checking pnp transitors

[scampo77]

how can i check a pnp or npn transistor while it is still in the board?

what should i be looking for? im mostly looking at the TO-92 cases.

i would think first check the center leg for voltage (neg or pos) if there is something there then the 2 outer legs should read the same voltage? is this correct?

[Perry Babin]

It's difficult to check them in the board definitively because current will flow (from your meter) through other components. A leaky transistor will typically be missed.

Generally, you can determine if both junctions are intact but again, leakage through other components can cause false readings.

Of course, you can find those that are shorted but they only short ~1/2 of the time. It's best to remove them for definitive testing (the most definitive with a multimeter, that is).

[scampo77]

i guess im just trying to better understand what i am looking at

if i have voltage readings on all 3 legs wont this tell me something?

example

B- 3v
C- 3v
E- 3v

vs

B- 3v
C- 3v
E- 0v

would this automatically mean the second is bad and the first is good?

when switching a transistor on and off do i use the same switching voltage as the what is passing through it?

[Perry Babin]

I'd suspect that the first one is shorted. The second wouldn't tell you much if this is a PNP transistor. The B-E junction is reverse biased so it can't do anything in the circuit.