Running Luxeons With 12v Auto?

How many blue luxeons should I run together to properly work with a 12v auto supply? Should I wire them in series or parallel?

georges80 has a number of 12v solutions like this one

Luxeun 1,3, or 5s?
We need more info to help.

1 or 3 watters. I tried running some numbers with the LED calculator I found here on CPF and it gave me a negative resistor recommendation, does that mean I dont need one?

If you got a negative answer you were trying too many in series. I.e. they won't light. I still recommend a regulated solution. However, if you really just want to use a resistor here it is. I'm sure its been covered many times before. An automotive electrical system is NOT 12 V. A fully charged auto battery without the engine running is 12.6 V. Running it will be in the area of 14.x V depending on tempurature. Calculate for 16 V. Never run LEDs in parallel if you can help it. I would run no more than 3 in series. To limit the current to the right amount, 350 mA for the 1.2 W, we use Ohm's law. R = E/I Google™ knows Ohm's law. The resistor must drop the remaining voltage, so we plug into Google™ (16 V - 3.42 V - 3.42 V - 3.42 V)/ 0.35 A = and we get 16.4 Ohms. The next highest standard value is 18 Ohm. We're not done. The resistor must be able to handle the power its going to get rid of. Two ways to figure. Both are just different ways of looking at Ohms law. The simplest to state is P = R * I^2. In Google™ this is 18 Ohm * ((.35 A)^2) = 2.20500 Watts. The more common way of looking at it is P = E * I The voltage across the resistor times the current flowing through it. Same answer! Typically you use a resistor of at least twice the wattage requirements. I'd say 4 - 7 W depending on whats available. This is going to be warm so protect it from anything getting against it, but still allow air to circulate around it. Actual current flowing will be less than 350 mA most of the time, so there may be a minor color shift.

BTW - remember it is illegal to have any blue light on any non-emergency vehicle in most (all?) states.

If you assume 3.8V per Luxeon for 700mA, four Luxeons will work well in series, even if the voltage did get close to 16v. gwbaltzell, why would you not recommend more than 3 in series? Too dim? The more you add, the less voltage per LED. 13.8V / 4 = 3.4V per LED. 16V / 4 = 4V per LED MAX.

In auto systems, you should always use a current limiting resistor or (even better) a regulator like an lm317 in current mode. Voltage in a car system can jump to 18V or higher at times. All it takes is a few ms of a high voltage pulse to fry one of your luxeons.

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evan9162 said:
In auto systems, you should always use a current limiting resistor or (even better) a regulator like an lm317 in current mode. Voltage in a car system can jump to 18V or higher at times. All it takes is a few ms of a high voltage pulse to fry one of your luxeons.

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ditto!


Edit: OK, I guess I should address the why not 4 question. Let see 3.42 V * 4 = 13.68 V. Battery, engine not running, 12.6 V. Hmmm... result little or no light. If you want to treat LEDs like incandescants, use incandescants. And I'm still betting that if we drove incandescants with a constant current source they would last at least 50% longer.

As has already been mentioned, use these and don't look back. You can put two emitters in series off of each driver. You are protected from all automotive V spikes, and can enjoy an efficient constant current driver. I've used 15 of them, and think they're dreamy.

Yes, you can run a few in series, but if you're going through the trouble anyway, you might as well do it right the first time!