Electronics > Beginners

Have I gone mad?

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king.oslo:
Hello there,

I thought this was correct: "NPN transistor, small base-emitter current enables large collector-emitter-current."? I thought so, until now. I am very confused:

I found two transistor in my parts bin with with C368 marking. I assume they are BC368 1A 20V NPN transistors.

My PSU has 1 ampere current limit. I connected 2K2 resistor between 5VDC and collector, and emitter straight to GND. When I applied 5VDC (via my Fluke 87V in current ampere mode) to the base, and my PSU goes straight to current limiting (1amp) which my multimeter confirm is flowing in through the base. Consequently, the transistors get very hot. This is the case for both my C368-marked transistors.

This surprised me so much. How come a large current plunge through the base to the emitter, when I have 2K2 resistor between 5VDC and collector.

This makes sense to me. Is my knowledge of NPN transistors to blame, or is there something else wrong?

Thank you for your time.

Kind regards,
Marius

IanB:
You have fallen into the classic error of logic:

"All dogs have four legs. The creature I see in front of me has four legs. Therefore the creature in front of me is a dog."

Do you see the problem?

IanB:
The above assumes you really have a BC368 transistor and that you have identified the pins correctly.

But also note there is no guarantee a component marked C368 is actually a BC368 transistor, nor that you have correctly identified which legs are B, C and E.

grumpydoc:
Nothing in your set-up limits the base current. At the end of the day the base-emmitter junction is just a diode and you more-or-less created a short circuit from the +VE terminal on your lab power supply, via the B-E junction to ground. No wonder it current limited, I'm surprised the transistor survived (did it?).

Putting a 2.2k resistor in the collector circuit will limit the collector current, not the base current.

IanB:
You wrote: "small base-emitter current enables large collector-emitter-current".

Maybe so.

But THIS DOES NOT MEAN that restricting the collector-emitter current will limit the base current.

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