Is this forward lighting (white), or rear lighting (red)? Either way, sweet! I am working on a project using 18 K2's per headlight (going kind of slow due to classes). Contact Andrewwynn here on CPF (PM him if you desire). He is pretty helpful! If you cannot get a hold of him, try Georges80 from TaskLED, or Dat2zip (Wayne Yamaguchi from the Sanwich Shoppe). They also have the expertise in LED drivers. Also give Newbie a try. He is very helpful and knowledgeable of almost any aspect of using any sort of high-power LED.
You are in good luck. I used a circuit that andrewwynn originally designed for use in high power incandescent flashlights, but was found to work in high current LED projects like mine. The circuit he designed (that I recommend) is called the
Hotdriver. It is pretty much a beefy voltage regulator (but do not let the fact that it is a voltage regulator scare you). It also has a current limiter so you can set the maximum current that you want to send to your LEDs to protect them from thermal runaway. The circuit is only about 1 inch in diameter. In addition, you can dim all the LEDs from an external potentiometer if you wish. I have mine set up so a small High beam/Low beam relay activates either of two trimmer pots that are preset (by me). With this set up, I can set each beam mode to have a certain brightness. You can have one fixed resistance, instead, if you would like.
This one circuit can handle two headlight clusters (18V @ ~12.5V) with ease, so a smaller scale project like yours will work with no problem at all. Of course, to use this circuit, you will have to set up your LEDs into a parallel/series configuration to work (four strings of 4 LEDs in series).
I bet price is a consideration, but have you considered fewer LED's at higher currents? With 12 Luxeon III LEDs, you may get a max of ~550lm @350mA. 12 * $3.45 (per emitter) = $41.40. You might as well go with the 'ol Luxeon I if you only plan to drive them at that current.
Maybe you should try the K2. These are great because of the lower thermal resistance and the higher temperature handling. These are tough and for the same price as the Luxeon III. Color consistency is awesome, too. I got TXOH when I ordered 36 of them around September last year, and every time I ordered more later on, I got the same bin. If you have to, you can up the current safely and get a bit more output. Sure these appear to be "inefficient" at max power, compared to others, but you can save money by going with only 6 of them powered at less than 1A each.
Try the Seoul Semiconductor P4. You can get the same brightness with only 4 of these beasts when powered at around 700mA. These are more efficient (and consume much less power) and will cost you about the same (or perhaps less) as the 12 Luxeon III emitters. 4 LEDs will be so much easier to wire up, also. Trust me on the wiring. 18 can be a pain to solder, as 12 may be. You can also use less optics and a much smaller heatsink. With more room, you can have the luxury of using bigger optics for an even brighter beam. If you desire, push 1A each and you will get 640lm out of just four of these things.
With the P4, you can use an even smaller circuit (I can think of anything at this exact moment) so you have room for you protection circuitry. The car's electrical power supply is so dirty and screwwy, but I can explain more later. I wish I knew your target brightness. If you are trying to get the most brighness with 12 emitters regardless, try 12 P4s. At 350mA, you will achieve ~1080 lumens. You can an absurd amount of brightness at higher currents (more than 2000lm)!
I am not trying to tell you what to do, I just hope that I can help you by making things easier and simpler, all while saving you some money. Might as well as make use of the latest technology. I am not caught up in the moment with the crees and P4s like some on here because others are. I am just sold on the P4 after some testing of my own. This thing is spectacular, and I cannot get over it. There is a place in my heart for the Luxeon I and III and K2, but I feel that it is just smart to utilize new technologies in such an application as yours. My next headlight for another friend will most likely use 7-10 P4's instead of the 18 power-hungry Lux III or K2.
Try this link. Reply to it if you would like to. It is the thread where I conceptualized, designed, and created my project. There are several nice pictures there to look at. I am still trying to finish it (with my very limited time and tools at college), but it is getting there. It is not an impossible idea!
http://www.candlepowerforums.com/vb/showthread.php?t=117661&page=1&pp=20
Please tell me what you think and keep us updated. I hope I was helpful. It may be helpful to give us as much info about your project as possible (the specific use or your project and your budget, etc). Whatever configuration of LEDs you choose, contact Andrewwynn about his circuit, the Hotdriver. It is well priced and it is a nice driver. I wish you the greatest luck with your project. Whew! I typed enough for tonight...
-Tony
