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4.10 Resistance Measurements
Voltage applied to a resistance range will cause reading errors if low and dam-
age the Instrument if high. When making in-circuit measurements, make certain
that the circuit is completely de-energized before making connections to it.
When making in-circuit measurements, circuit paths in parallel with the resis-
tance being measured may cause reading errors. Check circuit diagrams for the
presence of such components before assuming that the reading obtained is
correct.
When resistance is measured, the batteries furnish power for the circuit. Since
batteries are subject to variation in voltage and internal resistance, the Instru-
ment must be adjusted to zero before measuring a resistance, as follows:
a.
Turn range switch to desired ohms range.
b.
Plug the black test lead into the -COMMON jack and the red test lead into the
+ jack.
c.
Connect the ends of test leads together to short the VOM resistance circuit.
d.
Rotate the ZERO OHMS control until pointer indicates zero ohms. If pointer
cannot be adjusted to zero, one or both of the batteries must be replaced.
(For battery replacement, refer to Section 5.)
e.
Disconnect shorted test leads.
4.11 Measuring Resistance
a.
Before measuring resistance in a circuit, make sure the power is off to the
circuit being tested and all capacitors are discharged. Disconnect shunting
components from the circuit before measuring its resistance.
b.
Set the range switch to one of the resistance range positions as follows
(Figure 4-13):
1. Use R X 1 for resistance readings from 0 to 200 .
2. Use R X 100 for resistance readings from 200 to 20,000 .
3. Use R X 10,000 for resistance readings above 20,000 .
c.
Set the function switch at either -DC or +DC position: Operation is the same
in either position except if there are semiconductors in the circuit. (See para-
graph 4.12.) Adjust ZERO OHMS control for each resistance range.
d.
Observe the reading on the OHMS scale at the top of the dial.
NOTE:
The OHMS scale reads from right to left for increasing values of resis-
tance.
e.
To determine the actual resistance value, multiply the reading by the factor at
the switch position. (K on the OHMS scale equals one thousand.)
4.12 Resistance Measurement of Semiconductors
Make sure that the OHMS range being used will not damage any of the semicon-
ductors (refer to Table 1-1, item 8, Section 1).
If there is a “forward” and “backward” resistance such as in diodes, the resistance
should be relatively low in one direction (for forward polarity) and higher in the
opposite direction.
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