Lux III and 5W - how hot is too hot?

Hi all - I recently went on a short night hike with a group of parents and 5-year olds. As appropriate for any flashaholic, I had a bunch of lights with me, but I looked forward especially to using my new FireFly II, with stock LIII at 650ma.

Indeed, the light was excellent, and provided light for nearly the whole group - a few bobbing points of yellow light from their random flashlights in a pool of Luxeon white. However, after about 10 minutes, the FireFlyII become too hot to hold comfortably near the head, and after 15 or so, I shut it down and switched to my L4, as it was hot enough that I worried about the LED. It was also hot enough that it was uncomfortable to carry in my pants pocket after shut-off.

I'm bad at guessing measurements, but it was hot enough for real discomfort with only a couple second's touch - like concrete in the sun that you can barely walk on if you hop from one foot to another - I'd guess 120-130*F?

The L4 also got quite hot after 15 min, but not nearly so blisteringly hot, probably due to a much larger heat-sink mass, and then our hike was over.

So, my question is, how hot is too hot? At what point does the heat in a small flashlight like the FireFly get to be a real problem for the LED? It's clearly a problem for this user after about 15 min.

Sounds like it's hotter than 120-130F. If you can't comfortably hold it, then the LED die is probably in a bad state. Very small form factor lights like the FF should not be run in excess of 1.5W to 2W. It makes it a short run, short lived LED. I biased mine (TVOJ) at 500mA and I probably would not run it higher. I have run it continuously for runtime tests and it got uncomfortably warm (not hot) to the touch. A higher Vf LED like a K should probably be biased around 420-450mA or so.

One way to cool the light is to actually let the body's circulatory system draw heat from the light. That means holding it in your hands and letting "human cooling" take effect. This conduction is much better than letting air convection do the trick.

CM

Thanks - I did that until my hands got too toasty.

I think my FFII is now my '5 min or fewer light', which fortunately accounts for 95% of my usage.

Just to be clear - it was hot enough for discomfort, but nowhere near hot enough to burn. My experience is that things above 160-170 are really too darn hot to touch, much less hold without burning, thus the 120-130 estimate. Hot, but not dangerous, by any stretch.

Is there a good market for Luxeon powered handwarmers, ya think? /ubbthreads/images/graemlins/grin.gif

The L4 will get so hot its scary, and can actually allllmost burn you, (dont let it touch your cheek) eeeek !

I worry sometimes also, but Surefire tests these things, and they know they get hot, its not great for the led, but it can handle it, is just shortening its life alittle, compared to a much larger light, that can keep it cooler.

Isn't a hot flashlight body a sign of GOOD heatsinking?

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357 said:
Isn't a hot flashlight body a sign of GOOD heatsinking?

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No, not at all. This does seem to be a widely held misconception though. At steady state conditions and a given power level, the better the flashlight is transferring heat to the environment the cooler it will be.

I tis indeed, but the problem is mostly the transfer of the heat from the body to the air / human flesh. Big lights can do this better than small lights once both are hot.
bernhard

You want a flashlight body that gets warm quickly, which shows how fast heat is moving out of the LED. But it should remain comfortably warm rather than getting hot. This is because the metal in a flashlight can only absorb so much heat before its temperature starts rising. If the temperature starts rising, you can think of it like the heat is banking up in the body rather than leaving the body and dissipating to the environment. And like a choked toilet where the water banks up... /ubbthreads/images/graemlins/smoker5.gif (hey how come we don't have an exploding emoticon?) /ubbthreads/images/graemlins/grin.gif

So look for flashlights that warm up quickly but don't get too hot.

Errr... one caveat though, and that has to do with the mass of the flashlight. Naturally if the flashlight is massive (lots of metal), there is a larger thermal sink and so can take more heat before the temperature rises.

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This is because the metal in a flashlight can only absorb so much heat before its temperature starts rising.


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Actually, the temperature starts rising immediately. It's not like you pump heat into a mass, then at some point it starts getting warm. The moment you begin to put heat into a body, the temperature starts to rise. Higher thermal conductivity results in the entire mass maintaining a more even temperature. Low thermal conductivity means a hotspot develops at the source of heat. Given a heat source, and two thermal masses, the larger one will heat up more slowly, but both will being to warm up immediately.

If you attach a sensitive temperature probe to the outside of a modded mag lite, even with a 1W LED, the moment you turn on the light, the temperature starts rising.

The light's maximum temperature depends on the power input, airflow, and the thermal differential between the body and surrounding air. The LED pumps a certian amount of heat energy into the flashlight body. When the flashlight body becomes warmer than the surrounding air (which happens immediately, assuming the light was at ambient temperature), some heat energy is transferred from the body into the surrounding air. The rate depends on the airflow and delta-T between the light body and air. A higher delta-T means more heat energy is transferred from the body to the air. When this delta-T is high enough such that all of the heat energy produced by the LED (per unit time) is transferred into the surrounding air, the flashlight stops warming up. Colder air, high airflow, or larger surface area all contribute to causing the body to stabilize at a lower temperature. So does transferring heat into a lower-temperature thermal mass (such as your hand).

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Steelwolf said:
the metal in a flashlight can only absorb so much heat before its temperature starts rising

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It should be noted that unless the laws of physics have been repealed, that amount is *exactly* zero. /ubbthreads/images/graemlins/grin.gif

[edit]Oh wait, nm.

I think some of the hot overdriven LED's are going to die. I think well designed lights keep the drive within limits as well as tempature which may not be as bright as it could be. I guess thats why there is so much interest in Variable power lights. My 3 cell Nuwai's get pretty hot but my 3 cell L6 stays reasonable. It though has heat sink fins. we will see. wait a year and see if your light is alive.

NikolaTesla /ubbthreads/images/graemlins/smile.gif

An Arc lamp is the Spark that takes away the Dark--HID Forever!

My Lights

Hmmmm... somehow I don't think DSpeck or the Sandwich shop are big on warranty work for overcooked LED's...

Perhaps I'll just take to plunging my red-hot flashlights into cold water from time to time... does Aluminum temper? /ubbthreads/images/graemlins/rolleyes.gif