4-3
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:
CAUTION
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.
