How to Test a Transistor with a Multimeter

A bipolar junction transistor can be viewed as two semiconductor junctions sharing the base terminal. Diode-mode measurements can therefore reveal many common failures without powering the circuit.

Identify the transistor and pinout

Determine whether the device is NPN or PNP and confirm its base, collector and emitter pins from a reliable datasheet. Package appearance alone is not a dependable pinout guide.

Check the base junctions

For an NPN transistor, forward-biased measurements from base toward emitter and collector normally behave like diode junctions. A PNP device uses the opposite probe polarity. Exact forward readings depend on the device and temperature.

Check collector-to-emitter behavior

With the base not intentionally driven, a very low resistance or diode-mode reading between collector and emitter in both directions can indicate a shorted transistor.

Be careful with in-circuit paths

Base resistors, protection diodes and surrounding semiconductors can change the readings. Compare with an identical channel or isolate the device when necessary.