Re: Simplest current regulator

On Jan 28, 4:03 pm, Phil Endecott <spam_from_usenet_0...@xxxxxxxxxxxx>
wrote:

Of course I can use a zenner with a potential divider in an op-amp
circuit, but I need help to use it in something simpler.

Well, I'd think an LM10 and pass transistor is fairly
simple (half a square inch of circuit board), but there's
another option that hasn't been discussed yet: a current
mirror.

Just run a resistor from V+ to base of two transistors,
both emitters to GND, and connect collector 1 to base,
and LED from V+ to collector 2.

The trick is, the transistors have to be on a common heat sink
and the first has to have much smaller emitter area than the
second (the collector current is scaled by the emitter area,
all else being equal). LED current is N*((V+) - Vbe))/R
and the 'N' represents the area ratio.

ICs use this kind of trick all the time, but buying transistors in
onesies it's hard to know what N is going to be.
.

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