Luxeon I question

[blackops]

Hello all.  New to the board and looking for some help.

I am building a luxeon led bar which is going to be installed in a vehicle.  I will be running three luxeon I stars from the vehicles 14v (alternator) power source.  Would it be correct to use a 33 ohm, 4 watt resistor in this application (information obtained off of lumileds website)?  Also, would it be better to run the three leds in a series or parallel? 

I appreciate any help that you guys could offer.

Thank you.

[Rob]

Welcome to the boards.

3 is a magic number. You can easily run all three in a single series circuit, and stay within the voltage supply.

The resistor approach has the benefit of simplicity, but the car's charging system will create a range of voltages, and you'll need to size the resistor conservatively so that worst case currnent to the Luxeons is in the safe range.

This project fairly screams for a current-regulated driver. Check this out:
http://led.linear1.org/a-cheap-current-regulated-luxeon-star-driver-design/

With 14V in, one cheap regulator IC can easily handle 3 Luxeons in series. Total cost around 2-3 bucks. Current will be tightly controlled to the value you need. (I'd go with the 320 mA as shown.)

[blackops]

Thank you, Rob.   With my input voltage being near 14.0 volts would I still use the 3.9 ohm resistor and the lm317 regulator?  Thank you again for your help.

[justDIY]

here is a "problem" with the lm317, which is why I don't like them for automotive applications:

lets say your engine is running, charging system is normal, everything is good, 14v on the 12v bus
lm317 takes away 3v for itself ... so you're left with 11v ... 11v / 3 = 3.6v
3.6v is good ... if you figure around 300mA for the luxeon 1, your voltage is going to be about that for white (you didn't mention a color here, and it does make a difference)

now lets do a different scenario; its dark, headlights are on, defroster is running, heater fan is cranking, wipers wiping and you're stuck in traffic ... your 12v bus is now more like 11v
lm317 takes away 3v for itself ... so you're left with 8v ... 8v / 3 = 2.6v
2.6v is below the minimum for the white, blue and green luxeons, however, red, orange and amber will still work.

so in summary, the lm317 should be able to drive 3 luxeons in perfect conditions, but I would aim for 2 luxeons instead, to cover all the possibilities ... you can build two circuits, and run 4 leds this way, or have the 3rd led run off its own circuit.

there are other solutions, low drop-out regulators such as the LM1084 has a much lower "overhead" than the 317, requiring only 1.5v for itself ... the problem being, it costs 6x as much... $1.50 vs $0.25... not something to break the bank.

[Rob]

Good points there. For whatever reason, I imagined red LEDs even though the OP didn't say what color.

I like the 2x2 plan.

[blackops]

Forgot to mention in my first post that I would be building several in different colors. 

Looks like I will be going with the 2x2 design with the LM1084.

Thank you again guys for your help with my project!!

[justDIY]

sounds awesome - share some pictures when you're done!