COMMON TEST EQUIPMENT

LEARNING OBJECTIVES

Upon completing this chapter, you should be able to:

In the previous chapters, you have learned how to use some basic and miscellaneous measuring instruments to perform required maintenance and upkeep of electronic systems and components. You were also introduced to the construction and operation of basic meter movements in test equipment. This chapter will introduce you to some of the testing instruments commonly used in the Navy today.

MULTIMETERS

During troubleshooting, you will often be required to measure voltage, current, and resistance. Rather than using three or more separate meters for these measurements, you can use the MULTIMETER. The multimeter contains circuitry that allows it to be used as a voltmeter, an ammeter, or an ohmmeter. A multimeter is often called a VOLT-OHM-MILLIAMMETER (VOM).

One of the greatest advantages of a VOM is that no external power source is required for its operation; therefore, no warm-up is necessary. Other advantages are its portability, versatility, and freedom from calibration errors caused by aging tubes, line voltage variations, and so forth.

Q.1 What is one of the greatest advantages of a VOM?

Two disadvantages are that (1) the VOM tends to "load" the circuit under test, and (2) the meter movement is easily damaged as a result of improper testing procedures.

Never press down on or place any object on the glass face of any multimeter. This can disable the meter movement from operating properly or cause damage.

MEASURING RESISTANCE, VOLTAGE, AND CURRENT WITH A NONELECTRONIC VOM

In the discussion that follows, you will become familiar with the operation and use of the multimeter in measuring resistance, voltage, and current.

The meter selected for this discussion is the Simpson 260 multimeter, as shown in figure 4-1. The Simpson 260 is a typical VOM used in the Navy today.

Figure 4-1. - Simpson 260 Series 6XLP Volt-Ohm-Milliammeter (VOM)

The multimeter has two selector switches. The switch on the lower left is the function switch, and the one in the lower center is the range switch. The function switch selects the type of current you will be measuring (+dc, -dc, or ac). The range switch is a 12-position switch that selects the range of ohmmeter, voltmeter, or milliammeter measurements you will make.

The multimeter is equipped with a pair of test leads; red is the positive lead and black is the negative, or common, lead. Eight jacks are located on the lower part of the front panel. To prepare the meter for use, simply insert the test leads into the proper jacks to obtain the circuit and range desired for each application. In most applications, the black lead will be inserted into the jack marked at the lower left with a negative sign (-) or with the word COMMON.

Measuring Resistance

Before proceeding, you should be aware of the following important safety precaution that must be observed when using the ohmmeter function of a VOM:

Never connect an ohmmeter to a "hot" (energized) circuit. Be sure that no power is applied and that all capacitors are discharged.

Q.2 Before you connect a VOM in a circuit for an ohmmeter reading, in what condition must the circuit be?

The internal components of the multimeter use very little current and are protected from damage by an overload protection circuit (fuse or circuit breaker). However, damage may still occur if you neglect the safety precaution in the CAUTION instructions above.

Because no external power is applied to the component being tested in a resistance check, a logical question you may ask is, Where does the power for deflection of the ohmmeter come from? The multimeter contains its own two-battery power supply inside the case. The resistive components inside the multimeter are of such values that when the leads are connected together (no resistance), the meter indicates a full-scale deflection. Because there is no resistance between the shorted leads, full-scale deflection represents zero resistance.

Before making a measurement, you must zero the ohmmeter to ensure accurate readings. This is accomplished by shorting the leads together and adjusting the OHMS ADJ control so that the pointer is pointing directly at the zero mark on the OHMS scale. The ZERO OHMS control is continuously variable and is used to adjust the meter circuit sensitivity to compensate for battery aging in the ohmmeter circuits.

An important point to remember when you are making an accurate resistance measurement is to "zero" the meter each time you select a new range. If this is not done, the readings you obtain will probably be incorrect.

When making a resistance measurement on a resistor, you must give the following considerations to the resistor being tested:

The resistor must be electrically isolated. In some instances, a soldered connection will have to be disconnected to isolate the resistor. Generally, isolating one side of the resistor is satisfactory for you to make an accurate reading. The meter leads must make good electrical contact with the resistor leads. Points of contact should be checked for dirt, grease, varnish, paint or any other material that may affect current flow. Touch only the insulated portions of the test leads. Your body has a certain amount of resistance, which the ohmmeter will measure if you touch the uninsulated portions of the leads.

Figure 4-2 is a functional block diagram of the ohmmeter circuit in a VOM. The proper method of checking a resistor is to connect the red lead to one end of the resistor and the black lead to the other end of the resistor.

Figure 4-2. - Functional block diagram of an ohmmeter circuit.

Because zero resistance causes full-scale deflection, you should realize that the deflection of the meter is inversely proportional to the resistance being tested; that is, for a small resistance value, the deflection will be nearly full scale; and for a large resistance value, the deflection will be considerably less. This means that the left portion of the OHMS scale represents high resistance; the right side of the scale represents low resistance. Zero resistance (a short circuit) is indicated on the extreme right side of the scale; infinite resistance (an open circuit) is located on the extreme left side of the scale.

Notice that you read the OHMS scale on the multimeter from RIGHT to LEFT. For example, the pointer of the multimeter in figure 4-3 indicates 8.0 ohms. To determine the actual value of a resistor, multiply the reading on the meter scale by the range switch setting (R X 1, R X 100, or R X 10,000).

Figure 4-3. - Ohmmeter scale.

Notice that the scale marks are crowded on the left side of the OHMS scale, which makes them difficult to read. Therefore, the best range to select is one in which the pointer will fall in the space from midscale to slightly to the right side of midscale. The divisions in this area of the scale are evenly spaced and provide for easier reading and greater accuracy.

Q.3 When taking resistance readings with a VOM, you will obtain the most accurate readings at or near what part of the scale?