Is it possible to test a transistor while it is still in a circuit - at least to determine whether it's bad? I've searched for the answer online and found conflicting information. There are some YouTube videos showing people testing transistors on PCBs using the diode mode of a DMM. But I found a couple sites (here and here) saying it can only be done a different way. Those two descriptions seem to be describing similar methods, but I don't understand them. Do they require an external voltage source or powering-on the device being tested?

 

Not really conclusively, often if it is shorted it is possible, but it all depends on the type of circuit, devices attached to etc.

 

Is it possible to test a transistor while it is still in a circuit - at least to determine whether it's bad? I've searched for the answer online and found conflicting information. There are some YouTube videos showing people testing transistors on PCBs using the diode mode of a DMM. But I found a couple sites (here and here) saying it can only be done a different way. Those two descriptions seem to be describing similar methods, but I don't understand them. Do they require an external voltage source or powering-on the device being tested?

In general, no. Whatever measurement you are making is being affected not only by the transistor, but by everything else that is connected to whatever nodes you are probing. If you know enough about the circuit, then you can often put useful limits on what the measurements should be to divine useful information from them, but without that knowledge, and the effort to figure out what those limits are and what the measurement may or may not mean, it is pretty difficult. There's some low hanging fruit, of course. If you use the diode check on the base-emitter junction and it shows an open, that almost certainly means the transistor is bad. But if it shows what you would expect from a good transistor, that transistor may still be bad because your measurement may be measuring the diode drop of some other diode or transistor connected to the same traces.

 

What about go/no go testers like this?

 

Testing any component in circuit means testing the entire circuit connected to the DUT (device under test).
If you have information on the connecting circuit, you can make an informed decision on whether the device is good or bad.

A dead short, zero ohms or low resistance with an ohmmeter measuring in both directions is a good chance of a shorted component. Any other indication is not sufficient to make a valid diagnosis.

 

What about go/no go testers like this?

Same comment -- everything else in the circuit is going to affect the results of the test and it is going to be up to the person using it to determine whether the test results are ambiguous or not.

 

In a analog(linear) powered circuit it is possible to check the base to emitter voltage to see if it is about 0.7 volts. It is also possible to check the emitter voltage (across the emitter resistor If the voltage is not zero then probably the transistor is conducting. Likewise, cehecking the collector to emitter voltage and seeing it is less than the supply voltage will tell you that the transistor is conducting. But if it is zero, the transistor might be shorted. Or the circuit might be non-powered.
NOTE!!! Those tests would be done with a DIGITAL voltmeter with a very high input resistance. An analog meter will not be suitable.