Newbie here! Some questions on taillight retrofit with Luxeons.

[205GTi]

Hi everyone!

I was introduced here by JustDIY~ I have just spent about half an hour reading some of you guy's threads, I can tell I am gonna learn a lot here!

I am planning to retrofit my car's taillights with 3W ProLite Luxeons LEDs (For each LEDs, typ vf=2.20v; typ current=700mA).

My input voltage would be 9-16V (from a car) and I will use four 3W LEDs to replace one filament bulb.

I have been doing research on heat issues, drivers, light intensity, viewing angles...etc and I got these questions still in mind.... (Sorry if I am being dumb, I have never touched electronics before)

1) Constant current driver or voltage regulator... which is better for my application?
2) Usually, what is the max temperature that would still allow these Luxeon LEDs to operate properly? I see on some data sheet that it could go up to 100 degree celcius... is this for real?
3) How much heat will these LEDs generate when ran with correct current and voltage?

I will probably have plenty more questions to ask on this topic... I hope you guys won't find me annoying! 

[justDIY]

Hi John, welcome to the boards.

1) Constant current driver or voltage regulator... which is better for my application?

Constant current is the only way to go, since LEDs are current mode devices.   I'm a bit of a fiend for efficiency, so my recommendation would be constant-current switch-mode driver, but switch-mode is a bit complicated for a first time project!

2) Usually, what is the max temperature that would still allow these Luxeon LEDs to operate properly? I see on some data sheet that it could go up to 100 degree celcius... is this for real?

The junction might survive 100 deg for a few hours, but then it's shot.   You want the LED to feel no warmer its surroundings (assuming ambient is already over 25C).  Most of the time that's not possible, but you want to keep the LED around 35-40C which should be possible with a good passive heatsink.

3) How much heat will these LEDs generate when ran with correct current and voltage?

2.2v * 0.7a = 1.54 w ... LEDs are just little heaters that also give off light, so most of the wattage rating you can expect as heat instead of light.

[Rob]

Solid info.

205GTi, Welcome to the boards. We like questions here. And we even love questions when they show that you've put some thought into the matter. So keep em coming.

I'll expand on the "how much heat" thing--the above answer is precisely correct, but most people aren't used to thinking of heat in watts. Usually you'll want to know "how hot will the surroundings get" in degrees. So you can get to that number by means of the thermal resistance of the heatsink and thermal interface material. You multiply that by the heat power in watts to get degrees. Most of the time you're left estimating the thermal resistance though. You can find C/W ratings on heatsinks, but interfaces are always tricky.

A very thorough discussion of thermal design is here: http://lumileds.com/pdfs/AB05.pdf

I expect that may raise more questions than it answers, but I though I'd let you take a look at it to get a feel for what's involved.