Transistor Testing-An ohmmeter can be used to check the state of a transistor i.e., whether the transistor is
good or not. We know that base-emitter junction of a transistor is forward biased while
collector-base junction is reverse biased.
Therefore, forward biased base-emitter
junction should have low resistance and reverse biased collector-base junction should
register a much higher resistance. Fig. 1 shows the process of testing an n-p-n
transistor with an ohmmeter.
(i) The forward biased base-emitter junction (biased by internal supply)
should read a low resistance, typically 100 Ω to 1 kΩ as shown in Fig.1
(i). If that is so, the transistor is good. However, if it fails this check, the
transistor is faulty and it must be replaced.
(ii) The reverse biased collector-base junction (again reverse biased by
internal supply) should be checked as shown in Fig. 1(ii). If the reading
of the ohmmeter is 100 kΩ or higher, the transistor is good. If the
ohmmeter registers a small resistance, the transistor is faulty and requires
Note. When testing a pnp transistor, the ohmmeter leads must be
reversed. The results of the tests, however, will be the same.
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