General > General Technical Chat
Measuring hfe of "P2N2222A" varies wildly
eti:
I've got a cheap Vichy VC97 dmm, and another one by "AstroAI", and when I shove a P2N2222A in the hfe tester, I get wildly different gain readings. On one I get 253, and the other is 150 - any idea what gives? And yes, the pins are correct, I've tested it 5x
(and I'm also aware that the *P*2N2222A* variant has a different pinout.
Thanks.
IanB:
Seems like hFE should be in the range of 50 to 300, so both indicated values are within the expected range.
eti:
--- Quote from: IanB on November 05, 2021, 03:39:03 am ---Seems like hFE should be in the range of 50 to 300, so both indicated values are within the expected range.
--- End quote ---
Ah okay, but why the vast difference - the device isn't going to change its gain between meters, is it. Also the sockets are horrendous on these cheap DMMs 😂
Cerebus:
It is going to change its HFE at different collector currents or different VCEs and you have no guarantee that those conditions are the same for both meters (and knowing the calibre of the VC97 I don't expect you'll find those conditions listed in the meter's data sheet/manual, or the data sheet/manual for the even more anonymous AstroAI which I've never even heard of before).
This is a question about pretty basic transistor theory, or rather a lack of sufficient grasp of it, and as such would have been more at home under "beginners" where people are generally more minded to be forthcoming with material of a tutorial nature - which is what it looks like is needed here.
David Hess:
Hfe is not well controlled between units, and it also varies significantly with collector current, with different transistor types optimized for different collector currents. A lot of small signal transistors are optimized to operate with a collector current of about 10 milliamps, but low noise and high gain types will be optimized for 1 milliamp or even 100 microamps. You can get a good idea of the design current from the datasheet by looking at the hfe specifications to see which ones are fully specified at a specific collector current.
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