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Module 16—Introduction to Test Equipment

Chapter 1: Pages 1-21 through 1-33

Pages i - ix, 1-1 to 1-10, 1-11 to 1-20, 1-21 to 1-33, 2-1 to 2-10, 2-11 to 2-20, 2-21 to 2-27, 3-1 to 3-10, 3-11 to 3-20, 3-21 to 3-30,

3-31 to 3-34, 4-1 to 4-10, 4-11 to 4-20, 4-21 to 4-28, 5-1 to 5-10, 5-11 to 5-20, 5-21 to 5-30, 5-31 to 5-40, 6-1 to 6-10,

6-11 to 6-20, 6-21 to 6-30, 6-31 to 6-40, 6-41 to 6-46, Index

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 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.

1-21

Figure 1-5.—Wheatstone bridge.

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 R

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

and

With this information, we can figure the value of the unknown resistor R

We can simplify this equation:

1-22

then we multiply both sides of the expression by R

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

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.

Figure 1-6.—Resistance bridge (ac).

For example, if in figure 1-6 we know that R1 is 20 ohms, R2 is 40 ohms, and R

1-23

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. (

or

Figure 1-7.—Capacitance bridge.

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 C

1-24

Similarly, the following resistance ratio exists between the four arms of the bridge, just as in the resistance bridge expression discussed earlier:

or

Thus, both the unknown resistance and capacitance, R

In figure 1-7, for example, we know that R1 is 20 ohms, R2 is 40 ohms, Rs is 60 ohms, and C

and

Q-19. When a bridge is used to measure resistance, what is the value of R

1-25

Figure 1-8.—Inductance bridge.

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 L

(NOTE: The right side of this expression is NOT inverse as it was in the capacitance bridge.)

and

or

In figure 1-8, for example, the values of R1, R2, and R

1-26

and

Thus, both the unknown resistance and inductance can be estimated in terms of the known values for

R1, R2, R

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?

**SUMMARY**

The important points of this chapter are summarized in the following paragraphs:

The

The

The

The

1-27

The

The

The

1-28

The

The

The

1-29

The

1-30

The

The

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

1-31

The

An

1-32

**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.

1-33

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 Solid-State Devices and Power Supplies, Introduction to Amplifiers, Introduction to Wave-Generation and Wave-Shaping 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, Radio-Frequency Communications Principles, Radar Principles, The Technician's Handbook, Master Glossary, Test Methods and Practices, Introduction to Digital Computers, Magnetic Recording, Introduction to Fiber Optics