How to use a CREE LED in my car?

Hello people of Candlepowerforums,

I would like to use an XML T6 emitter or two in my car to replace the dim halogen inside my trunk, however, i'm not sure of the best way to wire it up on a ~12-14v car's electrical system.

All of the aftermarket car bulb LED's are very cheap and not very bright. I was thinking of wiring two XML T6 bulbs in series which would require a voltage of 7v, and dampening the remaining 5-7v (12 or 14 - 7 = 5 or 7) with the use of resistors. I am posting however, to inquire as to how the experts of candlelightpowerforums would do this?

Thanks!

Miles

There is bound to be something out there that might work for you. Or something that can be modified. If you want to build a driver yourself, I can help. Since the voltage will vary like you mentioned (12 to 14.5v), and if you want it to be constant current regulated regardless of the the varying input voltage then you're going to want to have a buck switching type of driver to step down the voltage to around 6v (equivalent to two XM-L's in series). A driver like this will have a very small resistor in series with the LEDs to measure the current and then an op-amp to level adjust that voltage up to the buck chip's feedback pin. That will hold the current level to whatever value you set it at or you can have multiple levels if you want.

I looked over at Ebay at some of those inexpensive DC-DC module boards that could work for you. Perhaps you could buy one of the LM2596 boards with adjustable output. They are dirt cheap. The LM2596 was originally a National Semiconductor part but I wouldn't be surprised if its been cloned. But doesn't matter as the data sheet is available and that way you could see how the rest of the parts on the board would be connected. Basically those have a pot to set the voltage, which you'd want to set it low (around 5v if using two XM-L's in series and slowly raise the voltage. The problem with using voltage as the regulation to drive the LED's is that there is a very small window of voltage across the LED in which its too dim before its too bright and can be burned out. The range is only a couple of tenths of a volt. But I think it would work and would be pretty simple and cheap. I'd at least make one change to that board and that is to change the resistance that is in series with that pot to a value that will minimize the range that the voltage can to be adjusted to. You can figure that out by experimenting with it.

Then if wanting to take it even a step further with current regulation is to place a small resistor between ground and your series LED's. Then take a wire from the positive side of that resistor back to the FB pin of the LM2596. You'll first have to disconnect how the FB pin is wired on the board though. The FB pin wants to see 1.23 volts to hold regulation. Thus for example if you wanted to run 500mA through your LEDs, then you'd want to size that resistor for 1.23v/0.5 = 2.46 ohms or 2.5 ohms whichever is available. Power through the resistor will be I^2*R or 0.615 watts (use a minimum of 1w resistor). Or to save power you use a very small resistor and an op amp to amplify the voltage up to 1.23v.........but that is more involved. The LM2596 will hold the output voltage to whatever value it has to to maintain that 1.23v at the feedback pin. And you know that it's 500mA through the 2.46 ohm resistor that assures that...........thus this example is current regulated at 500mA. The output voltage will be whatever it needs to be to maintain that which is probably going to be somewhere around 7.23volts (6volts across the LEDs and 1.23v across the current sense resistor).