Module 21  Test Methods and Practices
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531 to 535, AI1 to AI3, Index
MAXWELL BRIDGE The Maxwell bridge, shown in view C of figure 114, measures inductance by comparing it with a capacitance and (effectively) two resistances.] This bridge circuit is employed for measuring inductances having losses greater than 0.05 (expressed by the D dial reading). For such inductors it is necessary to introduce, in place of the series control (D dial), a new loss control (Q dial), which shunts the standard capacitor. This control, which becomes effective when the FUNCTION switch is turned to the L(Q) position, is conveniently calibrated in values of Q, the storage factor of the inductor under measurement. The balance for inductance is the same for either bridge circuit. This permits the use of the same markings on the RANGE switch for both the L(D) and L(Q) positions of the FUNCTION switch. REACTANCE MEASURING EQUIPMENT The reactance type of inductance measuring equipment makes use of the following principle: If an ac voltage of fixed frequency is applied across an inductor (and a resistor in series), the voltage drop produced across the reactance of the inductor by the resulting current flow is directly proportional to the value of the inductance. An inductance measurement using the reactance method is identical to capacitance measurements using the same method, except that current flow is directly proportional to the value of inductance, rather than inversely proportional as in the case of capacitance. It follows then that if a reactancetype capacitance measuring equipment is provided with a chart that converts the capacitance readings to equivalent inductance values and a proper range multiplying factor, the same test setup can be used to measure both capacitance and inductance. In practice, test equipment using the reactance method for capacitance measurements usually provides an inductance conversion chart. Because the current flowing through the inductance under test is directly proportional to the value of inductance, the reciprocals of the capacitance range multipliers must be used; for example, a multiplier of 0.1 becomes and a multiplier of 100 becomes The reactancetype equipment gives approximate values only. Like the analog multimeter, it is used only when portability and speed are more important than precision. If the ohmic resistance of the inductor is low, the inductance value obtained from the conversion chart can be used directly. If the ohmic value (as measured with an ohmmeter) is appreciable, a more accurate value of inductance can be obtained by use of the following formula:
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Q16. Is the current flow through an inductor directly proportional or inversely proportional to its inductance value? MEASUREMENT OF INDUCTANCE USING THE VTVM If you do not have a 250DE+1325 at your disposal, the inductance of a coil can be determined by using a VTVM and a decade resistance box, as shown in figure 115. In the following example the inductance of an unknown coil in the secondary winding of a 6.3volt filament transformer will be determined with a VTVM and decade resistance box. The unknown coil must be connected in series with the decade resistance box. The voltage across the decade box and across the coil must be monitored as the decade box is adjusted. When equal voltages are reached, read the resistance of the decade box. Since the voltage across the inductor equals the voltage across the decade box, the X_{L} of the coil must be equal to the resistance read on the decade box. For example, assume that the resistance reading on the decade box is 4 kilohms and the frequency is 60 hertz. This must mean that the X_{L} of the coil is also equal to 4,000 ohms. The inductance formula L = X_{L}/2πf Figure 115.  Determining inductance with a VTVM and decade resistance box. 122
SUMMARY This chapter has presented information on basic measurements. The information that follows summarizes the important points of this chapter. The five basic measurements are VOLTAGE, CURRENT, RESISTANCE, CAPACITANCE, and INDUCTANCE. The accuracy of all measurements depends upon YOUR SKILL as a technician and the accuracy of your TEST EQUIPMENT. Accuracy of different types of test equipment varies greatly and depends on design characteristics, tolerances of individual components, and YOUR KNOWLEDGE of test equipment applications. The METCAL program ensures that your calibrated test equipment meets established specifications. Most equipment technical manuals contain VOLTAGE CHARTS which list correct voltages that should be obtained at various test points. It is important to remember that the INPUT IMPEDANCE of your test equipment must be high enough to prevent circuit loading. When you are performing ac voltage measurements, an additional consideration that greatly affects the accuracy of your measurements is the FREQUENCY LIMITATIONS of your test equipment. Ac and dc CURRENT MEASUREMENTS can be performed using a wide variety of test equipment. Most current measurements require you to break the current path by unsoldering components and wires and inserting an ammeter in series with the current path. One alternative method is to compute (using OHM'S LAW) the current through a circuit by measuring the voltage drop across a known resistance. Another alternative is to use a CURRENT PROBE that requires no unsoldering.
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When performing resistance measurements, your primary concerns are the RANGE AND DEGREE OF ACCURACY of your test equipment. In most instances, an analog multimeter is accurate enough to perform basic troubleshooting. When measuring extremely large resistances, you are sometimes required to use a MEGGER or a DIFFERENTIAL VOLTMETER.
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When testing currentsensitive devices, you must be certain that the current produced by your test equipment does not exceed the current limitations of the device being tested. Capacitance and inductance measurements are seldom required in the course of troubleshooting. These measurements are usually performed with various types of BRIDGES or with a reactance type of measuring device. The bridge measuring techniques are more commonly used and are more accurate than reactance types of measurements. REFERENCES 8000A Digital Multimeter, NAVSEA 0969LP2799010, Naval Sea Systems Command, Washington, D.C., undated. EIMB, Test Methods and Practices Handbook, NAVSEA 0967LP0000130, Naval Sea Systems Command, Washington, D.C., 1980. EIBM, General, NAVSEA 09670000100, Naval Sea Systems Command, Washington, D.C., 1983. Instruction Manual for Universal Impedance Bridge, Model 250DE, 13202, Electro Scientific Industries, 13900 N. W. Science Park Drive, Portland, Oregon 97229, March 1971. 125
Instruction Manual, Model 893A/AR ACDC Differential Voltmeter, NAVSEA 0969LP2797010, Naval Sea Systems Command, Washington, D.C., 1969. Operation and Maintenance Instruction, Current Tracer 547A, NAVAIR 16453103, Naval Air Systems Command, Washington, D.C., 1979. Operation and Maintenance Instructions, VoltOhmMilliammeter, 260 Series 6P, NAVSEA 0969LP2861010, Naval Sea Systems Command, Washington, D.C., 1974. ANSWERS TO QUESTIONS Q1. THROUGH Q16. A1. Its calibration. A2. 10 to 1 A3. Increased input impedance, greater accuracy, and increased voltage range. A4. Midscale. A5. Accuracy and high input impedance. A6. The range of frequencies that can accurately be measured. A7. At least 60% of the vertical trace. A8. Decreased internal meter resistance, greater accuracy, and greater current range. A9. Current probes enable you to perform current measurements without disconnecting wires. Current probes are clamped around the insulated wire. A10. By zeroing the meter with the test leads shorted. A11. The current flow through the component is limited to 1 milliamp. A12. True. A13. Bridge type. A14. Magneticmetal core. A15. A capacitor. A16. Directly proportional. 126
NEETS Table of Contents
 Introduction to Matter, Energy,
and Direct Current
 Introduction to Alternating Current and Transformers
 Introduction to Circuit Protection,
Control, and Measurement
 Introduction to Electrical Conductors, Wiring
Techniques, and Schematic Reading
 Introduction to Generators and Motors
 Introduction to Electronic Emission, Tubes,
and Power Supplies
 Introduction to SolidState Devices and
Power Supplies
 Introduction to Amplifiers
 Introduction to WaveGeneration and WaveShaping
Circuits
 Introduction to Wave Propagation, Transmission
Lines, and Antennas
 Microwave Principles
 Modulation Principles
 Introduction to Number Systems and Logic Circuits
 Introduction to Microelectronics
 Principles of Synchros, Servos, and Gyros
 Introduction to Test Equipment
 RadioFrequency Communications Principles
 Radar Principles
 The Technician's Handbook, Master Glossary
 Test Methods and Practices
 Introduction to Digital Computers
 Magnetic Recording
 Introduction to Fiber Optics
