Multimeter Diode Test on Transistors??

[Robbie010]

Evening all,

I got myself a cheap Multimeter today that has a Diode test setting. I read online that this can be used for checking transistors but when I test one it just gives me a number without telling me what it is i.e V, mV etc etc, so what exactly is this test telling me?

When testing an MPS L01 (BJT NPN) Transistor I get the following readings:

Leg 2-3 = 518
Leg 2-1 = 521

Is it forward voltage?

Thanks

[mikerj]

That will be the forward voltage in millivolts.

[Robbie010]

Thank you very much.

And is this number variable dependent upon the voltage in or would it remain constant (to a certain point)?

[David Hess]

Those numbers are the forward voltage drop of the two junctions.  If your measurements are good enough, which includes not holding the transistor in your fingers while making the measurements because the forward voltage drops change by about -2mV/C, then I believe the emitter junction will have a slightly higher voltage drop than the collector junction.  But the diode test is mostly useful for verifying that neither junction is open or shorted rather than revealing anything about performance.

[Aubrey999]

Yes, from my experience, I find that the resistance of the Emitter is usually the higher one, regardless if it is NPN or PNP.
So that is a way to work out the polarity of the transistor, and the lead-outs.