Powering 3 watt LED

Thread Starter


Joined Oct 30, 2009
audi. would you need a resistor also with the LM317t?Because I want to do the same with 3 watt LED's.mine are 3watt,input is 3.0/3.27v-current 700ma-80 lumes.I would like to use 3 in seires .But I think I would have to use a LM338T,for the higher amp.but would I need a resister on that also.
Im just looking for a small cir.for either 2 or 3 in seires .From useing a car battary 12vdc.


Joined Mar 24, 2008
You need to provide the specs of the LEDs. We also need to know if this is part of an automotive circuit, since a cars electrical system voltage varies a lot more than a battery, and even with a battery by itself you can run into trouble with chargeing the battery (which causes voltage to fluxuate). The regulator can compensate for that just fine, but it has to be working on the low voltage end of the operation.

Using a LM337T is no big deal, both will work for this application. It will be how hot the regulator gets that is important.


Joined Feb 12, 2009
From what you say, I assume the LED needs a current of 700mA. This is fairly standard for this type of LED. You need to make a constant-current driver. This can be a "linear" or a switchmode" circuit. Basically, a switchmode circuit will be much more efficient and will run much cooler. But is more complicated to make.

You also say that at this current the LED drops 3.0 to 3.27V. at 700mA You want to run 3 of these. With a constant-current driver you can simply connect these LEDs in series. The total voltage drop will then be 9.0 to 9.81V. If you want to run these three from a 12V supply then a linear circuit will be OK because there will only be just over 2V drop. Power dissipated will be only about 1.4W

Can't draw you a circuit now but it's fairly simple. Do a bit of searching on the web or perhaps someone else will come up with one!