Re: Simplest current regulator

On Jan 28, 1:56 am, Phil Endecott <spam_from_usenet_0...@xxxxxxxxxxxx>
wrote:
Hi All,

I have a bright white LED that takes 300 mA with a forward voltage of
about 3.4 V.  I'm powering it from 3 AA cells, and it's clear that a
series resistor is not good enough as a regulator: choosing a value to
suit 1.5V per cell, I only get half the current once the per-cell
voltage has dropped to 1.3V.

There are a lot of white LED driver integrated circuits around. A
quick session with Google threw up this

http://www.kin-track.com/led/ZD850.pdf

which might do what you want - the current sense resistor drops 0.2V.

You could do almost as well using an National Semiconductor LM10CLN to
control a PNP series transistor

http://www.ortodoxism.ro/datasheets/nationalsemiconductor/DS005652.PDF

The LM10 includes a 200mV voltage reference. The output stage can't
handle 300mA so you've got to drive some kind of buffer - a MOSFET
with a low drop-out voltage would do, but a decent-sized PNP
transsitor would probably be cheaper.

The LM334 only requires a 64mV drop across the current sensing
resistor - the reference voltage is temperature dependent (see the
graph at the bottom of page 3 of the LM334 data ***), but probably
won't move enough to worry you if you use it indoors.

http://www.worldtorch.com/LDO-fixed-current.php

http://www.ortodoxism.ro/datasheets/nationalsemiconductor/DS005697.PDF

--
Bill Sloman, Nijmegen
.

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