COLLECTOR-TO- EMITTER TEST
Figure 7-11.--Testing a transistor's gain with an ohmmeter.
the test lies in the fact that little or no current flows in a
transistor between emitter and collector until the
emitter-base junction is forward biased.
Ensure that the output of the tester does not exceed
the maximum collector-emitter breakdown
voltage of the transistor.
With the switch in the open position, as shown in
figure 7-11, no voltage is applied to the PNP
transistor's base; thus, 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. This causes a
lower resistance reading on the ohmmeter. A 10 to 1
resistance ratio in this test between meter readings
NOTE: REVERSING THE METER LEADS
indicates a normal gain for an audio-frequency
GIVES A LOW READING
Figure 7-12.--Testing a transistor's leakage with an
To test an NPN transistor by using this circuit,
simply reverse the ohmmeter leads and carry out the
procedure described earlier.
An ohmmeter can be used to test for transistor
When making these measurements, avoid
leakage (an undesirable flow of current) by measuring
using the R 1 scale on the meter or a meter with a
the base-emitter, base-collector, and collector-emitter
high internal battery voltage. Either of these
forward and reverse resistances.
conditions can damage a low-power transistor.
In each view of figure 7-12, consider the transistor
under test as two diodes connected back to back. Each
diode has a low-forward resistance and a high-reverse
Now consider the possible transistor problems that
resistance. By measuring these resistances with an
could exist if the indicated readings are not obtained. A
ohmmeter, as shown in the figure, you can determine if
the transistor is leaking current through its junctions.
list of these problems is shown in table 7-1.