[sdiy] voltage and LEDs

[db]

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.

I'm guessing I need something bigger than a 2n5088, and maybe better to run 
this all on 9v, rather than the 12 in my schematic.



At 2/11/2010 07:41 AM, George Mattson wrote:

>The forward voltage is what the diode is going to use up across the PN
>junction. It just "goes away"
>Actually, that's the voltage required to get the current to cross the
>barrier region. Consider it the PN road bump.
>
>Using 5 V, subtract the 3.9V Vf from 5V to get the difference of 1.1V.
>1000mA max current is 1 Amp. Using ohms law divide 1.1V by 1 Amp to get 1.1
>ohms to limit the current.
>
>That's the minimum resistance necessary to prevent burning up the LED in
>regards to the specs. Anything less will just dim it down. Usually, you can
>get by with half the current. Anything above a certain limit doesn't let the
>LED get any brighter, just dissipates as heat at the junction and shortens
>the life of the semiconductor.
>
>But, 1 Amp is also the high limit of the 7805
>
>You'd either need a huge heat sink and a fan to keep it cool. Or, use two in
>parallel and let them split the load.
>
>The 1.1 ohm resistor at 1 Amp is going to dissipate 1.1 Watts of power
>(P=IE)
>
>I'd suggest using a 2 to 5 W resistor and mounting them above the board to
>let them get some air to cool down.
>
>
>My $0.02
>
>George Mattson
>
>
>I want to drive a 3watt led
># 1000mA - Max Drive Current
># 3.90Vf - Forward Voltage
>
>Can just use a 7805 to get 5v, and then drop it with two diode in series to
>get 3.6v?
>Or do I still need resistors in series with these LEDS?
>
>
>dennis.barton:skylab2000[socal]
>
>
>
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