Electronics > Beginners
LED constant current circuit
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Liquid-light:
It's 1.87V
But by the way it's in reality just 27mA or so. Probably because of the part tolerances.
Benta:
Your Rg is way too high. If you read the article you linked to, it suggests around 1 mA through Rg.

The problem is, that your BC546 has nothing to do, meaning almost no collector current. The base current will be so low, that VBE will be down to 0.4...0.5 V. The 0.7 V postulated only make sense at IC and IB currents that are in the normal operating range of the transistor. And even then, 0.65 V is more likely.

That being said, I don't like the circuit. Relying on VBE for current limiting is probably the most imprecise way around.

Liquid-light:
Thanks.
I've just tried an Rg with only 1k. Unfortunately still not working. Everything is the same as before.
StillTrying:
Are you sure RG is connected to Vcc V+ and not to the LED/drain, it would sort of explain the constant 2.5V.
Liquid-light:
What a stupid beginner mistake. |O
You are spot on, that was the reason for the problem.

Thank you very much!!!  :-+
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