Module 16—Introduction to Test Equipment
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COMMON TEST EQUIPMENT
Upon completing this chapter, you should be able to:
1. Describe the proper operating
procedures for using the multimeter.
2. Describe the proper operating procedures for using the digital
3. Describe the proper operating procedures for using the differential voltmeter.
4. Describe the proper operation of the transistor tester.
5. Describe the proper procedure for using
the RCL bridge to measure resistance, capacitance, and inductance.
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.
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
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 VOM
In the discussion that
follows, you will become familiar with the operation and use of the multimeter in measuring resistance, voltage,
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
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
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
When making a resistance measurement on a resistor, you must give the following considerations to the resistor
· 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.
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?
To explain the relationship between the
meter readings and the range switch setting, let’s use an example. Suppose you have a 2,400-ohm resistor, which
you have identified by the resistor color code. With the range switch in the R ´ 1 position, you connect the meter
across the resistor. The meter point then deflects between 200 and the point labeled with the infinity symbol (¥)
on the extreme left side of the scale. Because the R ´ 1 range is selected, you multiply the reading by 1.
Obviously, the scale reading is not accurate enough. Therefore, you move the range selector switch to the next
higher scale position (R x 100) to obtain a more easily read value.
In the R x 100 position, you again
zero the meter. This time, the pointer moves to the 24 mark on the scale. Because the R x 100 scale is selected,
the reading is multiplied by 100. This gives a more accurate reading of 2,400 ohms (24 times 100).
position the range switch to the R x 10,000 scale, accuracy decreases. The most accurate readings are obtained at
or near midscale. Other VOM instruments have ranges with other settings, such as R x 10, R x 100, or R x 1,000, to
make it easier to make such readings.
Another thing to remember when you are measuring resistance is the tolerance of the resistor. If the tolerance of
the resistor in the preceding example is 10 percent, we would expect a reading between approximately 2,160 and
2,640 ohms. If the reading is not within these limits, the resistor has probably changed value and should be
An open resistor will indicate no deflection on the meter. A shorted resistor causes full-scale deflection to the
right on the lowest range scale, such as if the leads were shorted together.
Measuring dc Voltages
You set the multimeter to operate as a dc voltmeter by
placing the function switch in either of two positions: +DC or -DC. The meter leads, as in the case of the
ohmmeter function, must be connected to the proper meter jacks. When you measure dc voltages, be sure the red lead
is the positive lead and the black lead is the negative, or common, lead. View A of figure 4-4 is a functional
block diagram of dc voltage circuits in a multimeter. View B shows the jacks and switch positions for measuring dc
Figure 4-4.—Functional block diagram of dc voltage circuits.
When the meter is connected in a circuit, it becomes a circuit component. Because all meters have some
resistance, they alter the circuit by changing the current. The resistance presented by the voltmeter depends on
the amount of voltage being measured and the position of the function switch.
Some multimeters use a
20,000 ohms-per-volt meter sensitivity for measuring dc voltage and a 5,000 ohms-per-volt sensitivity for
measuring ac voltage. The higher the meter resistance, the less it will load the circuit. The idea is to keep
circuit loading to an absolute minimum so that the circuit under test is unaffected by the meter. In this way, you
can get a clearer picture of what the circuit malfunction is, not the effect of the meter on the circuit.
Again, refer to figure 4-4. With the function switch set to either +DC or -DC, let’s consider the effect of the
range switch on the meter scale to be used. When measuring dc voltages, you have eight voltage ranges available:
.25V, 2.5V, 10V, 50V, 250V, and 500V (1- and 1,000-volt special application plug-ins are also available). The
setting of the range switch determines the maximum value represented on the meter. When measuring dc voltages, use
the scale marked DC (figure 4-3). The last number at the extreme right side of the DC scale indicates the maximum
value of the range being used. When the range switch is in the 2.5V position, the scale represents a maximum of
To simplify the relationship between the digits on the meter scale and the setting of the range
switch, always use the multiple of the full-scale-deflection digits on the meter face that correspond to the
numbers of the range switch. For example, use the 250 scale for the 250MV jack, 2.5V, and 250V ranges; the 50 on
the scale for 50V and 500V ranges; and the 10 on the scale for the 10V and 1,000V ranges.
For explanation purposes, let’s assume you wish to measure 30 volts dc. In this case, select the next
higher range position, 50V. When you place the range switch to the 50V position (as shown in view B of figure
4-4), the meter pointer should rise from a little more than midscale to 30, which represents 30 volts dc.
When measuring a known dc voltage, position the range switch to a setting that will cause approximately midscale
deflection. Readings taken near the center of the scale are the most accurate. When measuring an unknown dc
voltage, always begin on the highest voltage range. Using the range switch, work down to an appropriate range. If
the meter pointer moves to the left, you should reverse the polarity of the function switch.
Always check the polarity before connecting the meter.
Q-4. Besides setting up the meter for expected voltage ranges, what must be strictly observed when
taking dc voltage readings?
Now let’s discuss how you take a voltage measurement on a component within a
circuit. As an example, let’s measure the voltage drop across the resistor shown in figure 4-5.
Figure 4-5.—Measuring the voltage drop of a resistor.
When measuring a dc voltage drop across a component in a circuit, you must connect the voltmeter in
parallel with the component. As you can see in figure 4-5, the positive (red) lead is connected to the positive
side of the resistor, and the negative (black) lead is connected to the negative side. A voltage reading is
obtained on the meter when current flows through the resistor.
Some voltmeter readings will require the
use of a ground as a reference point. Under these conditions, one voltmeter lead is connected to the equipment
ground, and the other lead is connected to the test point where voltage is to be measured. Be sure to observe
Measuring ac Voltages
To measure ac voltages, you must set the function switch to the AC
position. The same procedure used to measure dc voltages applies, except that in reading the voltage, you use the
AC volts scale (the polarity of the test leads is not important). When measuring very low or high frequencies of
ac voltages, you should be aware that the multimeter has a tendency to be inaccurate. View A of figure 4-6 is a
functional block diagram of the ac and output voltage circuits in the multimeter. View B shows the
jacks and switch positions used to measure ac voltages.
Figure 4-6.—Functional block diagram of ac and output voltage circuits.
Measuring Output Voltages
You will often measure the ac component of an output
voltage where both ac and dc voltage levels exist. This occurs primarily in amplifier circuits.
The multimeter has a 0.1-microfarad, 400-volt blocking capacitor in series with the OUTPUT jack. The capacitor
blocks the dc component of the current in the circuit under test, but allows the ac component to pass on to the
When using OUTPUT, do not attempt to use the meter in a circuit in which the dc voltage
component exceeds the 400-volt rating of the blocking capacitor.
To use the multimeter to measure output voltage, you must follow these steps:
1. Set the
function switch to AC.
2. Plug the black test lead into the COMMON jack and the red test lead into the
3. Set the range switch at the appropriate range position, marked as 2.5V, 10V, 50V, or
4. Connect the test leads to the component being measured with the black test lead to the negative
side of the component.
5. Turn on the power in the test circuit. Read the output voltage on the appropriate ac voltage
scale. For the 2.5V range, read the value directly on the scale marked 2.5. For the 10V, 50V, or 250V range, use
the red scale marked AC and read the black figures immediately above the scale. Measuring Current
The multimeter can function as an ammeter to measure current flow.
When using the multimeter as a current-indicating instrument, NEVER connect the test
leads directly across a voltage. ALWAYS connect the instrument in series with the load.
To use the multimeter as an ammeter, you must take the following steps:
1. Set the function
switch at +DC (assuming the current to be positive).
2. Plug the black test lead in the COMMON jack and
the red test lead into the + jack.
3. Set the range switch at one of the five ampere-range positions.
4. Ensure the equipment is OFF and then physically open the circuit in which the current is being measured.
5. Connect the VOM in series with the circuit, ensuring that proper polarity is observed when making this
6. Turn the equipment ON and then read the current on the DC scale. (This is the same scale
used to measure dc voltages.)
The setting of the range switch determines the maximum value represented by the DC scale. Always use the range
scale that corresponds to the range switch setting.
Never attempt to measure currents greater than the setting of the range switch.
Increase the range with a shunt, if necessary, but do not exceed the marked current.
When measuring unknown currents, follow the same procedures as when measuring voltages. Always start
with the highest range available and work down. Use the range that gives approximately half-scale deflection. If
this procedure isn’t followed, the meter could be burned out.
Figure 4-7 is a functional block diagram of
the dc current circuits in a multimeter.
Introduction to Matter, Energy, and Direct Current,
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,
, Introduction to Number Systems and Logic Circuits, Introduction
to Microelectronics, Principles of Synchros, Servos, and Gyros
Introduction to Test Equipment
, Radar Principles,
The Technician's Handbook,
Master Glossary, Test Methods and Practices,
Introduction to Digital Computers,
Magnetic Recording, Introduction to Fiber Optics