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Old 17th June 2005
Lives for gear
brianroth's Avatar

I assume an analog meter vs. a digital, since the voltages used at the test leads for the latter don't work correctly for this sort of test much of the time. Hence, I carry both a beat-to-hell Rat Shack analog meter in my tool kit as well as a newer digi-meter.

Between base and emitter and base and collector, you should see a diode ("medium" ohms with leads one way, infinity the other), while between emitter and collector it should be infinity ohms regardless of which way the leads are connected.

Which lead goes where depends upon the internal battery polarity of the meter, but *typically* black (minus) lead to base, then red to collector or emitter should see the "conducting diode", with reversed leads seeing open circuit, on a NPN transistor. Collector to emitter should be an open circuit regardless of the test lead polarity. Flip the test lead colors for a PNP.

With my Rat Shack meter, I use the Rx1 range when looking for the "diodes", and a higher range (maybe Rx100) when checking emitter to collector.

This test is usually invalid if the transistor is in-circuit since other directly connected parts will provide "sneak paths".