2-34
Transistor testers are nothing more than the solid-state equivalent of electron-tube testers (although
they do not operate on the same principle). With most transistor testers, it is possible to test the transistor
in or out of the circuit.
There are four basic tests required for transistors in practical troubleshooting: gain, leakage,
breakdown, and switching time. For maintenance and repair, however, a check of two or three parameters
is usually sufficient to determine whether a transistor needs to be replaced.
Since it is impractical to cover all the different types of transistor testers and since each tester comes
with its own operator's manual, we will move on to something you will use more frequently for testing
transistors-the ohmmeter.
Testing Transistors with an Ohmmeter
Two tests that can be done with an ohmmeter are gain, and junction resistance. Tests of a transistor's
junction resistance will reveal leakage, shorts, and opens.
TRANSISTOR GAIN TEST.—A basic transistor gain test can be made using an ohmmeter and a
simple test circuit. The test circuit can be made with just a couple of resistors and a switch, as shown in
figure 2-18. The principle behind the test lies in the fact that little or no current will flow in a transistor
between emitter and collector until the emitter-base junction is forward biased. The only precaution you
should observe is with the ohmmeter. Any internal battery may be used in the meter provided that it does
not exceed the maximum collector-emitter breakdown voltage.
Figure 2-18.—Testing a transistor's gain with an ohmmeter.
With the switch in figure 2-18 in the open position as shown, no voltage is applied to the PNP
transistor's base, and the emitter-base junction is not forward biased. Therefore, the ohmmeter should read
a high resistance, as indicated on the meter. When the switch is closed, the emitter-base circuit is forward
biased by the voltage across R1 and R2. Current now flows in the emitter-collector circuit, which causes a
lower resistance reading on the ohmmeter. A 10-to-1 resistance ratio in this test between meter readings
indicates a normal gain for an audio-frequency transistor.
To test an NPN transistor using this circuit, simply reverse the ohmmeter leads and carry out the
procedure described earlier.
