Re: Simplest current regulator

whit3rd wrote:
On Jan 28, 2:39 am, Phil Endecott <spam_from_usenet_0...@xxxxxxxxxxxx>
wrote:
Does anyone know why we can't buy discrete zenners (or similar) down to
65mV ?

Zener knees get soft below 4V or so. If you want a 65 mV reference,
you can always use a jFET with gate/source connected
as a current reference, into a suitable load resistor.

You can also use a resistor divider to drop the 200 mV
reference to something more convenient in terms of LED
current sense resistor dissipation.

Can someone show me how I could use a potential divider to get a lower reference voltage than a zenner can provide in this sort of current source?

\
/
\ Load
/ |
| |
| |/
+------|
| |\>
_|_| |
| ^ \
/_\ / R
| \
| /
| |
--+--------+


V_zenner = V_be + IR

If I want R to be small, I need V_zenner to be just a little more than V_be.

Or I could put a regular diode in series with the zenner, and have V_zenner equal the voltage across the sense resistor. Or use a second trasistor.

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.

Thanks for all the useful suggestions.

Phil.
.

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