characteristics of matter, energy, and direct current (dc). It also describes some of the general safety precautions and first-aid procedures that should be common knowledge for a person working in the field of electricity. Related safety hints are located throughout the rest of the series, as well.
Module 2, Introduction to Alternating Current and Transformers, is an introduction to alternating current (ac) and transformers, including basic ac theory and fundamentals of electromagnetism, inductance, capacitance, impedance, and transformers.
Module 3, Introduction to Circuit Protection, Control, and Measurement, encompasses circuit breakers, fuses, and current limiters used in circuit protection, as well as the theory and use of meters as electrical measuring devices.
Module 4, Introduction to Electrical Conductors, Wiring Techniques, and Schematic Reading, presents conductor usage, insulation used as wire covering, splicing, termination of wiring, soldering, and reading electrical wiring diagrams.
Module 5, Introduction to Generators and Motors, is an introduction to generators and motors, and covers the uses of ac and dc generators and motors in the conversion of electrical and mechanical energies.
Module 6, Introduction to Electronic Emission, Tubes, and Power Supplies, ties the first five modules together in an introduction to vacuum tubes and vacuum-tube power supplies.
Module 7, Introduction to Solid-State Devices and Power Supplies, is similar to module 6, but it is in reference to solid-state devices.
Module 8, Introduction to Amplifiers, covers amplifiers. Module 9, Introduction to Wave-Generation and Wave-Shaping Circuits, discusses wave generation and wave-shaping circuits.
Module 10, Introduction to Wave Propagation, Transmission Lines, and Antennas, presents the characteristics of wave propagation, transmission lines, and antennas.
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Module 11,Microwave Principles, explains microwave oscillators, amplifiers, and waveguides.
Module 12,Modulation Principles, discusses the principles of modulation.
Module 13, Introduction to Number Systems and Logic Circuits, presents the fundamental concepts of number systems, Boolean algebra, and logic circuits, all of which pertain to digital computers.
Module 14, Introduction to Microelectronics, covers microelectronics technology and miniature and microminiature circuit repair.
Module 15, Principles of Synchros, Servos, and Gyros, provides the basic principles, operations, functions, and applications of synchro, servo, and gyro mechanisms.
Module 16, Introduction to Test Equipment, is an introduction to some of the more commonly used test equipments and their applications.
Module 17, Radio-Frequency Communications Principles, presents the fundamentals of a radio- frequency communications system.
Module 18, Radar Principles, covers the fundamentals of a radar system.
Module 19, The Technician's Handbook, is a handy reference of commonly used general information, such as electrical and electronic formulas, color coding, and naval supply system data.
Module 20,Master Glossary, is the glossary of terms for the series.
Module 21, Test Methods and Practices, describes basic test methods and practices.
Module 22, Introduction to Digital Computers, is an introduction to digital computers.
Module 23,Magnetic Recording, is an introduction to the use and maintenance of magnetic recorders and the concepts of recording on magnetic tape and disks.
Module 24, Introduction to Fiber Optics, is an introduction to fiber optics.
Embedded questions are inserted throughout each module, except for modules 19 and 20, which are reference books. If you have any difficulty in answering any of the questions, restudy the applicable section.
Although an attempt has been made to use simple language, various technical words and phrases have necessarily been included. Specific terms are defined in Module 20,Master Glossary.
Considerable emphasis has been placed on illustrations to provide a maximum amount of information. In some instances, a knowledge of basic algebra may be required.
Assignments are provided for each module, with the exceptions of Module 19, The Technician's Handbook; and Module 20, Master Glossary. Course descriptions and ordering information are in NAVEDTRA 12061, Catalog of Nonresident Training Courses.
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Throughout the text of this course and while using technical manuals associated with the equipment you will be working on, you will find the below notations at the end of some paragraphs. The notations are used to emphasize that safety hazards exist and care must be taken or observed.
AN OPERATING PROCEDURE, PRACTICE, OR CONDITION, ETC., WHICH MAY RESULT IN INJURY OR DEATH IF NOT CAREFULLY OBSERVED OR FOLLOWED.
AN OPERATING PROCEDURE, PRACTICE, OR CONDITION, ETC., WHICH MAY RESULT IN DAMAGE TO EQUIPMENT IF NOT CAREFULLY OBSERVED OR FOLLOWED.
An operating procedure, practice, or condition, etc., which is essential to emphasize.
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The text pages that you are to study are listed at the beginning of each assignment. Study these pages carefully before attempting to answer the questions. Pay close attention to tables and illustrations and read the learning objectives. The learning objectives state what you should be able to do after studying the material. Answering the questions correctly helps you accomplish the objectives.
Read each question carefully, then select the BEST answer. You may refer freely to the text. The answers must be the result of your own work and decisions. You are prohibited from referring to or copying the answers of others and from giving answers to anyone else taking the course.
To have your assignments graded, you must be enrolled in the course with the Nonresident Training Course Administration Branch at the Naval Education and Training Professional Development and Technology Center (NETPDTC). Following enrollment, there are two ways of having your assignments graded: (1) use the Internet to submit your assignments as you complete them, or (2) send all the assignments at one time by mail to NETPDTC.
Grading on the Internet: Advantages to Internet grading are:
• you get your results faster; usually by the next working day (approximately 24 hours).
assignments. To submit your assignment answers via the Internet, go to:
http://courses.cnet.navy.mil
Grading by Mail: When you submit answer sheets by mail, send all of your assignments at one time. Do NOT submit individual answer sheets for grading. Mail all of your assignments in an envelope, which you either provide yourself or obtain from your nearest Educational Services Officer (ESO). Submit answer sheets to:
Answer Sheets: All courses include one “scannable” answer sheet for each assignment. These answer sheets are preprinted with your SSN, name, assignment number, and course number. Explanations for completing the answer sheets are on the answer sheet.
Do not use answer sheet reproductions: Use only the original answer sheets that we provide—reproductions will not work with our scanning equipment and cannot be processed.
Follow the instructions for marking your answers on the answer sheet. Be sure that blocks 1, 2, and 3 are filled in correctly. This information is necessary for your course to be properly processed and for you to receive credit for your work.
Courses must be completed within 12 months from the date of enrollment. This includes time required to resubmit failed assignments.
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If your overall course score is 3.2 or higher, you will pass the course and will not be required to resubmit assignments. Once your assignments have been graded you will receive course completion confirmation.
If you receive less than a 3.2 on any assignment and your overall course score is below 3.2, you will be given the opportunity to resubmit failed assignments. You may resubmit failed assignments only once. Internet students will receive notification when they have failed an assignment--they may then resubmit failed assignments on the web site. Internet students may view and print results for failed assignments from the web site. Students who submit by mail will receive a failing result letter and a new answer sheet for resubmission of each failed assignment.
After successfully completing this course, you will receive a letter of completion.
Errata are used to correct minor errors or delete obsolete information in a course. Errata may also be used to provide instructions to the student. If a course has an errata, it will be included as the first page(s) after the front cover. Errata for all courses can be accessed and viewed/downloaded at:
http://www.advancement.cnet.navy.mil
We value your suggestions, questions, and criticisms on our courses. If you would like to communicate with us regarding this course, we encourage you, if possible, to use e-mail. If you write or fax, please use a copy of the Student Comment form that follows this page.
For subject matter questions:
E-mail<EMAIL_ADDRESS>Phone: Comm:<PHONE_NUMBER>, ext. 1728
DSN: 922-1001, ext. 1728 FAX:<PHONE_NUMBER> (Do not fax answer sheets.)
E-mail<EMAIL_ADDRESS>Phone: Toll Free:<PHONE_NUMBER>
Comm:<PHONE_NUMBER>/1181/1859 DSN: 922-1511/1181/1859 FAX:<PHONE_NUMBER> (Do not fax answer sheets.)
If you are a member of the Naval Reserve, you will receive retirement points if you are authorized to receive them under current directives governing retirement of Naval Reserve personnel. For Naval Reserve retirement, this course is evaluated at 5 points. (Refer to Administrative Procedures for Naval Reservists on Inactive Duty, BUPERSINST 1001.39, for more information about retirement points.)
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NAVEDTRA: 14188 Date:
We need some information about you:
Your comments, suggestions, etc.:
Privacy Act Statement: Under authority of Title 5, USC 301, information regarding your military status is requested in processing your comments and in preparing a reply. This information will not be divulged without written authorization to anyone other than those within DOD for official use in determining performance.
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Learning objectives are stated at the beginning of each chapter. These learning objectives serve as a preview of the information you are expected to learn in the chapter. The comprehensive check questions included are based on the objectives. By successfully completing the NRTC, you indicate that you have met the objectives and have learned the information.
Upon completing this chapter, you should be able to:
1. Describe the Ship Configuration and Logistic Support Information System (SCLSIS). 2. State the differences between calibration and repair.
3. Explain the various calibration status labels used by the Navy.
4. List the procedures for obtaining repairs to test equipment.
5. Describe the Metrology Automated System for Uniform Recall and Reporting (MEASURE) System and the purpose of the Metrology Equipment Recall and Reporting (METER) card and recall schedule.
6. Describe major test equipment references available to you. 7. Explain the purposes and benefits of testing.
8. State the safety precautions involved in working with test equipment.
9. List three precautions you should observe to avoid damaging electric measuring instruments.
10. State the correct procedures for using a safety shorting probe.
11. Describe resistance, voltage, and current measurements in terms of purposes, methods, and instruments used.
12. Describe how capacitance and inductance are measured.
13. Explain the operation of bridges in the measurement of unknown resistances, capacitances, and inductances.
One purpose of this chapter is to acquaint you with the practical use of test equipment. The presence of adequate test equipment in your shop is not in itself a "cure-all" for making repairs to complex electronic equipment. You must know how to best use the equipment available. First, however, you must understand the basis of electronic theory and be able to apply it to the system under repair.
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Another purpose of this chapter is to introduce you to calibration and repair procedures, and basic voltage and current measurements. You will also learn how ac bridges are used for precise measurements of resistance, capacitance, and inductance.
Much of the theory of operation and practical applications of the basic types of test instruments used in electrical and electronic circuits are found in the instruction books and technical manuals that accompany various equipments. You should read and understand these books before you attempt to use any test instrument. You should also know the established safety precautions to ensure your safety and safe equipment operating procedures to protect equipment from damage.
electronic equipment and components by their appropriate nomenclatures. You will find that several methods are used to identify test equipment used; this may be somewhat confusing to you at first. For example, a Tektronix Model 541A oscilloscope can also be identified as a CBTV-541A. The Joint Electronics Type Designation System (JETDS) is used by all branches of the military to identify equipment by a system of standardized nomenclatures.
Q-1. What system is currently used by all branches of the military to identify test equipment?
use of undesirable electronic test equipment (ETE) in fleet and shore activities. The board classifies electronic test equipments as GENERAL PURPOSE (GPETE) or SPECIAL PURPOSE (SPETE) and assigns responsibility for their management. Items classified as general purpose are managed by the Space and Warfare Systems Command (SPAWARSYSCOM). Items classified as special purpose are managed by the individual systems command that generates the requirement.
GPETE is test equipment that has the capability, without modification, to generate, modify, or measure a range of parameters of electronic functions required to test two or more equipments or systems of basically different design.
Special-purpose electronic test equipment (SPETE) is specifically designed to generate, modify, or measure a range of parameters of electronic functions of a specific or peculiar nature required to test a single system or equipment. These special test equipments are procured by the systems command that has the responsibility for the system/equipment requiring the SPETE for maintenance.
Q-2. Name the two classes of test equipment. Q-3. What test equipment is designed to generate, modify, or measure a range of parameters of
electronic functions of a specific nature required to test a single system or equipment? Until the ETE classification board was established, the uncontrolled increase in use of nonstandard
GPETE had resulted in loss of inventory control and increased support costs. NESEA has the responsibility for evaluating requests to purchase nonstandard GPETE and for recommending its approval or disapproval to NAVSEA. NAVSEA will then forward its final decision to the originating command for such requests.
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SHIP CONFIGURATION AND LOGISTIC INFORMATION SYSTEM (SCLSIS) PROGRAM The Navy must maintain, update, and calibrate thousands of pieces of equipment. To do this, the
SHIP CONFIGURATION AND LOGISTIC SUPPORT INFORMATION SYSTEM (SCLSIS) program was designed to keep track of all installed and portable equipment in the fleet. SCLSIS is used to keep up with the existence, location, and changes made to equipment. The SCLSIS program seeks to improve the quality of equipment reporting, provide information needed by other Navy management systems, and reduce record keeping. It is also designed to assist Navy supply systems that furnish spares, documentation, and training necessary to support installed and portable equipment.
Therefore, the inventory of assigned test equipment on board ship is directly related to SCLSIS records. Properly maintained SCLSIS records also show the complete inventory of test equipment on board by quantity, serial number, and location. The SCLSIS program has two basic elements: (1) VALIDATION, to establish a baseline data inventory, and (2) INVENTORY UPDATING, to correct errors or omissions and to document configuration changes.
Q-4. Name the two basic elements of the SCLSIS program.
The difference between the terms calibration and repair needs to be addressed before we proceed further. Calibration is little more than checking, adjusting, or systematically aligning a test instrument to a known standard. To do this, you must ensure that the equipment you send to the calibration lab is in working order.
The calibration lab is where actual repair work becomes important. Obvious problems, such as open power cords, burned components, broken meters, and missing hardware, should be repaired or replaced before sending equipment to the calibration lab. Most calibration labs with which you will deal will be part of an intermediate maintenance activity (IMA) on board a tender. CALIBRATION STATUS
You can determine the calibration status of any test equipment by checking the calibration label or tag located on the equipment. These calibration labels or tags advise you as to whether the item is usable and within its calibration interval. Tags and labels to be used in the METROLOGY CALIBRATION (METCAL) coordination program are listed in the following paragraphs. No other calibration labels or tags are authorized to be placed on test equipment.
The CALIBRATED label, shown in view A of figure 1-1, has black lettering and a white background and comes in two sizes. It is the most commonly used label in the METCAL program. This label indicates that the instrument to which it is attached is within its applicable tolerance on all parameters. If there are any qualifying conditions for use of the instrument, one of the other labels described in the next paragraphs should be used.
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The CALIBRATED—REFER TO REPORT label, shown in view B of figure 1-1, has red lettering and a white back ground. It comes in two sizes and is used when you must know the actual measurement values to use the instrument.
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Q-5. What calibration label is used when actual measurement values must be known to use the test equipment?
Two SPECIAL CALIBRATION labels are shown in view C of figure 1-1 that have black lettering and a yellow background; the size and content of the labels are different. A SPECIAL CALIBRATION tag (figure 1-1, view C) is used with the smaller of the two labels. These labels or tag are used when some unusual or special condition in the calibration should be drawn to your attention.
Such special conditions may be deviations from usual calibration tolerances, multiple calibration intervals, or a requirement for in-place calibration. The special condition that resulted in the SPECIAL CALIBRATION label should be described on the large label when sufficient space is available on the instrument or on the tag when the small label is used. Brief descriptions of special conditions are provided in the following paragraphs.
Q-6. An instrument that must be calibrated in place requires what type of calibration label? In cases where you do not require full instrument capability, the calibration can be performed with
reduced tolerances or cover less than all ranges and parameters. This approach is often used when the instrument does not meet full calibration tolerances on certain ranges or parameters, but can still meet user requirements. On the other hand, the special calibration may be for higher accuracy than usual on a short-term basis upon your specific request.
MULTIPLE CALIBRATION INTERVALS.—Some instruments have components that require calibration less frequently than the rest of the instrument. For example, the attenuator in a signal generator may require calibration every 12 months, whereas the rest of the instrument parameters should be calibrated every 4 months. Since the attenuator calibration is time consuming and may require unavailable standards, use of the multiple-interval approach can save considerable time (man-hours) as well as permit the more frequent calibration to be performed at a lower level laboratory.
When a specific instrument has been designed for multiple calibration intervals, such information is provided in the applicable calibration procedure. The SPECIAL CALIBRATION label or tag is annotated with the words MULTIPLE INTERVAL, and the type of calibration performed is indicated; for example, partial 1 of 2, 2 of 2, complete calibration, and so forth. The calibration due date reflects the due date of the next partial or complete calibration.
CALIBRATION IN-PLACE.—Some instruments should be calibrated in-place. Annotation on the SPECIAL CALIBRATION label or tag will alert both you and the calibrator that the instrument should not be removed, but should be calibrated in-place.
Some test and measuring equipment (T&ME) should be calibrated by you instead of your referring the instrument to a calibration facility. For example, some instruments, such as hardness testers and densitometers, are provided with their own standards and should be calibrated each time used, or at least very frequently. Some instruments, such as oscillographic recorders, may require calibration before, during, and after each use.
Other automatic test equipment (ATE) have self-calibration tests that should be performed each time used or each day of use. Still other instruments are calibrated as part of checkout procedures performed daily or weekly and recorded in maintenance logs. Whenever recognized, the requirement for calibration
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by the user and the calibration interval (each use—daily, weekly, every 100 hours—each overhaul, and so forth) is indicated in the Metrology Requirements List (METRL).
The USER CALIBRATION label, shown in view D of figure 1-1, has black lettering and a white background and is affixed when the calibration is performed by the user; however, this label is not replaced at each calibration. When the label is first attached to the instrument, it is annotated as to the appropriate calibration interval. Records of calibrations performed, when other than each time used, should be by normal maintenance practices; that is, in the maintenance log, on maintenance action forms, and so forth.
In the event that an individual instrument due for recalibration will not be used for some time in the future, you may indefinitely postpone the recalibration by affixing an inactive label to the instrument. As shown in view E of figure 1-1, the INACTIVE—CALIBRATE BEFORE USE label has green lettering and a white background. The INACTIVE label remains on the instrument until it is recalibrated. The instrument is not to be used while bearing this label.
Test equipment standards and T&ME not requiring calibration are shown as CALIBRATION NOT REQUIRED. This label, shown in view F of figure 1-1, has orange letters and a white background. It is attached to and should remain on the instrument indefinitely unless its calibration requirements change. If the instrument is not listed in METRL, you should use the following criteria when placing instruments in the CALIBRATION NOT REQUIRED category:
• Instrument does not make quantitative measurements nor provide quantified outputs.
• The device is "fail-safe"; that is, operation beyond specified tolerances will be apparent to the user.
• All measurement/stimulus circuits are monitored during use by calibrated instruments or are dependent on external known or calibrated sources for performance within required limits. (When determining that an instrument falls into the CALIBRATION NOT REQUIRED category, you should annotate the label as to the authority for the decision, such as METRL, technical manual, letter or message from higher authority.)
adequately repaired, a REJECTED—REFER TO ATTACHED TAG label is placed on the instrument and all other servicing labels removed. This label, as shown in view G of figure 1-1, has black letters and a red background. In addition to the REJECTED label, a REJECTED tag, giving the reason for rejection and other information as required, is attached to the instrument. Both the label and tag remain on the instrument until it is repaired and recalibrated. The instrument is not to be used while bearing a REJECTED label.
The CALIBRATION VOID IF SEAL BROKEN label, shown in view H of figure 1-1, has black letters and a white background. It is placed over readily accessible (usually exterior) adjustments to prevent tampering by the user when such tampering could affect the calibration. The label should not be
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used to cover adjustments or controls that are part of the normal use and operation of the instrument. This label may also be used to prevent removal and/or interchange of plug-ins, modules, subassemblies, and so forth, when such removal or interchange would affect the calibration.
If you are unable to replace a known failed component with onboard spares, you can often locate the replacement component from other supply sources. The replacement component can then be delivered, along with the inoperative equipment, to the IMA. So by sending the repair part along with the equipment, you can reduce repair time considerably. This is particularly true when your unit is getting under way and no time is available for you to complete the repair before calibration. Most operational commands have a higher supply priority for purchase of repair parts than the IMA can use.
timely manner. The "no reject" policy says, in effect, that test equipment submitted to the IMA for calibration, which is later found to require repair, will be repaired by the repair department of the IMA. Before this policy, any equipment found inoperative by the calibration lab was marked REJECTED, the reasons stated, and the equipment returned uncalibrated to the ship for repairs. The "no reject" policy does not relieve you of your responsibility to ensure your equipment is in working order prior to submitting it for calibration. Its purpose is to streamline the procedure and cut down delays in returning your equipment to you calibrated and ready to use.
Generally, the responsibility for repair and maintenance of test equipment is placed on maintenance personnel. In some cases, however, maintenance personnel are not authorized to make repairs. Then the test instrument must be sent to a shore repair/calibration facility.
Q-7. Responsibility for repair and maintenance of test equipment generally rests with what group of personnel?
When test equipment is sent for calibration and repair, all accessories, such as probes, adapters, and calibration sheets, should be included. Only in emergencies or special situations should partial repair or calibration be attempted on test equipment designated as nonrepairable. Such emergency repairs should be noted on a tag attached to the unit and an entry made on the MEASURE card (discussed shortly). The equipment should then be sent at the earliest opportunity to an authorized facility so that permanent repairs can be made and the unit calibrated.
STOWAGE AND HANDLING OF TEST EQUIPMENT Most electronic test equipment is precision equipment. Such equipment must be handled with care to
properly perform its designed functions. Rough handling, excessive heat, moisture, and dust all affect the useful life of the equipment. Bumping or dropping a test instrument may ruin the calibration of a meter, cause short circuits, or damage electronic elements inside the case. Sharp bends, creases, or dents in coaxial test cables can alter the expected attenuating effect and cause false meter readings or measurements. Forced air cooling, dust filters, and heaters are used in many pieces of equipment. This test equipment requires clean air filters for proper ventilation and a warm-up period that permits units in the equipment to maintain calibrated standards.
Electronic test equipment should be stowed in a dry location with the dust cover (if provided) in place. Dust covers for spare plug-in units should be constructed for such stowage. For ease in performing
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maintenance, the test equipment should be stowed at a location convenient to equipment spaces. If possible, related test equipment should be mounted in the equipment spaces. This reduces the problem of finding adequate stowage space elsewhere.
In stowage spaces, individual pieces of test equipment should be held in place by stretch seat-belt- type straps. If bars are used to hold equipment on shelves, meters and control knobs should be protected by blocking the equipment to prevent it from rolling and sliding on the shelf. Test equipment too large for shelf stowage should be kept in stowage cases and tie-downs provided to secure the cases. Refer to Stowage Guide for Portable Test Equipment, NAVSEA ST000-AB-GYD-010/GPETE, to determine adequate stowage space and proper weight support requirements.
For the sake of simplicity, we will use the more commonly used acronym MEASURE instead of the full name to describe this system in the next discussion.
MEASURE is a data processing system designed to provide a standardized system for the recall and scheduling of test, measurement, and diagnostic equipment (TMDE) into calibration facilities. It also provides for the documentation of data pertaining to the calibration actions performed by these facilities.
The primary reference document that describes the operation of the MEASURE system is Metrology Automated System for Uniform Recall and Reporting (MEASURE) Users Manual, OP 43P6A. The Chief of Naval Operations oversees this program and establishes policy and guidelines.
Q-8. What Navy office oversees the MEASURE program? Each naval activity must ensure that the test equipment for which it has been assigned primary
responsibility is submitted on a timely basis to a calibration activity for required calibration.
The MEASURE program is designed to assist these naval activities in the fulfillment of this responsibility. MEASURE does this by providing for the automatic scheduling and recall of all such test equipment for calibration.
Each activity submits an initial inventory, using the form shown in figure 1-2, to its Metrology Calibration Representative (METCALREP) for approval. The METCALREP then forwards the inventory to the Measure Operational Control Center (MOCC). The MOCC, based on the information contained on these inventory report forms, provides the necessary preprinted Metrology Equipment Recall and Reporting (METER) card. Figure 1-3 illustrates a MEASURE METER card.
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In part, the METER card is preprinted with information taken from the initial inventory data submitted on the inventory report forms together with such updated data as may appear on any prior METER card. The remaining information required is entered on the card by the user of the equipment or the calibration activity, as appropriate.
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The METER card is used to report changes, additions, or deletions to the user activity’s inventory. It is also used to report changes in custody of the item of test equipment. The procedure for filling out the METER card is outlined in the appendixes of the MEASURE Users Manual. Blank METER cards can be obtained through the responsible METCALREP.
A computer printout recall schedule is also generated by the MEASURE system. The purpose of this printout is to list those items of equipment that are due for calibration. Each recall schedule is composed of a set of four identical copies. One set is provided to the calibration activity as an aid to workload planning; a second set is sent to the user’s activity. The recall schedule is one of several products/formats sent automatically by the MEASURE Operation Control Center to the user activity on a regular basis. The MOCC automatically distributes the following products to user activities at the intervals shown:
maintained aboard ship by type commander instructions. These requirements are usually found in the inspection checkoff list. Other publications, while not required by directive, are necessary to you as reference and study material so you will be able to administer an effective test equipment program. Technicians should become familiar with the publications/directives listed in appendix II of this module.
Our military forces increasingly rely on electrical and electronic equipment to help perform their mission. The effectiveness of our tactical forces depends on many types of electronic systems, such as communications systems, detection systems, and fire control systems. The reliability of such equipment is determined by many factors; however, the primary factors are the quality of the equipment in use, the availability of spare parts, and the ability of maintenance personnel to perform adequate maintenance.
Maintenance is work done to correct, reduce, or counteract wear, failure, and damage to equipment. Maintenance of electrical and electronic equipment is divided into two main categories: PREVENTIVE (routine) and CORRECTIVE maintenance. Preventive maintenance consists of mechanical, electrical, and electronic checks to determine whether equipment is operating properly. It also consists of visual inspections of cabling and equipment for damage and to determine if lubrication is needed. Corrective maintenance isolates equipment failure by means of test techniques and practices; it also replaces defective parts and realigns or readjusts equipment to bring it back to proper performance.
Q-9. What are the two main categories of maintenance? Q-10. What type of maintenance involves isolating equipment troubles and replacing defective parts?
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Testing and troubleshooting are the areas of maintenance that require the greatest technical skill. Testing procedures are referred to as measurements, tests, and checks. The definitions of these terms often overlap, depending on their use and the results obtained. For example, a power measurement and a frequency check could constitute a test of the operation of the same radio transmitter.
Troubleshooting is a term which we in the electronics field use daily. But what does it mean? Troubleshooting is sometimes thought to be the simple repair of a piece of equipment when it fails to function properly. This, however, is only part of the picture. In addition to repair, you, as a troubleshooter, must be able to evaluate equipment performance. You evaluate performance by comparing your knowledge of how the equipment should operate with the way it is actually performing. You must evaluate equipment both before and after repairs are accomplished.
Equipment performance data, along with other general information for various electronic equipments, is available to help you in making comparisons. This information is provided in performance standards books for each piece of equipment. It illustrates what a particular waveform should look like at a given test point or what amplitude a voltage should be, and so forth. This data aids you in making intelligent comparisons of current and baseline operating characteristics for the specific equipment assigned to you for maintenance. ("Baseline" refers to the initial operating conditions of the equipment on installation or after overhaul when it is operating according to design.)
Remember, maintenance refers to all actions you perform on equipment to retain it in a serviceable condition or to restore it to proper operation. This involves inspecting, testing, servicing, repairing, rebuilding, and so forth. Proper maintenance can be performed only by trained personnel who are thoroughly familiar with the equipment. This familiarity requires a thorough knowledge of the theory of operation of the equipment.
A logical and systematic approach to troubleshooting is of the utmost importance in your performance of electronics maintenance. Many hours have been lost because of time-consuming "hit-or- miss" (often referred to as "easter-egging") methods of troubleshooting. GENERAL TEST EQUIPMENT INFORMATION
In any maintenance training program, one of your most important tasks is to learn the use of test equipment in all types of maintenance work. To be effective in maintenance work, you must become familiar not only with the common types of measuring instruments, but also with the more specialized equipment. Some examples of common types of typical measuring instruments are the ammeter, voltmeter, and ohmmeter; examples of specialized test equipment are the spectrum analyzer, dual-trace oscilloscope, and power and frequency meters.
require certain handling precautions to prevent damage and to ensure accurate readings. In addition, to prevent injury to personnel, you must observe precautions while using test equipment. You can find a list of applicable instructions in appendix II of this module.
To prevent damage to electrical measuring instruments, you should observe the precautions relating to three hazards: mechanical shock, exposure to magnetic fields, and excessive current flow.
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MECHANICAL SHOCK.—Instruments contain permanent magnets, meters, and other components that are sensitive to shock. Heavy vibrations or severe shock can cause these instruments to lose their calibration accuracy.
EXPOSURE TO STRONG MAGNETIC FIELDS.—Strong magnetic fields may permanently impair the accuracy of a test instrument. These fields may impress permanent magnetic effects on permanent magnets, moving-coil instruments, iron parts of moving-iron instruments, or in the magnetic materials used to shield instruments.
EXCESSIVE CURRENT FLOW.—This includes various precautions, depending on the type of instrument. When in doubt, use the maximum range scale on the first measurement and shift to lower range scales only after you verify that the reading can be made on a lower range. If possible, connections should be made while the circuit is de-energized. All connections should be checked to ensure that the instrument will not be overloaded before the circuit is reenergized.
Precautions to be observed to prevent instrument damage include the following:
• Keep in mind that the coils of wattmeters, frequency meters, and power meters may be carrying large quantities of current even when the meter pointer is on scale.
• Never open secondaries of current transformers when the primary is energized.
• Never short-circuit secondaries of potential transformers the primary is energized.
• Never leave an instrument connected with its pointer off-scale or deflected in the wrong direction.
• Ensure that meters in motor circuits can handle the motor starting current. This may be as high as six to eight times the normal running current.
• Never attempt to measure the internal resistance of a meter movement with an ohmmeter since the movement may be damaged by the current output from the ohmmeter.
• Never advance the intensity control of an oscilloscope to a position that causes an excessively bright spot on the screen; never permit a sharply focused spot to remain stationary for any period of time. This results in burn spots on the face of the cathode-ray tube (CRT).
• In checking electron tubes with a tube tester that has a separate "short test," always make the short test first. If the tube is shorted, no further test should be made.
• Before measuring resistance, always discharge any capacitors in the circuit to be tested. Note and record any points not having bleeder resistors or discharge paths for capacitors.
• Always disconnect voltmeters from field generating or other highly inductive circuits before you open the circuit.
Q-11. Which quantity (voltage or current) determines the intensity of an electrical shock? Situations can arise during the use of test equipment that are extremely dangerous to personnel. For
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another, and a soldering iron in still another. Also, you may be using an extension cord for some equipments and not others or may be using other possible combinations. Some of the hazards presented by situations such as these include contact with live terminals or test leads. In addition, cords and test leads may be cross connected in such a manner that a potential difference exists between the metal cases of the instruments. This potential difference may cause serious or fatal shocks.
Test leads attached to test equipment should, if possible, extend from the back of the instruments away from the observer. If this is not possible, they should be clamped to the bench or table near the instruments.
At times, you may use instruments at locations where vibration is present, such as near a diesel engine. At such times, the instruments should be placed on pads of folded cloth, felt, or similar shock- absorbing material.
Insofar as is practical, you should NOT undertake repair work on energized circuits and equipment. However, it could become necessary, such as when you make adjustments on operating equipment. In such cases, obtain permission from your supervisor, then proceed with your work, but carefully observe the following safety precautions:
• DO NOT WORK ALONE.
• Station an assistant near the main switch or circuit breaker so the equipment can be immediately de-energized in case of an emergency.
• Someone qualified in first aid for electrical shock should be standing by during the entire operation.
• Ensure that you have adequate lighting. You must be able to see clearly if you are to perform the job safely and properly.
• Be sure that you are insulated from ground by an approved rubber mat or layers of dry canvas and/or wood.
• Where practical, use only one hand, keeping the other either behind you or in your pocket.
• If you expect voltage to exceed 150 volts, wear rubber gloves.
• DO NOT work on any type of electrical apparatus when you are wearing wet clothing or if your hands are wet.
• DO NOT wear loose or flapping clothing.
• The use of thin-soled shoes and shoes with metal plates or hobnails is prohibited.
• Flammable articles, such as celluloid cap visors, should not be worn.
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• Remove all rings, wristwatches, bracelets, and similar metal items before working on the equipment. Also ensure that your clothing does not contain exposed metal fasteners, such as zippers, snaps, buttons, and pins.
• Do not tamper with interlock switches; that is, do not defeat their purpose by shorting them or blocking them open.
• Ensure that equipment is properly grounded before energizing.
• De-energize equipment before attaching alligator clips to any circuit.
• Use only approved meters and other indicating devices to check for the presence of voltage.
• Observe the following procedures when measuring voltages in excess of 300 volts:
� Turn off the equipment power.
� Short-circuit or ground the terminals of all components capable of retaining a charge.
� Connect the meter leads to the points to be measured.
� Remove any terminal grounds previously connected.
� Turn on the power and observe the voltage reading.
� Turn off the power.
� Short circuit or ground all components capable of retaining a charge.
� Disconnect the meter leads.
• On all circuits where the voltage is in excess of 30 volts and where decks, bulkheads, or workbenches are made of metal, you should insulate yourself from accidental grounding by using approved insulating material. The insulating material should have the following qualities:
� It should be dry, without holes, and should not contain conducting materials.
� The voltage rating for which it is made should be clearly marked on the material. The proper material should be used so that adequate protection from the voltage can be supplied.
� Dry wood may be used or, as an alternative, several layers of dry canvas, sheets of phenolic (resin or plastic) insulating material, or suitable rubber mats.
� Care should be exercised to ensure that moisture, dust, metal chips, and so forth, which may collect on insulating material, are removed at once. Small deposits of such materials can become electrical hazards.
� All insulating materials on machinery and in the area should be kept free of oil, grease, carbon dust, and so forth, since such deposits destroy insulation.
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A representative shorting probe is shown in figure 1-4. An approved shorting probe is shown in NAVSEA 0967-LP-000-0100, EIMB, General, Section 3.
Capacitors and cathode-ray tubes may retain their charge for a considerable period of time after having been disconnected from the power source.
Always assume there is a voltage present when working with circuits having high capacitance, even when the circuit has been disconnected from its power source.
An approved type of shorting probe should be used to discharge capacitors and cathode-ray tubes individually.
When using the safety shorting probe, always be sure to first connect the test clip to a good ground (if necessary, scrape the paint off the grounding metal to make a good contact). Then hold the safety shorting probe by the insulated handle and touch the probe end of the shorting rod to the point to be shorted out. The probe end is fashioned so that it can be hooked over the part or terminal to provide a constant connection by the weight of the handle alone. Always take care not to touch any of the metal parts of the safety shorting probe while touching the probe to the exposed "hot" terminal. It pays to be safe; use the safety shorting probe with care.
Some equipments are provided with walk-around shorting devices, such as fixed grounding studs or permanently attached grounding rods. When that is the case, the walk-around shorting devices should be used rather than the safety shorting probe.
Q-12. What tool is used to de-energize capacitors in a circuit that has been disconnected from its power source?
When any electronic equipment is to be repaired or overhauled, certain general safety precautions should be observed. They are as follows:
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• Remember that electrical and electronic circuits often have more than one source of power. Take time to study the schematics or wiring diagrams of the entire system to ensure that all sources of power have been disconnected
• If pertinent, inform the remote station regarding the circuit on which work will be performed.
• Use one hand when turning switches on or off.
• Safety devices, such as interlocks, overload relays, and fuses, should never be altered or disconnected except for replacement. In addition, they should never be changed or modified in any way without specific authorization.
• Fuses should be removed and replaced only after the circuit has been de-energized. When a fuse "blows," the replacement should be of the same type and have the same current and voltage ratings. A fuse puller should be used to remove and replace cartridge fuses.
• All circuit breakers and switches from which power could possibly be supplied should be secured (locked if possible) in the OPEN or OFF (safe) position and danger tagged in accordance with procedures in the Standard Organization and Regulations of the U.S. Navy, OPNAVINST 3120.32.
• After the work has been completed, the tag (or tags) should be removed only by the same person who signed it (them) when the work began.
• Keep clothing, hands, and feet dry if at all possible. When you must work in wet or damp locations, place a rubber mat or other nonconductive material on top of a dry, wooden platform or stool; then use the platform or stool to sit and stand on. Use insulated tools and insulated flashlights of the molded type when you are required to work on exposed parts.
metal bases, and other structural parts of electrical and electronic equipment are at ground potential.
Normally, on steel-hull vessels, such grounds are inherently provided because the metal cases or frames of the equipment are in contact with one another and with the metal structure of the vessel. In some instances where such inherent grounding is not provided by the mounting arrangements, such as equipment supported on shock mounts, suitable ground connections must be provided.
The grounding wire used for this purpose is generally made of flexible material (copper or aluminum) that provides sufficient current-carrying capacity to ensure an effective ground. In this manner, equipment cases and frames that are not intended to be above ground potential are effectively grounded; also, the possibility of electrical shock to personnel coming in contact with metal parts of the equipment is minimized. The secondary purpose of grounding equipment is to improve the operation and continuity of service of all equipments.
Paint, grease, or other foreign matter can interfere with the positive metal-to-metal contact at the ground connection point. Therefore, all bonding surfaces (connection points or metallic junctions) must be securely fastened and free of such matter. In all instances where equipment grounding is provided, certain general precautions and preventive maintenance measures must be taken. A few of these precautions are listed below:
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• Periodically clean all strap-and-clamp connectors to ensure that all direct metal-to-metal contacts are free from foreign matter.
• Check all mounting hardware for mechanical failure or loose connections.
• Replace any faulty, rusted, or otherwise unfit grounding strap, clamp, connection, or component between the equipment and the ground to the ship’s hull.
• When replacing a part of the ground connection, make certain that the metallic contact surfaces are clean and that electrical continuity is re-established.
• After completing the foregoing steps, recheck to be sure that the connection is securely fastened with the correct mounting hardware. Paint the ground strap and hardware in accordance with current procedures.
Because of the electrical shock hazards that could be encountered aboard ship, plugs and convenience outlets for use with portable equipment and power tools normally are standard three-prong type. Both plugs and outlets are keyed so that the plug must be in the correct position before it can be inserted into the receptacle. To ensure that the safety factors incorporated in these devices are in serviceable condition and are safe for use, you must perform the following precautions and inspections:
• Inspect the pins of the plug to see that they are firmly in place and are not bent or damaged.
• Check the wiring terminals and connections of the plug. Loose connections and frayed wires on the plug surface must be corrected and any foreign matter removed before the plug is inserted into the receptacle.
• Use a meter to ensure that the ground pin has a resistance of less than 1 ohm equipment ground.
• Do not attempt to insert a grounded-type plug into a grounded receptacle without first aligning the plug properly.
Never use a power tool or a piece of portable test equipment unless you are absolutely sure that it is equipped with a properly grounded conductor.
Electronic measurements involve the fundamental electrical quantities of voltage and current and the inherent characteristics of resistance, capacitance, and inductance. In circuits being tested, voltage and current are dependent upon resistance, capacitance, and inductance for their distribution; therefore, voltage and current measurements are valuable aids in determining circuit component conditions and in the evaluation of symptoms. Practically any reading obtained from the use of test equipment will depend on these basic measured quantities of resistance, capacitance, and inductance.
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Voltage measurements may be made as part of either preventive or corrective maintenance. These measurements are made using a voltmeter. When compared with voltage charts, these measurements are a valuable aid in locating a trouble quickly and easily. However, if the sensitivity of the test voltmeter differs from that of the voltmeter used in preparing the chart, the voltage measurements must be evaluated before the true circuit conditions can be determined. (Sensitivity in voltmeters was discussed in NEETS, Module 3, Introduction to Circuit Protection, Control, and Measurement.)
Since many of the troubles you find in equipments and systems are the result of abnormal voltages, voltage measurements are a valuable aid in locating trouble. You can measure voltage with a voltmeter without interrupting circuit operation.
Point-to-point voltage measurement charts, usually found in equipment technical manuals, contain the normal operating voltages found in the various stages of the equipment. These voltages are usually measured between indicated points and ground unless otherwise stated. When you begin recording voltage measurements, it is a smart and safe practice to set the voltmeter on the highest range before measuring. This ensures that excessive voltages existing in the circuit will not cause overloading of the meter.
Q-13. On what range should you set the voltmeter prior to taking a voltage measurement? To increase accuracy, you should then set the voltmeter to the appropriate range for the proper
comparison with the expected voltage in the voltage charts. When checking voltages, remember that a voltage reading can be obtained across a resistance, even if that resistance is open. The resistance of the meter itself forms a circuit resistance when the meter probes are placed across the open resistance. Therefore, the voltage across the component may appear to be normal or near-normal as you read the meter, but may actually be abnormal when the meter is disconnected from the circuit.
If the internal resistance of the voltmeter is approximately the same value as the resistance being tested, it will indicate a considerably lower voltage than the actual voltage present when the meter is removed from the circuit. The sensitivity (in ohms per volt) of the voltmeter used to prepare the voltage charts is provided on those charts. If a meter of similar sensitivity is available, you should use it to reduce the effects of loading.
The following precautions are general safety measures that apply to the measurement of voltages. Remember that nearly all voltages are dangerous and have often proved fatal to careless technicians. When measuring voltages, be sure to observe the following precautions:
• Set test equipment to the HIGHEST range.
• Make sure safety observer knows where to secure power for the equipment under test.
• Connect the ground lead of the voltmeter first.
• Use only one hand to take measurements (when possible), and put the other hand in your pocket or behind your back.
• If the voltage to be measured is less than 300 volts, place the end of the test probe on the point to be tested; use the polarity switch to select positive or negative readings.
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• If the voltage to be measured is more than 300 volts, proceed as follows:
1.
2. Discharge all filter capacitors with a shorting probe.
3. Temporarily ground the point to be measured.
4. Connect (clip on) the proper test lead to the high-voltage point. 5. Move away from the voltmeter.
6. Turn on circuit power and read the voltmeter.
7. Turn off circuit power.
8. Discharge all capacitors before disconnecting the meter.
Q-14. When taking a voltage measurement, which lead of the voltmeter should you connect to the circuit first?
Current measurements are not often taken in the course of preventive maintenance or testing. This is because the ammeter (or other current-measuring instrument) must become an actual part of the equipment being tested. The circuit must be opened (desoldered) to connect the ammeter in series with the circuit being tested. Usually, you can take a voltage measurement and use this factor to calculate the circuit current by applying Ohm’s law.
Q-15. Is an ammeter connected in series or in parallel with the circuit under test?  Maintenance handbooks for the equipment can often be used to help you take these measurements. These handbooks often contain resistance charts that are referenced to accessible test points within the equipment. Without these charts, taking resistance measurements in a complex circuit is a slow process. The process is slow because one side of the circuit component must often be desoldered to get a true resistance measurement. However, resistance tolerances vary so widely that approximate resistance readings are adequate for most jobs.
Once the most accessible test point is found, an ohmmeter is usually used to take the resistance measurement. Because of the degree of accuracy needed when an ohmmeter is used, proper calibration and understanding of the meter scales is a must. (Topic 2 of this module will discuss these requirements in detail.) When using an ohmmeter, you must observe the following precautions:
• The circuit being tested must be completely de-energized.
• Any meters or transistors which can be damaged by the ohmmeter current must be removed before any measurement is made.
Q-16. What must be done to a circuit before you can use an ohmmeter for testing?
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Capacitance measurements are usually taken with a capacitance meter. Capacitance tolerances vary even more widely than resistance tolerances. Capacitance tolerances depend on the type of capacitor, the value of capacitance, and the voltage rating. The actual measurement of capacitance is very simple; however, you must make the important decision of whether to reject or to continue to use the capacitor after it has been tested.
The POWER FACTOR of a capacitor is important because it is an indication of the various losses of a capacitor. Power losses can be traced to the dielectric, such as current leakage and dielectric absorption. Current leakage is of considerable importance, especially in electrolytic capacitors.
Q-17. What is the term used to refer to the losses which can be traced to the dielectric of a capacitor? INDUCTANCE MEASUREMENTS
Inductance measurements are seldom required in the course of troubleshooting. However, inductance measurements are useful in some cases; therefore, bridges (discussed in the next section) are available for making this test. You will find that many capacitance test sets can be used to measure inductance. Most capacitance test sets are furnished with inductance conversion charts if the test equipment scale is not calibrated to read the value of inductance directly.
You can measure capacitance, inductance, and resistance for precise accuracy by using ac bridges. These bridges are composed of capacitors, inductors, and resistors in a wide variety of combinations. These bridges are operated on the principle of a dc bridge called a WHEATSTONE BRIDGE.
The Wheatstone bridge is widely used for precision measurements of resistance. The circuit diagram for a Wheatstone bridge is shown in figure 1-5. Resistors R1, R2, and R3 are precision, variable resistors. The value of Rx is an unknown value of resistance that must be determined. After the bridge has been properly balanced (galvanometer G reads zero), the unknown resistance may be determined by means of a simple formula. The galvanometer (an instrument that measures small amounts of current) is inserted across terminals b and d to indicate the condition of balance. When the bridge is properly balanced, no difference in potential exists across terminals b and d; when switch S2 is closed, the galvanometer reading is zero.
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The operation of the bridge is explained in a few logical steps. When the battery switch S1 is closed, electrons flow from the negative terminal of the battery to point a. Here the current divides as it would in any parallel circuit. Part of it passes through R1 and R2; the remainder passes through R3 and Rx. The two currents, I1 and I2, unite at point c and return to the positive terminal of the battery. The value of I1 depends on the sum of resistance R1 and R2, and the value of I2 depends on the sum of resistances R3 and Rx. In each case, according to Ohm’s law, the current is inversely proportional to the resistance.
R1, R2, and R3 are adjusted so that when S1 is closed, no current flows through G. When the galvanometer shows no deflection, there is no difference of potential between points b and d. All of I
1 follows the a b c path and all I2 follows the a b c path. This means that a voltage drop E1 (across R1 between points a and b) is the same as voltage drop E3 (across R3 between points a and d). Similarly, the voltage drops across R2 and Rx (E2 and Ex) are also equal. Expressed algebraically,
With this information, we can figure the value of the unknown resistor Rx. Divide the voltage drops across R1 and R3 by their respective voltage drops across R2 and Rx as follows:
We can simplify this equation:
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then we multiply both sides of the expression by Rx to separate it:
For example, in figure 1-5, we know that R1 is 60 ohms, R2 is 100 ohms, and R3 is 200 ohms. To find the value of R
x, we can use our formula as follows:
A wide variety of ac bridge circuits (such as the Wheatstone) may be used for the precision measurement of ac resistance, capacitance, and inductance. Let’s look at ac bridges in terms of functions they perform.
RESISTANCE BRIDGE.—An ac signal generator, as shown in figure 1-6, is used as the source of voltage. Current from the generator passes through resistors R1 and R2, which are known as the ratio arms, and through Rs and Rx. Again, Rx is known as resistance. Rs has a standard value and replaces R3 in figure 1-6. When the voltage drops across R2 and Rs are equal, the voltage drops across R2 and Rx are also equal; no difference of potential exists across the meter and no current flows through it. As we discovered with the Wheatstone bridge, when no voltage appears across the meter, the following ratio is true:
For example, if in figure 1-6 we know that R1 is 20 ohms, R2 is 40 ohms, and Rs is 60 ohms, we can find the value of Rx using our formula as follows:
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With the ac signal applied to the bridge, R1 and R2 are varied until a zero reading is seen on the meter. Zero deflection indicates that the bridge is balanced. (NOTE: In actual practice, the variables are adjusted for a minimum reading since the phase difference between the two legs will not always allow a zero reading.)
CAPACITANCE BRIDGE.—Because current varies inversely with resistance and directly with capacitance, an inverse proportion exists between the four arms of the bridge in figure 1-7; the right side of our expression is inverted from the resistance bridge expression as follows:
Q-18. What effect does an increase in capacitance have on a capacitor’s opposition to current flow? Because R1 and R2 are expressed in the same units, the equation R1/R2 becomes a simple
multiplication factor. This equation provides a numerical value for Cx and will be in the same units as Cs (farad, microfarad, and so forth).
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Similarly, the following resistance ratio exists between the four arms of the bridge, just as in the resistance bridge expression discussed earlier:
resistance R1, R2, Rs, and known capacitance Cs.
In figure 1-7, for example, we know that R1 is 20 ohms, R2 is 40 ohms, Rs is 60 ohms, and Cs is 10 microfarads. We can find the values of Cx and Rx by using the respective formulas as follows:
Q-19. When a bridge is used to measure resistance, what is the value of Rx if R1 equals 80 ohms, R2 equals 120 ohms, and R3 equals 280 ohms?
INDUCTANCE BRIDGE.—The value of the unknown inductance Lx may be determined by means of the simple bridge circuit shown in figure 1-8. Ratio arms R1 and R2 are accurately calibrated resistors. Ls is a standard inductor with a known inductance; Rs is the known resistance, and Rx represents the resistance of the unknown inductor.
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The ac signal is applied to the bridge, and variable resistors R1 and R2 are adjusted for a minimum or zero deflection of the meter, indicating a condition of balance. When the bridge is balanced, the following formulas may be used to find Lx.
(NOTE: The right side of this expression is NOT inverse as it was in the capacitance bridge.)
In figure 1-8, for example, the values of R1, R2, and Rs are 20, 40, and 60 ohms, respectively. The value of Ls is 10 millihenries. We can find the values of Rx and Lx by using their respective formulas as follows:
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Thus, both the unknown resistance and inductance can be estimated in terms of the known values for R1, R2, Rs, and Ls.
Q-20. When an unknown capacitance is tested with a bridge, what is the value of Cx if R1 equals 70 ohms, R2 equals 150 ohms, and Cs equals 550 microfarads?
The important points of this chapter are summarized in the following paragraphs:
The JETDS SYSTEM is jointly used by all branches of the military to identify equipments by a system of standardized nomenclatures.
GPETE is test equipment that has the capability, without modifications, to generate, modify, or measure a range of parameters of electronic functions required to test two or more equipments or systems of basically different design. All GPETE are listed in Standard General Purpose Electronic Test Equipment, MIL-STD-1364 (series).
SPETE is test equipment that is specifically designed to generate, modify, or measure a range of parameters of electronic functions of a specific or peculiar nature required to test a single equipment or system.
The SHIP CONFIGURATION AND LOGISTICS INFORMATION SYSTEM (SCLSIS) program is designed to keep track of equipment configuration changes in the fleet.
The SCLSIS program has two basic elements, VALIDATION and INVENTORY UPDATING.
The CALIBRATION STATUS of any items of test equipment can be determined by the information recorded on the calibration label or tag located on the equipment.
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The CALIBRATED label, with black lettering on a white background, indicates the instrument to which it is attached is within tolerance on all scales.
The CALIBRATED—REFER TO REPORT label, with red lettering on a white background, is used when actual measurement values must be known to use the instrument.
The SPECIAL CALIBRATION label, with black lettering on a yellow background, is used when some unusual or special condition in the calibration should be drawn to your attention.
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The USER CALIBRATION label indicates that you should calibrate the test and measuring instrument instead of sending the instrument to a calibration facility.
The INACTIVE—CALIBRATE BEFORE USE label is used when a piece of test equipment due for recalibration will not be used for some time in the future.
The CALIBRATION NOT REQUIRED label is used on test instruments listed in the Metrology Requirements List (METRL) as not requiring calibration.
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The REJECTED label is attached to a test instrument that fails to meet the acceptance criteria during calibration and cannot be repaired.
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The CALIBRATION VOID IF SEAL BROKEN label is placed over readily accessible adjustments to prevent tampering by the user when such tampering could affect the calibration.
The MEASURE system is designed to standardize the recall and scheduling of test, measurement, and diagnostic equipment into calibration facilities and for the documentation of actions performed by the calibration facility.
MAINTENANCE is work done to correct, reduce, or counteract wear and damage to equipment.
PREVENTIVE MAINTENANCE consists of checks to determine weather equipment is functioning properly. It also consists of visual inspections of cabling and equipment for damage and to determine if lubrication is needed.
CORRECTIVE MAINTENANCE is used to isolate troubles by means of test techniques and practices that realign or readjust equipment or otherwise bring the equipment back up to proper performance.
SENSITIVITY of the voltmeter is always given on the voltage charts for a particular piece of equipment. You should always use a voltmeter of similar sensitivity to the equipment to diminish the effects of circuit loading.
CURRENT MEASUREMENTS are not often taken in the course of testing because the ammeter (or other current measuring device) must become an actual part of the equipment being tested. The circuit must be opened for necessary connection of the meter. Usually you can use a voltage measurement to calculate the circuit current by applying Ohm’s law.
You should observe the following PRECAUTIONS when using an ohmmeter:
1. The circuit being tested must be completely de-energized.
2. Any circuit components which can be damaged by ohmmeter current must be removed before any measurement is made.
The WHEATSTONE BRIDGE is used for precise measurement of resistance.
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The CAPACITANCE BRIDGE is used for measuring an unknown capacitance.
An INDUCTANCE BRIDGE is used to find the value of an unknown inductance.
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ANSWERS TO QUESTIONS Q1. THROUGH Q20. A-1. Joint Electronics Type Designation System (JETDS). A-2. General-purpose electronic test equipment (GPETE) and special-purpose electronic test
equipment (SPETE). A-3. Special-purpose electronic test equipment.
A-4. Validation and updating.
A-5. CALIBRATED—REFER TO REPORT.
A-6. SPECIAL CALIBRATION label.
A-7. Maintenance personnel.
A-8. The Chief of Naval Operations. A-9. Preventive and corrective maintenance.
A-10. Corrective maintenance.
A-11. Current.
A-12. Shorting probe.
A-13. Highest.
A-14. Ground.
A-15. In series.
A-16. It must be de-energized.
A-17. Power losses.
A-18. Opposition to current flow decreases. A-19. 420 ohms.
A-20. 256 microfarads.
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Upon completing this chapter, you should be able to:
1. Define and explain the use of the terms "dB" and "dBm" as they apply to power measurements.
2. Describe the use of resistive loads, bolometers, and thermocouples in power measurements.
3. Explain the measurement of mechanical rotation using the tachometer, stroboscope, and the strobotac.
4. Explain the measurement of frequency in various ranges using vibrating reeds, tuned circuits, heterodyne frequency meters, absorption wavemeters, cavity wavemeters, and frequency counters.
5. Describe the use of frequency-measurement devices, oscilloscopes, and spectrum analyzers in waveform analysis and maintenance.
6. Describe semiconductor testing and applicable terms in maintenance.
In chapter 1, you studied test equipment administration and the basic measurements that all technicians are responsible for performing. Chapter 2 presents miscellaneous measurements that are fairly common; keep in mind, however, that you may not routinely perform these measurements in your particular job. This chapter introduces you to several test instruments and components found in those test instruments. It will also serve as a review of some of the basics of electronic theory related to test equipment.
You may be required to check the power consumption and the input-signal power levels of electronic equipment. The determination of dc power is fairly simple; recall that the unit of power, the watt, is the product of the potential in volts and the current in amperes (P = E · I).
As discussed in NEETS, Module 2, Introduction to Alternating Current and Transformers, the phase angle of the voltage and current must be considered for accurate ac power measurements. The measurement of ac power is further complicated by the frequency limitations of various power meters. If there is no phase angle difference, you can compute ac power in the same manner as dc power; that is, by determining the effective value of the product of the voltage and current.
For equipments that operate in the audio-frequency (af) range, power levels have to be determined in the performance of routine checks and during corrective maintenance procedures.
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Power measurements for af circuits are usually indicated in terms of decibels (dB) or decibels referenced to 1 milliwatt (dBm). Because the actual calculation of decibel measurements is seldom required, the following explanation is somewhat simplified. Most test equipment is designed to measure and indicate decibels directly. This eliminates the need for you to perform complicated calculations. Nevertheless, a basic explanation of the decibel measurement system is necessary for you to understand the significance of dB readings and amplifier-gain ratings that are expressed in decibels.
The basic unit of measurement in the system is not the decibel; it is the bel. The bel is a unit that expresses the logarithmic ratio between the input and the output of any given component, circuit, or system. It may be expressed in terms of voltage, current, or power. Most often, it is used to show the ratio between input and output power to figure gain. You can express the power gain of the amplifier (N) in bels by dividing the output (P1) by the input (P2) and taking the base 10 logarithm of the resulting quotient. The formula for determining this gain is:
If an amplifier doubles the input power, the quotient of P1 to P2 will be 2. If you consult a logarithm table, you will find that the base 10 logarithm of 2 is 0.3, making the power gain of the amplifier 0.3 bel.
Q-1. What is the logarithmic ratio between the input and output of a given circuit called? Experience has shown that because the bel is a rather large unit, it is difficult to apply. A more
practical unit, and one that can be used more easily, is the decibel (1/10 bel). You can convert any figure expressed in bels to decibels by multiplying that figure by 10 or simply by moving the decimal point one place to the right. Applying this rule, we find that the above ratio of 0.3 bel is equal to 3 decibels.
The decibel (dB) cannot be used to represent actual power; only the ratio of one power compared to another. To say that an amplifier has a 3 dB gain means that the output power is twice the input power. This gives no indication of the actual power represented. You must be able to state the input power for it to be meaningful. In many applications, a mathematical expression represents the actual power, not a power ratio. One standard reference is the dBm.
The dBm is an abbreviation used to represent power levels above or below 1 milliwatt. Negative dBm ( - dBm) represents power levels below 1 milliwatt, and positive dBm (+dBm) represents power levels above 1 milliwatt. In other words, a dBm value is a specific amount of power; 0 dBm is equal to 1 milliwatt. Briefly stated, the amount of power in a given value of dBm is the power which results if 1 milliwatt is amplified or attenuated by that dB value. For example, 40 dBm represents an actual power level (watts or milliwatts) that is 40 dB above 1 milliwatt, whereas - 10 dBm represents a power level that is 10 dB below 1 milliwatt. The formula for finding dBm is a variation of the dB power formula:
Q-2. What term is used to represent power levels above or below a 1-milliwatt reference?
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You do not need to use the formula in most applications. The following shows conversions of dBm to mW:
+20dBm = 100mW +10dBm = 10mW +7dBm = 5mW +6dBm = 4mW +4dBm = 2.5mW +3dBm = 2mW 0dBm = 1mW -3dBm = .5mW -10dBm = .1mW
For a +10 dBm level, start with the 1 milliwatt reference and move the decimal point one place to the right (+10 dBm = 10 mW). Another 10 dB increment brings the power level to +20 dBm, thereby moving the decimal point another place to the right (+20 dBm = 100 mW). For a - 10 dBm level, again start with 1 milliwatt, but this time move the decimal point one place to the left ( - 10 dBm = .1 mW). An additional 10 dB decrease results in another decimal point shift to the left ( - 20 dBm = .01 mW).
For a 3 dB increase, you double the power. For a 3 dB decrease, you reduce the power by one-half (+3 dBm = 2 mW and - 3 dBm = .5 mW). A +6 dBm level is an additional 3 dB change from +3 dBm. In this case, you just double the power level of the +3 dBm (+6 dBm = 4 mW).
Q-3. What milliwatt value is equal to +6 dBm? The dB change can be made in either direction. For example, +7 dBm is a decrease from +10 dBm.
Reducing the +10 dBm power by one-half, we have +7 dBm, or 5 mW. A +4 dBm power level is a 3 dB decrease from +7 dBm (+4 dBm - 2.5 mW). By using this simple method, you can quickly find any power level that corresponds to a given dBm.
Some test instruments you will be using are calibrated in decibels and have a 1 milliwatt zero reference level. Figure 2-1 illustrates such an instrument. Notice that this is an ac voltmeter in which the upper scale of the meter indicates ac voltage and the lower scale indicates decibels. The zero power-level indicator on the decibel scale is located at, or near, center scale. If the power in the line being measured is more than the reference value, the meter will indicate a value to the right of the zero mark (+dB). If the power is less than the reference value, the meter will indicate a value to the left of the zero mark ( - dB). Such meters are useful when recording measurements where a direct indication in decibels is desired. However, you must remember that this meter is still a voltmeter and that power measurements are not meaningful unless the circuit impedance is known. If you feel the need to review how to calculate power in ac circuits, refer to NEETS, Module 2.
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At radio frequencies below the UHF range, power is usually determined by voltage, current, and impedance measurements. One common method used to determine the output power of radio-frequency (rf) oscillators and radio transmitters consists of connecting a known resistance to the equipment output terminals. Current flowing through this resistance is then measured and the power is calculated as the product of I2R.
Because power is proportional to the current squared, the meter scale can be calibrated to indicate power units directly. A THERMOCOUPLE AMMETER can be used in this manner for measuring rf power. The resistor used to replace the normal load is specially designed to have low reactance and the ability to dissipate the required amount of power. Such resistors are commonly called DUMMY LOADS or DUMMY ANTENNAS.
Q-4. What name is given to a resistor used to replace the normal load in a circuit? In the UHF and SHF frequency ranges, accurately measuring the voltage, current, and resistance is
difficult. These basic measurements can vary greatly, depending on where in the circuit the measurements are made. They are also affected by small changes in parts placement in the vicinity of tuned circuits.
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To measure the output of microwave radio or radar transmitters, you can use test instruments that convert rf power to another form of energy, such as light or heat. These instruments can be used to indirectly measure the power. A method used to measure the effect of a resistor load on a stream of passing air can also be used to indirectly measure power. Accurate measurement of large-magnitude power also can be achieved by measuring the temperature change of a water load. The most common type of power meter for use in this frequency range employs a BOLOMETER.
The bolometer is a loading device that undergoes changes of resistance as changes in dissipated power occur. The two types of bolometers are the BARRETTER and the THERMISTOR. The barretter is characterized by an increase in resistance as the dissipated power rises. The thermistor decreases in resistance as the power increases. In either case, resistance is measured before and after the application of rf power. If the same change in resistance is then produced by a variable dc source of power, then the rf power is equal to the measured dc power. This relationship makes possible the direct calibration of a bridge circuit in units of power. In other words, one condition of balance exists when no rf power is applied; but in the presence of power, a second condition of balance exists because of the resistance changes of the bolometer. It is this change of resistance that is calibrated in power.
Q-5. What are the two types of bolometers? BARRETTER.—The construction of a typical barretter is shown in figure 2-2. The fine wire
(usually tungsten) is extremely small in diameter. This thin diameter allows the rf current to penetrate to the center of the wire. The wire is supported in an insulating capsule between two metallic ends, which act as connectors. Because of these physical characteristics, the barretter resembles a cartridge-type fuse. The enclosure is a quartz capsule made in two parts. One part is an insert cemented in place after the tungsten wire has been mounted. In operation, the barretter is matched to the rf line after power is applied.
THERMISTOR.—A high degree of precision is made possible by the thermistor; therefore, it is widely used. Figure 2-3 shows the typical construction of a bead-type thermistor. The negative- temperature coefficient comes from the use of a semiconductor as the active material. Notice that the
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active material is shaped in the form of a bead. It is supported between two pigtail leads by connecting wires. The pigtail ends are embedded in the ends of the surrounding glass capsule.
The negative-resistance temperature coefficient of thermistors is desirable. This is because excessive power has the effect of changing the resistance of the thermistor to an extent that causes a pronounced rf mismatch. The resulting decrease in power transfer reduces the likelihood of burnout.
Figure 2-4, views A and B, is an example of a THERMISTOR BRIDGE used for rf power measurements. A thermistor bridge circuit includes other thermistor elements, referred to as compensating thermistors. These thermistors respond to fluctuations in ambient temperature so that the bridge balances and calibration are maintained over a wide temperature range. Compensating thermistors are usually in disc form so that they can be mounted on a flat metal surface, such as a chassis or a waveguide. The thermistor bridge in view B is located in the terminating section of a waveguide and contains RT-2, a bead thermistor, and two compensating thermistors, RT-1 and RT-3, on the outside of the waveguide (view A). RA-1 in view B, a calibrated attenuator, controls the amount of rf energy applied to RT-2.
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Before power is applied, R1 and R2 in view A of figure 2-4 are used to adjust the current through RT-2. When the resistance of RT-2 reaches the equivalent parallel resistance of R6 and RT1 (122.4 ohms), the bridge is balanced. Meter M-1 reads 0 at this time. The rf signal being measured is connected to the test set and applied via the calibrated attenuator to RT-2. This causes the temperature of RT-2 to increase, thus reducing its resistance. The bridge becomes unbalanced, causing meter M-1 to deflect an amount proportional to the decrease in resistance of RT-2. Meter M-1, because of the operation of RT-2, reads average power.
Q-6. As the dissipated power increases, what effect does this have on the resistance of a thermistor? If the ambient temperature rises, the resistance of RT-1 decreases. This shunts more current around
the bridge network and allows RT-2 to cool. The resistance of RT-3 decreases, maintaining meter sensitivity independent of temperature changes. Cavity Z-1 in view B of figure 2-4 is an ABSORPTION- TYPE FREQUENCY METER. This type of meter will be discussed later.
and electronic equipment. Some examples of the various frequency measurements follow:
• Rotation frequencies of some electro-mechanical devices, such as electric motors, must be determined.
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• The output frequency of electric power generators is checked when the engine is started and during preventive maintenance routines.
• Equipment that operates in the af range must be adjusted to operate at the correct frequencies.
• Radio transmitters must be accurately tuned to the assigned frequencies to provide reliable communications and to avoid interference with radio circuits operating on other frequencies.
• Radar sets must be properly tuned to obtain satisfactory performance.
As you can see from the above examples, frequency measurement does indeed play a valuable role in maintenance. These measurements can be divided into two broad categories: MECHANICAL- ROTATION FREQUENCY measurement and ELECTRICAL-OUTPUT FREQUENCY measurement. Depending upon your job and/or the type of command to which you are assigned, you may be tasked with performing one or both of these types of measurements.
driven generators, as well as the blade speed in turbines, is measured with devices called TACHOMETERS, STROBOSCOPES, and STROBOTACS.
A tachometer is an instrument that measures the rate at which a shaft is turning. Although tachometers are installed on machinery, such as generators and engines, you may need to determine the speed of a rotating machine that is not equipped with a tachometer. In these instances, you will be required to use a PORTABLE TACHOMETER. Portable hand-held tachometers measure speed by direct contact with the shaft of the measured unit. Portable tachometers are for use only during testing and should not be used continuously. The common types of portable tachometers are the CENTRIFUGAL and the CHRONOMETRIC.
CENTRIFUGAL TACHOMETER.—A centrifugal-type tachometer is illustrated in figure 2-5, view A. View B shows the internal arrangement of the centrifugal tachometer; refer to view B in this discussion. In the centrifugal tachometer, centrifugal force acts upon fly weights that are connected by links to upper and lower collars. The upper collar is affixed to a drive shaft; the lower collar is free to move up and down the shaft. A spring, which fits over the shaft, connects the upper and lower collars.
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Each portable centrifugal tachometer has a small rubber-covered wheel and a number of hard rubber tips. You fit the appropriate tip or wheel on the end of the tachometer drive shaft, and hold it against the shaft to measure speed of rotation. As the drive shaft begins to rotate, the fly weights rotate with it. Centrifugal force tends to pull the fly weights away from the center, causing the lower collar to rise and compress the spring. The lower collar is attached to a pointer, and its upward motion, restricted by the spring tension, causes an increase in the indication on the dial face.
When properly used, a centrifugal tachometer will indicate correct shaft speed as long as it is in contact with the machine shaft under test. A portable centrifugal tachometer has three ranges: low (50 to 500 rpm), medium (500 to 5,000 rpm), and high (5,000 to 50,000 rpm).
CHRONOMETRIC TACHOMETER.—The chronometric tachometer (figure 2-6) is a combination watch and revolution counter. It measures the average number of revolutions of a shaft per minute. The chronometric tachometer also comes with hard rubber tips, which must be inserted over the drive shaft.
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When applied to a rotating shaft, the outer drive shaft of this tachometer runs free until a starting button is depressed to start the timing element. In figure 2-6, note the starting button beneath the index finger. The chronometric tachometer retains readings on its dial after its drive shaft has been disengaged from a rotating shaft and until the pointers are returned to 0 by the reset button (usually the starting button). The range of a chronometric tachometer is usually from 0 to 10,000 rpm and from 0 to 3,000 feet per minute (fpm). Stroboscope
The rotation frequencies of recording devices and teletypewriter motors can be measured by the use of a STROBOSCOPE. The stroboscope is an instrument that allows you to view rotating or reciprocating objects intermittently and produces the optical effect of a slowing down or stopping motion. For example, electric fan blades revolving at 1,800 rpm will appear stationary if you look at them under a light that flashes uniformly 1,800 times per minute. At 1,799 flashes per minute, the blades will appear to rotate forward at 1 rpm; at 1,801 flashes per minute, they will appear to rotate backward at 1 rpm.
When the flashing rate of the light is adjustable, you can calibrate the control in flashes (or revolutions) per minute. The stationary image you see when the rate of the lamp and the rotational rate of a shaft are equal lets you record a very precise speed measurement.
The STROBOTAC (figure 2-7) is an electronic flash device in which the flash duration is very short (a few millionths of a second). (Table 2-1 contains a description of the controls and indicators shown on the strobotac in figure 2-7.) Because of this short flash duration, the strobotac can measure very rapid motion. The box contains a swivel mount with a STROBOTRON LAMP in a reflector, an electronic pulse generator to control the flashing rate, and a power supply that operates from the ac power line. The flashing rate is controlled by the large knob; the corresponding speed (rpm) is indicated on an illuminated dial that is viewed through windows in the knob.
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Table 2-1.—Strobotac Controls and Indicators
CONTROLS AND INDICATORS (see figure 2-7) NO NAME USE 1.  2. RPM control Controls the flashing rate of light as the fluted rim is rotated.
Dial is calibrated directly in revolutions per minute (rpm). 3. Range switch Selects any of three rpm (internal oscillator) ranges, plus three
4. CALibration indicator lamp Indicates the correct setting of CALibration adjustments for calibrating the RPM dial to power-line frequency.
5. HIGH CAL, LOW CAL Calibration adjustments used to calibrate the RPM dial. 6. OUTPUT TRIGGER jack A trigger pulse is available at this jack for strobotac types
1531, 1538, stroboslave type 1539, and strobolume type 1532. 7. INPUT jack Used for connecting the stroboscope to an external
synchronizing signal from the electrical device or mechanical contactor.
8. Reflector-lamp assembly Produces and aims the flashing light. 9. Power cord A permanently attached 6-foot power cord. For storage, the
cord is wound clockwise around the range-switch knob and reflector. The plug is secured by sliding it onto the holding pin.
10. Holder pin Used to secure the plug-end of the power cord when unit is to be stored in its case.
* Flashes at 3600 rpm until external signal is plugged in.
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The normal speed range is from 110 to 25,000 rpm.  The life of the strobotron lamp is approximately 250 hours if used at flashing speeds of less than 5,000 rpm, or 100 hours if used at higher speeds.
ELECTRICAL OUTPUT FREQUENCY All alternating voltage sources are generated at a set frequency or range of frequencies. A
FREQUENCY METER provides a means of measuring this frequency. The electrical output frequency of ac power generators can be measured by a vibrating reed, a tuned circuit, or by a crossed-coil, iron-vane type meter. The vibrating-reed device is the simplest type of frequency meter. It has the advantage of being rugged enough to be mounted on generator control panels.
A simplified diagram of a vibrating-reed frequency meter is shown in figure 2-8, views A through D. In view A, you can see that the current to be measured flows through the coil and exerts maximum attraction on the soft-iron armature twice during each cycle. The armature is attached to the bar, which is mounted on a flexible support. Reeds of suitable dimensions to have natural vibration frequencies of 110, 112, 114, and so forth, up to 130 hertz are mounted on the bar (view B). The reed with a frequency of 110 hertz is marked 55 hertz; the one with a frequency of 112 hertz is marked 56 hertz; the one with a frequency of 120 hertz is marked 60 hertz, and so forth.
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When the coil is energized by a current with a frequency between 55 and 65 hertz, all the reeds are vibrated slightly; but, the reed having a natural frequency closest to that of the energizing current vibrates through a larger amplitude. The frequency is read from the scale value opposite the reed having the greatest amplitude of vibration.
In some instruments, the reeds are the same length; but they are weighted by different amounts at the top so they will have different natural rates of vibration. An end view of the reeds in the indicator is shown in view C. If the energizing current has a frequency of 60 hertz, the reed marked 60 will vibrate the greatest amount, as shown. View D shows a hand-held vibrating-reed frequency meter mounted on the casing of a motor-generator.
TUNED CIRCUITS are used as filters for the passage or rejection of specific frequencies. BANDPASS FILTERS and BAND-REJECT FILTERS are examples of this type. Tuned circuits have certain characteristics that make them ideal for certain types of filters, especially where a high degree of selectivity is desired. A series-tuned circuit offers a low impedance to currents of the particular frequency to which the circuit is tuned and a relatively high impedance to currents of all other frequencies. A parallel-tuned circuit, on the other hand, offers a very high impedance to currents of its natural, or resonant, frequency and a relatively low impedance to others. If you feel you need to review the subject of tuned circuits at this time, refer to NEETS, Module 9, Introduction to Wave-Generation and Wave- Shaping Circuits, for more information on these circuits and their applications.
reading frequency meter. Frequency comparisons can be made by the use of a calibrated af generator in conjunction with either an oscilloscope or a modulator and a zero-beat indicating device. Direct-reading frequency measurements can be made by instruments using series, frequency-selective electrical networks, bridge test sets having null indicators, or counting-type frequency meters.
Heterodyne frequency meters are available in several varieties. They measure the frequency of the unknown signal by matching the unknown signal with a locally generated signal of the same frequency obtained from a calibrated, precision oscillator. This method is normally referred to as zero beating. When a perfect frequency match is obtained, it is indicated by the absence of a beat note (zero beat). The technician generally uses a set of headphones to detect a zero-beat condition in the equipment being tested.
The basic heterodyne meter (figure 2-9) is a calibrated variable oscillator, which heterodynes against the frequency to be measured. Coupling is accomplished between the frequency meter and the output of the equipment under test. (NOTE: This coupling should be in accordance with the step-by-step procedures listed in the technical manual for the frequency meter.) The calibrated oscillator is then tuned so that the difference between the oscillator frequency and the unknown frequency is in the af range. This difference in frequency is known as the BEAT FREQUENCY. As the two frequencies are brought closer to the same value, the tone in the headset will decrease in pitch until it is replaced by a series of rapid clicks. As the process is continued, the clicks will decrease in rapidity until they stop altogether. This is the point of zero beat; that is, the point at which the frequency generated in the oscillator of the frequency meter is equal to the frequency of the unknown signal being measured.
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Q-7. In a heterodyne-type frequency meter, what is the difference between the oscillator frequency and the unknown frequency?
For all practical purposes, the point of zero beat can be assumed when the clicks are heard at infrequent intervals. Figure 2-10 illustrates the zero-beat concept. Maintaining a condition of absolute silence in the earphones is extremely difficult when you are making this measurement. When the incoming signal is strong, the clicks are sharp and distinct. When the signal is weak, the zero-beat condition is evidenced by a slowly changing "swishing" or "rushing" sound in the headset. After the zero beat is obtained, the dial reading corresponds to the frequency measured.
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The manufacturer’s calibration book is a very important part of the frequency meter package; in fact, the book is so important that it bears the same serial number as the heterodyne-type frequency meter itself. Contained in this book is a list of the dial settings and the corresponding frequencies produced by that meter at those dial settings. Operating instructions for the meter are also included.
WAVEMETERS are calibrated resonant circuits used to measure frequency. The accuracy of wavemeters is not as high as that of heterodyne-type frequency meters; however, they have the advantage of being comparatively simple and can be easily carried.
Q-8. What equipment uses a calibrated resonant circuit to measure frequency? Any type of resonant circuit can be used in wavemeter applications. The exact kind of circuit used
depends on the frequency range for which the meter is intended. Resonant circuits consisting of coils and capacitors are used for VLF through VHF wavemeters.
The simplified illustration of an absorption wavemeter, shown in figure 2-11, consists of a pickup coil, a fixed capacitor, a lamp, a variable capacitor, and a calibrated dial. When the wavemeter’s components are at resonance, maximum current flows in the loop, illuminating the lamp to maximum brilliance. The calibrated dial setting is converted to a frequency by means of a chart, or graph, in the instruction manual. If the lamp glows very brightly, the wavemeter should be coupled more loosely to the circuit. For greatest accuracy, the wavemeter should be coupled so that its indicator lamp provides only a faint glow when tuned to the resonant frequency.
measured by resonant, cavity-type wavemeters or resonant, coaxial-line-type wavemeters. When properly calibrated, resonant-cavity and resonant-coaxial line wavemeters are more accurate and have better stability than wavemeters used for measurements in the LF to VHF ranges. These frequency-measuring instruments are often furnished as part of the equipment. They are also available as general-purpose test sets.
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Although many wavemeters are used in performing various functions, the cavity-type wavemeter is the type most commonly used. Only this type is discussed in some detail.
Figure 2-12 shows a typical CAVITY WAVEMETER. The wavemeter is of the type commonly used for the measurement of microwave frequencies. The device uses a resonant cavity. The resonant frequency of the cavity is varied by means of a plunger, which is mechanically connected to a micrometer mechanism. Movement of the plunger into the cavity reduces the cavity size and increases the resonant frequency. Conversely, an increase in the size of the cavity (made by withdrawing the plunger) lowers the resonant frequency. The microwave energy from the equipment being tested is fed into the wavemeter through one of two inputs, A or B. The crystal rectifier then detects (rectifies) the signal. The rectified current is indicated on current meter M.
Another device used to measure frequencies above the audio range is the ELECTRONIC FREQUENCY COUNTER. Since this instrument will be covered in detail in a later chapter, only a brief description is provided at this time.
The electronic frequency counter is a high-speed electronic counter with an accurate, crystal- controlled time base. This combination provides a frequency counter that automatically counts and displays the number of events occurring in a precise time interval. The frequency counter itself does not generate any signal; it merely counts the recurring pulses fed to it.
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WAVEFORM ANALYSIS can be made by observing displays of voltage and current variations with respect to time or by harmonic analysis of complex signals. Waveform displays are particularly valuable for adjusting and testing pulse-generating, pulse-forming, and pulse-amplifying circuits. The waveform visual display is also useful for determining signal distortion, phase shift, modulation factor, frequency, and peak-to-peak voltage.
Waveform analysis is used in various electrical and electronic equipment troubleshooting. This section will briefly discuss the oscilloscope and spectrum analyzer to provide you with basic knowledge of this test equipment.
Q-9. Name two instruments used to analyze waveforms.
The CATHODE-RAY OSCILLOSCOPE (CRO or O-SCOPE) is commonly used for the analysis of waveforms generated by electronic equipment. Several types of cathode-ray oscilloscopes are available for making waveform analysis. The oscilloscope required for a particular test is determined by characteristics such as input-frequency response, input impedance, sensitivity, sweep rate, and the methods of sweep control. The SYNCHROSCOPE is an adaptation of the cathode-ray oscilloscope. It features a wide-band amplifier, triggered sweep, and retrace blanking circuits. These circuits are desirable for the analysis of pulse waveforms.
Oscilloscopes are also part of some harmonic analysis test equipments that display harmonic energy levels. To effectively analyze waveform displays, you must know the correct wave shape. The maintenance instructions manual for each piece of equipment illustrates what waveforms you should observe at the various test points throughout the equipment. Waveforms that will be observed at any one selected test point will differ; each waveform will depend on whether the operation of the equipment is normal or abnormal.
The display observed on a cathode-ray oscilloscope is ordinarily one similar to those shown in figure 2-13. Views A and B show the instantaneous voltage of the wave plotted against time. Elapsed time (view A) is indicated by horizontal distance, from left to right, across the etched grid (graticule) placed over the face of the tube. The amplitude (view B) of the wave is measured vertically on the graph.
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The oscilloscope is also used to picture changes in quantities other than simply the voltages in electric circuits. For example, if you need to see the changes in waveform of an electric current, you must first send the current through a small resistor. You can then use the oscilloscope to view the voltage wave across the resistor. Other quantities, such as temperatures, pressures, speeds, and accelerations, can be translated into voltages by means of suitable transducers and then viewed on the oscilloscope. A detailed discussion of the oscilloscope is presented in chapter 6 of this module.
The SPECTRUM ANALYZER is a device that sweeps over a band of frequencies to determine (1) what frequencies are being produced by a specific circuit under test and (2) the amplitude of each frequency component. To accomplish this, the spectrum analyzer first presents a pattern on a display. Then the relative amplitudes of the various frequencies of the spectrum of the pattern are plotted (see figure 2-14). On the vertical, or Y axis, the amplitudes are plotted; on the horizontal, or X axis, the frequencies (time base) are plotted. The overall pattern of this display indicates the proportion of power present at the various frequencies within the SPECTRUM (fundamental frequency with sideband frequencies).
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Q-10. What device sweeps a band of frequencies to determine frequencies and amplitudes of each frequency component?
The spectrum analyzer is used to examine the frequency spectrum of radar transmissions, local oscillators, test sets, and other equipment operating within its frequency range. Proper interpretation of the displayed frequency spectrum enables you to determine the degree of efficiency of the equipment under test. With experience, you will be able to determine definite areas of malfunctioning components within equipment. In any event, successful spectrum analysis depends on the proper operation of a spectrum analyzer and your ability to correctly interpret the displayed frequencies. Later, in chapter 6, we will discuss the various controls, indicators, and connectors contained on the spectrum analyzer.
Because of the reliability of semiconductor devices, servicing techniques developed for transistorized equipment differ from those normally used for electron-tube circuits. Electron tubes are usually considered to be the circuit component most susceptible to failure and are normally the first components to be tested. Transistors, however, are capable of operating in excess of 30,000 hours at maximum rating without failure. They are often soldered in the circuit in much the same manner as resistors and capacitors. Therefore, they are NOT so quickly removed for testing as tubes.
Substitution of a semiconductor diode or transistor known to be in good condition is one method of determining the quality of a questionable semiconductor device. This method should be used only after you have made voltage and resistance measurements. This ensures the circuit has no defect that might
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damage the substitute semiconductor device. If more than one defective semiconductor is present in the equipment section where trouble has been localized, the semiconductor replacement method becomes cumbersome. Several semiconductors may have to be replaced before the trouble is corrected. To determine which stage(s) failed and which semiconductors are not defective, you must test all the removed semiconductors. You can do this by observing whether the equipment operates correctly as you reinsert each of the removed semiconductor devices into the equipment.
Semiconductor diodes, such as general-purpose germanium and silicon diodes, power silicon diodes, and microwave silicon diodes, can be tested effectively under actual operating conditions. However, crystal-rectifier testers are available to determine dc characteristics that provide an indication of crystal- diode quality.
A common type of crystal-diode test set is a combination ohmmeter-ammeter. Measurements of forward resistance, back resistance, and reverse current can be made with this equipment. Using the results of these measurements, you can determine the relative condition of these components by comparing their measured values with typical values obtained from test information furnished with the test set or from the manufacturer’s data sheets. A check that provides a rough indication of the rectifying property of a diode is the comparison of the back-and-forward resistance of the diode at a specified voltage. A typical back-to-forward-resistance ratio is on the order of 10 to 1, and a forward-resistance value of 50 to 80 ohms is common.
Q-11. What is the typical back-to-forward resistance ratio of a good-quality diode? Testing Diodes with an Ohmmeter
A convenient test for a semiconductor diode requires only an ohmmeter. The back-and-forward resistance can be measured at a voltage determined by the battery potential of the ohmmeter and the resistance range at which the meter is set. When the test leads of the ohmmeter are connected to the diode, a resistance will be measured that is different from the resistance indicated if the leads are reversed. The smaller value is called the FORWARD RESISTANCE, and the larger value is called the BACK RESISTANCE. If the ratio of back-to-forward resistance is greater than 10 to 1, the diode should be capable of functioning as a rectifier. However, keep in mind that this is a very limited test that does not take into account the action of the diode at voltages of different magnitudes and frequencies. (NOTE: This test should never be used to test crystal mixer diodes in radars.  Testing Diodes with Oscilloscopes
An oscilloscope can be used to graphically display the back-and-forward resistance characteristics of a crystal diode. A circuit used in conjunction with an oscilloscope to make this test is shown in figure 2- 15. This circuit uses the oscilloscope line-test voltage as the test signal. A series circuit (composed of resistor R1 and the internal resistance in the line-test circuit) decreases a 3-volt, open-circuit test voltage to a value of approximately 2 volts peak to peak.
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The test signal applied to the crystal diode is also connected to the horizontal input of the oscilloscope. The horizontal sweep represents the voltage applied to the diode under test. The voltage developed across current-measuring resistor R2 is applied to the vertical input of the oscilloscope. Because this voltage is proportional to the current through the diode being tested, the vertical deflection will indicate crystal current. The resulting oscilloscope trace for a normal diode is similar to the curve shown in figure 2-16.
To test Zener diodes, you must use a higher voltage than the oscilloscope line-test signal. This test can be made with a diode test set or with the circuit shown in figure 2-17. In this circuit, rheostat R1 is used to adjust the input voltage to a suitable value for the Zener diode being tested. Resistor R2 limits the current through the diode. The signal voltage applied to the diode is also connected to the horizontal input of the oscilloscope. The voltage developed across current-measuring resistor R3 is applied to the vertical input of the oscilloscope. The horizontal sweep represents the applied voltage, and the vertical deflection indicates the current through the diode being tested. Figure 2-18 shows the characteristic pattern of a Zener diode. Note the sharp increase in current at the Zener voltage (avalanche) point. For the Zener diode to be usable, this voltage must be within limits specified by the manufacturer.
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Most transistorized equipments use printed circuit boards on which components are neatly arranged. This arrangement makes the transistors and other components easy to reach while you are troubleshooting and servicing the equipment. While investigating with test probes, however, you must be careful to prevent damage to the printed wiring.
One of the outstanding advantages of transistors is their reliability. Tube failures account for over 90 percent of the failures in electron-tube equipments. Transistors, however, are long lived. This factor, among others, decreases maintenance required to keep transistorized equipment operating. The techniques used in testing transistorized equipment are similar to those for maintaining electron-tube circuits. Basically, these techniques include several checks and inspections.
When using test equipment to localize a trouble, you should check the power supply to see that its output voltages are present and of the correct values. Improper power supply voltages can cause odd effects. You will prevent many headaches by checking the power supply first.
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Visual inspection is a good maintenance technique. Occasionally, you will find loose wires or faulty connections, making extensive voltage checks unnecessary.
Transistors can be checked by substitution. Transistors, however, have a characteristic known as leakage current, which may affect the results obtained when the substitution method is used.
The leakage current may influence the current gain or amplification factor of the transistor. Therefore, a particular transistor might operate properly in one circuit and not in another. This characteristic is more critical in certain applications than in others. As the transistor ages, the amount of leakage current tends to increase. One type of transistor checker used is the semiconductor test set. This test set can be used either for in-circuit or out-of-circuit tests or for collector leakage current or current gain. You should use extreme care when substituting transistors. More and more transistors have specific current and breakdown voltage requirements that may affect how they operate within a given circuit.
Q-12. As a transistor ages, what happens to the leakage current? Voltage Checks
Voltage measurements provide a means of checking circuit conditions in a transistorized circuit just as they do in checking conditions in a tube circuit. The voltages, however, are much lower than in a tube circuit. The bias voltage between the base and emitter, for instance, is usually 0.05 to 0.20 volts. When making checks, observe polarity.
Transistors have little tendency to burn or change value because of low voltage in their circuits. They can, however, be permanently damaged by high-voltage conditions that occur when the collector voltage is increased. They can also be permanently damaged when the ambient temperature increases and causes excessive collector current flow. Transistors are easily damaged by high current; therefore, resistance measurements must not be taken with an ohmmeter that provides a maximum current output in excess of 1 milliampere. If you are not sure that the range of ohmmeter you want to use is below the 1 milliampere level, connect the ohmmeter to a milliammeter and check it. See figure 2-19 for a method of measuring the current from an ohmmeter.
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Resistance measurements usually are not made in transistorized circuits, except when you are checking for open windings in transformers and coils. When a resistance check is required, the transistors are usually removed from the circuit. Resistance checks cannot test all the characteristics of transistors, especially transistors designed for high frequencies or fast switching. The ohmmeter is capable of making simple transistor tests, such as open and short tests.
Refer to NEETS, Module 7, Introduction to Solid-State Devices and Power Supplies, for a review of transistor and semiconductor terms and theory.
The important points of this chapter are summarized in the following paragraphs:
The BEL is a unit that expresses the logarithmic ratio between the input and output of any given component, circuit, or system and can be expressed in terms of voltage, current, or power.
Any figure expressed in bels can be converted to DECIBELS by multiplying the figure by 10. The decibel cannot be used to represent actual power, only a ratio of one power to another.
The abbreviation dBm is used to represent power levels above or below a 1 milliwatt reference level.
A BOLOMETER is a device that undergoes changes in resistance as changes in dissipated power occur. The two types of bolometers most often used are the barretter and the thermistor.
FREQUENCY MEASUREMENTS can be divided into two broad categories: mechanical-rotation frequency and electrical-output frequency measurements.
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MECHANICAL ROTATION frequency is measured using a device called a TACHOMETER. Three basic tachometers are used for measuring mechanical rotation frequency—the CENTRIFUGAL tachometer, the CHRONOMETRIC tachometer, and the STROBOSCOPIC tachometer.
ELECTRICAL-OUTPUT frequencies of ac generators can be measured by VIBRATING-REED devices or TUNED CIRCUITS.
AUDIO FREQUENCIES can be measured by a process known as ZERO BEATING. This is done by matching an unknown signal with a locally generated signal of the same frequency obtained from a calibrated high-precision oscillator. As the two frequencies are brought closer to the same value, they reach a point of zero beat. This is when the frequency generated in the oscillator is equal to the frequency of the unknown signal being measured. Another term for zero beating is HETERODYNING.
WAVEMETERS are calibrated resonant circuits used to measure frequency. Any type of resonant circuit can be used in wavemeter applications. The type used depends on the frequency range for which the meter is intended.
For measuring frequencies in the microwave range, the CAVITY WAVEMETER is the type most commonly used.
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The CATHODE-RAY OSCILLOSCOPE and the SPECTRUM ANALYZER are used to perform WAVEFORM ANALYSIS.
A typical BACK-TO-FORWARD-RESISTANCE ratio for a diode is 10 to 1.
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ANSWERS TO QUESTIONS Q1. THROUGH Q12. A-1. Bel.
A-2. dBm.
A-3. 4 mW.
A-4. Dummy load or dummy antenna.
A-5. Barretter and thermistor.
A-6. It increases.
A-7. Beat frequency. A-8. Wavemeter.
A-9. Oscilloscope and spectrum analyzer.
A-10. Spectrum analyzer.
A-11. 10-to-1 ratio.
A-12. It tends to increase.
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Upon completing this chapter, you should be able to:
1. Describe the basic theory of the galvanometer.
2. Describe the basic theory of the D’Arsonval meter movement.
3. State the proper procedure for connecting an ammeter to a circuit.
4. Define ammeter sensitivity.
5. State the proper procedure for connecting a voltmeter to a circuit.
6. Describe possible effects on a circuit caused by the connection of a voltmeter.
7. Define voltmeter sensitivity.
8. Describe the internal operation of an ohmmeter with the use of a block diagram.
9. Describe the operating procedure for using a megohmmeter.
10. Describe the use of the electrodynamometer-type meter as a voltmeter, ammeter, and wattmeter.
11. Describe the factors that limit wattmeter capability.
12. Describe an open circuit, a ground, a short, and the tests used to check for these conditions.
When troubleshooting, testing, or repairing electronic equipment, you will use various meters and other types of test equipment to check for proper circuit voltages, currents, resistances, and to determine if the wiring is defective. You may be able to connect these test instruments to a circuit and take readings without knowing just how the instruments operate. However, to be a competent technician, you need to be able to do more than merely read a test instrument. You need a basic knowledge of how test instruments operate. This chapter discusses the operating principles of some of the test instruments you will use in equipment troubleshooting.
The best and most expensive measuring instrument is of no use to you unless you know what you are measuring and what each reading indicates. Remember that the purpose of a meter is to measure quantities existing within a circuit. For this reason, when the meter is connected to the circuit, it must not change the condition of the circuit.
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Meters are either SELF-EXCITED or EXTERNALLY EXCITED. Self-excited meters operate from their own power sources. Externally excited meters get their power from the circuit to which they are connected. Most common meters (voltmeters, ammeters, and ohmmeters) that you use in your work operate on the electromagnetic principle. All measuring instruments must have some form of indicating device, usually a meter, to be of any use to you. The most basic indicating device used in instruments that measure current and voltage operates by using the interaction between the magnetic fields associated with current flow in the circuit. Before continuing, you might want to review the properties of magnetism and electromagnetism in NEETS, Module 1, Introduction to Matter, Energy, and Direct Current.
Q-1. What meters operate from their own power sources? BASIC METER MOVEMENT
A stationary, permanent-magnet, moving-coil meter is the basic meter movement used in most measuring instruments used for servicing electrical equipment. When current flows through the coil, a resulting magnetic field reacts with the magnetic field of the permanent magnet and causes the movable coil to rotate. The greater the intensity of current flow through the coil, the stronger the magnetic field produced; the stronger the magnetic field produced, the greater the rotation of the coil. The GALVANOMETER is an example of one type of stationary, permanent-magnet, moving-coil measuring instrument.
The galvanometer is used to measure very low currents, such as those in bridge circuits. In modified form, the galvanometer has the highest sensitivity of any of the various types of meters in use today. A simplified diagram of a galvanometer is shown in figure 3-1. It is different from other instruments used for the same purpose because its movable coil is suspended by means of metal ribbons instead of a shaft and jewel-bearing arrangement often used in other instruments.
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The movable coil is wrapped around the aluminum frame of the galvanometer. The coil is suspended between the poles of the magnet by means of thin, flat ribbons of phosphor bronze. These ribbons provide a conduction path for the current between the circuit being tested and the movable coil. The ribbons allow the coil to twist in response to the interaction of the applied current through the coil and the magnetic field of the permanent magnet. They also provide the restoring force for the coil. Basically, the restoring force is that force necessary to return the movable frame to its resting position after a reading. The ribbons restrain or provide a counterforce to the magnetic force acting on the coil. When the driving force of the coil current is removed, the restoring force provided by the ribbons returns the coil to its zero position.
Q-2. What physical component of a galvanometer provides the restoring force for the coil? To determine the amount of current flow, we must have a means to indicate the amount of coil
rotation. Either of two methods may be used: (1) the POINTER arrangement or (2) the LIGHT AND MIRROR arrangement.
Q-3. In a galvanometer, what two methods are used to indicate the amount of coil rotation? In the pointer arrangement, one end of the pointer is mechanically connected to the rotating coil; as
the coil moves, the pointer also moves. The other end of the pointer moves across a graduated scale and indicates the amount of current flow. The overall simplicity of this arrangement is its main advantage. However, a disadvantage of this arrangement is that it introduces a mechanical coil balancing problem, especially if the pointer is long.
Q-4. What is the primary disadvantage of the pointer arrangement for indicating coil rotation? In the light and mirror arrangement, the use of a mirror and a beam of light simplifies the problem of
coil balance. When this arrangement is used to measure the turning of the coil, a small mirror is mounted on the supporting ribbon, as shown in figure 3-1. An internal light source is directed to the mirror and then reflected to the scale of the meter. As the movable coil turns, so does the mirror. This causes the light reflection to move across the graduated scale of the meter. The movement of the reflection is proportional to the movement of the coil; therefore, the intensity of the current being measured by the meter is accurately indicated.
If the beam of light and mirror arrangement is used, the beam of light is swept to the right or left across a translucent screen (scale). The translucent screen is divided uniformly with the zero reading located at center scale. If the pointer arrangement is used, the pointer is moved in a horizontal plane to the right or left across a scale that is divided uniformly with the zero reading at the center. The direction in which the beam of light or the pointer moves depends on the direction (polarity) of current through the coil.
Most dc instruments use meters based on some form of the D’Arsonval meter movement. In D’Arsonval-type meters, the length of the conductor and the strength of the field between the poles of the magnet are fixed. Therefore, any change in current causes a proportional change in the force acting on the coil. Figure 3-2 is a simplified diagram showing the principle of the D’Arsonval movement.
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