Transistor Test -- Different Results with Different Methods

[JockomoFeeno]

I tested a B950A PNP transistor pulled from an amp that I was working on. Using a DMM, the collector-to-emitter reads .540v. A good transistor should read OL. However, when I test the same transistor with my Drok transistor tester, it seems to be okay. Am I reading the results wrong or am I doing something wrong? Pics of results are attached. I'm learning, so thanks for helping.

 

[ecluser]

Consult the datasheet of the 2SB950A. You will see a flyback diode connected between the collector and emitter. It is normal to measure a low voltage from the collector to emitter, but you should have an OL reading from the emitter to collector.

 

[JockomoFeeno]

Thanks for clearing that up. I'll pay attention to the datasheet next time.

 

[audiodummy]

Nice thing about those modern transistortesters like the one you have in the picture actually shows that diode connected -- and you should "see" that diode with your DMM!

That diode is very common in power transistors in general, most modern transistors will have one integrated. Especially MOSFETs that diode tends to be parasitic due to manufacture, but nevertheless helpful in a lot of situations!

 

[ecluser]

Nice thing about those modern transistortesters like the one you have in the picture actually shows that diode connected

But that "modern transistortester" shows the diode in the wrong direction, according to common sense. It seems that the digital meter has the red probe on the collector, and the black probe on the emitter. This is how the flyback diode is connected in a pnp transistor.

 

[petehall347]

yep its backwards ..never trust anything .. https://datasheetspdf.com/pdf/741171/InchangeSemiconductor/2SB950A/1
not only that ..

 

[audiodummy]

good catch, yeah the transistortester schematic is indeed wrong. Probably made an assumption that it's drawn the same way as NPN but that's an error -- it should be opposite, anode connected to collector in pnp versus cathode to collector on npn.
Funny thing is sometimes you can't even trust datasheets either, have to look at it carefully and ask yourself if it makes sense or not...

Then you wonder if there's a firmware update... (I'll need to see if mine gets pnp darlingtons and p-ch mosfets right ...)



Fortunately this firmware is correct, though hFE is a bit low (it's a B673 pnp darlington)