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TM 9-254
8-2.
Meter Usage - Continued
(b)
Before attempting to make a resistance measurement, touch the ends of the leads together and
zero adjust the ohmmeter.
(c)
If it becomes necessary to change the range setting of the ohmmeter, always zero adjust the meter
before proceeding with the measurement.
Figure 8-3. Proper Ohmmeter Connection
f.
Transistor Test Technique with an Ohmmeter. The ohmmeter can do more than check for shorted or open
circuit conditions. It can also be used to check the operating condition of transistors. Since the ohmmeter function of a
multimeter has the same basic function as a transistor tester, the ohmmeter can be a valuable piece of test equipment
when it is used to check transistor characteristics.
(1)
Test requirements.
(a)
Before the instrument’s test leads are connected across a transistor, a few characteristics of the
ohmmeter must be understood. Its internal test potential may be anywhere from 1.5 volts to beyond
6 volts. The higher potential, if used, is likely to be involved only on the highest resistance ranges.
Some transistors may be damaged when potentials in excess of 6 volts are applied. To avoid
damage due to excessive current, we would also want some current-limiting resistance in series
with the ohmmeter’s test voltage. An inspection of the ohmmeter circuit will indicate which
resistance ranges may be used and which to avoid. In general, avoiding the highest resistance
ranges (possibly excessive voltage) and the lowest (possibly excessive current) will be sufficient
precaution. The R x 10 or R x 100 scales may be considered safe.
(b)
Polarity of the internal battery must also be considered. In most instruments, the ground or
common lead (black) goes to the negative side of the battery, with the hot or red lead going to the
positive side. If this situation is reversed, the user must be aware of the fact. In all tests to be
discussed, references to a positive lead or connection, or to a negative lead or connection, are
always related to the polarity of the battery rather than the identification of the lead, where these two
factors do not agree with each other.
Change 1 8-4
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