hfe measurement beginners puzzele

registracii · Post by **registracii** » 24 Jul 2012, 15:10

I got a new multimeter that has the hfe tester - so I decided to test it.
I get a value of 223.. which starts to drop slowly - I waited until it dropped to 214 but Im sure I will continue to drop this takes about a minute.
is that a normal reading or somethings wrong with the transistors or the multimeter ?

juanro · Post by **juanro** » 24 Jul 2012, 16:19

What transistor is it? Seems about right, for a medium gain transistor (like 2N3904)... If you have warmed it (the transistor) with your fingers, Hfe will (slightly) rise, and then slowly go down as it "cools" down to room temperature again. Try touching it while you're measuring and see if value goes up

Juanro

registracii · Post by **registracii** » 24 Jul 2012, 18:09

it's a 2N7000.
Thanks for the explanation.

juanro · Post by **juanro** » 24 Jul 2012, 18:19

Ah! But that's different... a 2N7000 is a mosfet, it doesn't have "Hfe", wich is the ratio of two currents... the one going into the base (to emitter), and the one from collector to emitter. Hfe = Ice / Ibe
Mosfets are "transconductance" devices, they allow a certain current from Drain to Source depending on a _voltage_ applied to gate, wich is electrically isolated from the other two terminals and thus current will not flow (under normal circunstances, at least)
I don't know if your meter has a specific setting to measure mosfets; if no, the value you're seeing is a spurious one (probably some drift current from gate charge buildup)

Regards,
Juanro

registracii · Post by **registracii** » 24 Jul 2012, 18:24

I think it does.. however I will look into it in more detail. there is much to be learned.