How to Test a Transistor using Multimeter

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 shows the process of testing an npn transistor with an ohmmeter.

(i) The forward biased base-emitter junction (biased by internal supply) should read a low resistance, typically 100 to 1 k as shown in Fig. (i). If that is so, the transistor is good. However, if it fails this check, the transistor is faulty and it must be replaced.

How to Test a Transistor using Multimeter

(ii) The reverse biased collector-base junction (again reverse biased by internal supply) should be checked as shown in Fig. (ii). If the reading of the ohmmeter is 100 k  or higher, the transistor is good. If the ohmmeter registers a small resistance, the transistor is faulty and requires replacement.

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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