Help with specific led driver

[hornebrett0]

Hi all,

I am currently building a high output 3w UV smd led kit for a friends pc case.
I have been researching a few different circuits and for cost and basic ability I have gone for lm317. I have seen two different circuits one regulates voltage via a resistor from pin2 to ground and a second resistor from ground to output. Another circuit I have seen uses the same setup but a single resistor across pin 1 and 2 to regulate current. Would it be possible to use both together to get a regulated voltage and current. I know you can buy premade units to do this but I like to tinker around.

Any help would be greatly appreciated

I have about a week to build it.

 

[MikeMl]

Why do you think that limiting both voltage and current is necessary or desirable?

LEDs should be driven with a constant current; the LED's forward voltage will take care of itself.

The only voltage limit is how low the input to the LM317 can go without the LM317 dropping out of regulation (look up Vdo on the data sheet). The voltage drop across the current sensing resistor is 1.2V, which also lowers the maximum available voltage to the LED. If your LED has a max forward drop of Vfd, then the minimum input voltage must be Vfd+1.2+Vdo.

The biggest problem will be getting rid of the heat from the LM317 and its resistor.