XM-L temp question. HOT HOT HOT

y0bailey

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Apr 21, 2008
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So I dusted off the old headlamp now that the days are getting shorter. I was testing some batteries I had laying around and noticed just how stinking hot my headlamp gets.

I basically have 2 XM-L's slapped with thermal tape to a heatsink that I velcro to my bike helmet. They are being driven at 3000ma with a TaskLed B3Flex. 16AA Eneloops wired in series power the whole shebang (sitting in my camelbak).

The setup gets too hot to hold for more than 5-10 seconds. This hasn't bothered me in the past because it is usually <50degrees F ambient and I am on my bike constantly moving...so it doesn't get that hot. But inside at 75degrees sitting on my workbench it reached 150 degrees F with my IR thermometer.

The XM-L spec sheet says T-Max = 150degrees C....but something that is too hot to touch worries me.

Should I care? I almost never need to run both LEDs at full blown 3000ma, but if I want to does this temp seem too high to run safely?

Thanks!
 

alpg88

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for passive cooling 2 xml driven at 3A, need a heatsink that is pbly half the size of your head, exact calculations are complex and have many varuables, but from experience building lights, with 2-5 xml, your headlamp heatsink does not have even 1\10 of the surfice area needed for sufficient passive cooling of 20 watts.
150 is themp. of junction, that is inside the led, each termal connection, (star\tape\heatsink) has losses, i don't know exact data for temp gradient of your materials , but i'd say you loose pbly 30-50C or more on them alone, if your heatsink is 70c the junction is pbly at its limit.
cut current in half. or even to 1\3,
 
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IMSabbel

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"too hot to touch" can be as low as 50C, depending on how sensitive you are...
 

y0bailey

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Too hot to touch for me is a confirmed 150 degrees F...because I used the IR THERM.
 

Christexan

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Really too little information to determine if your setup is "too hot". Ultimately, it MAY be too hot, indoors, not moving. But if used as a bikelight, moving, in cooler air, that can dramatically drop the temps (and reduces the heatsink area needed). So long as you can drop back the power (or turn off) when stationary, you'll likely be fine if you can still hold it for several seconds as you described, but never hurts to keep it cooler.

Assuming any decent chip-sized heatsinks (sounds like you used some type of cpu/microchip heatsinks I'm guessing?), they are likely 20-40C differential (Tj-a), since you are starting at around 65C (150F), assuming thermal tape is resisting another 10C (hopefully much smaller, but just going for "really bad" figures here), you are probably looking at up to +50C at the junction, so a total of 115C, hot, but well within tolerances. Also aim your IR thermometer right at the LED base (not the dome, the dome is highly thermally insulating), to confirm the temp as close to the LED as you physically can. So long as that's consistent, should be okay.
 

y0bailey

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Cool. Yea...for my testing I will just setup a big assed-fan to blow over it while on the work-bench. I am just trying to figure out real world run times, etc.

Thanks for the info. I will be fine. If I melt some LEDs, I will just upgrade to XM-L2's and call it necessary....lol.
 

DIWdiver

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A couple things about IR thermometers:

They measure average temp over an area, not a small spot. At close range, the area is probably 10+ mm diameter. If they have a laser dot, the reading area is likely to be near the dot, not on it, especially at close range.


They can't accurately measure the temperature of some surfaces, at least without help. For example, bare aluminum will give a MUCH lower reading (> 10C) than actual temperature. Conveniently, black electrical tape is a pretty good surface. Slap a piece of it on bare aluminum, and the reading accuracy is MUCH better.


For spot readings, a thermocouple glued down with arctic silver is way better. Or even taped or clamped down with some thermal compound around it. TC meters with probes can be had very cheap on fleabay.
 

alpg88

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So long as you can drop back the power (or turn off) when stationary, you'll likely be fine if you can still hold it for several seconds as you described, but never hurts to keep it cooler.

lol, every post of yours is a copy paste from someone elses post.
 

BestPCB

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Oct 14, 2013
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Maybe you have turn on the electric fans when you ran 2 XML at full 3000, so that the wind can accelerate the process, similar like you ride bicycles outside.
 
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