Help please with LED in a car

OK so the car runs at 14.4 volts while running... an LED runs on around 3-3.5 generally.

http://www.dealextreme.com/details.dx/sku.1445

I want to use those in the car, so what size resistor should I use? I remember using a 680 ohm to use the old school 5mm LEDs in the car, but I want the voltage to be more exact in this case, say 3.8 net.

The formula for this would be nice if any of you know.

Thanks

This is about as foolproof as you can get...

Link

Minjin said:

This is about as foolproof as you can get...

Link

Click to expand...

Wow thanks...

So for the forward voltage I'd put in 3.8 and for the current I'd use...? What people use in flashlights? 900ma?

Use the resistor calculator:


http://www.luxeonstar.com/application-notes.php

Your desired drive current will really depend on how your LED gets rid of its heat. If you can attach the star to a good heat sink, then go ahead and drive it at 700 mA or so. I just replaced a dome light with one of these and am driving it at 350 mA, because there is no good place for the heat to go if the light is left on for a while.

While you can use a resistor, keep in mind that driving a single LED from 14 volts at 350 mA means that your resistor will be dissipating about 4 watts and your LED a little over 1 watt. You'll have to have someplace for all that heat to go. Even with a resistor, the voltage spikes seen on automotive 12v grids could still kill your LED over time.

Your best bet would be to buy a constant current driver board that is protected from the dirt on your car's 12v system. Take a look at George's 350 mA (CC1W) and 700 mA (cc3w) drivers at www.taskled.com

GetSome said:

So for the forward voltage I'd put in 3.8 and for the current I'd use...? What people use in flashlights? 900ma?

Click to expand...

You know, that DX page has a link to the datasheet for that LED...