Testing Transistors In the Circuit

People have lots of trouble trying to figure out whether transistors are good. This is easy for ordinary PNP and NPN types.

For an ordinary NPN type, the collector will be at a higher voltage than the emitter, perhaps through some collector resistance, Rc. The emitter may have some Re to the most negative supply. It's almost certain that you won't have both no Rc and no Re, as there's no way to get signal out of that.

The base will be biased some way; we represent this as a resistor Rb to a bias voltage. Usually Rb is fairly big, 10K to as much as a half meg. It may be two resistors in series from +V to ground; same thing.

To test, hook your voltmeter up as shown, positive to collector and negative to emitter. Using a clip lead, connect one end to the base, and momentararily short the base to the emitter - just touch it and let it go. The voltmeter should show a suddenly increased voltage, as you've just turned the transistor off. Now use the same  clip lead and attach a 10K resistor to the free end. Momentarily touch the free end of the 10K resistor to the collector. The

voltmeter should show a sudden dip in voltage because you've just added a great deal of base current. Note that although the test does cause a lot of current flow when you short the base to collector, it is almost never fatal to the transistor if you touch the wire just long enough to read the voltmeter. DO NOT try this with power transistors in power amps, though, or with transistors that drive a transformer. It's safe with signal circuits that have resistances between the collector and the power supply.

For a PNP, the situation is the same, only the polarities have been reversed. The collector is now more negative than the emitter and base. Use the clip lead to momentarily touch the base to emitter; the voltmeter will go to a bigger voltage because you're turning the transistor off. Then touch the base to collector, and watch for the voltmeter to indicate a low voltage, under 1V.

If you don't get the changes you expect, either the transistor is bad, or something is already holding the transistor off or on, and you can make good guesses which it is by which way something changed.