[sdiy] voltage and LEDs

[Simon Brouwer]

 db wrote:
>
> I now see lm317 regulators can be set up as constant current source..
> Thanks to many of you.
>
> How about if my existing circuit used an lm317 to power the LED?
> http://img25.imageshack.us/img25/4098/leddriver.jpg
>
> I'd set the regulator to output 3v or so to match the LED selected.

The whole point of using the regulator as a constant current source is
that you set the current to 1000 mA and leave it to the LED what voltage
drop it will have at that current.

The LM317 regulates its output voltage to 1.25V higher than the voltage on
the adjust terminal. So to get 1000 mA you would need a resistor between
these terminals of 1.25 ohm.
Note that the current comes out of the LM317 output terminal, so almost
all of it will go through this resistor, which should have a power rating
of at least 1.25W.

Be aware that the power loss in this circuit will probably be more than
the consumption of the LED itself. A switching regulator would be much
more power efficient. See for example:
http://www.ebv.com/fileadmin/products/Products/ON_Semi/NCP3066/NCP3066-D.pdf


-- 
Vriendelijke groet,

Simon Brouwer
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