# Cooling LED's



## CTR (Aug 9, 2007)

Has anyone come across any other means of cooling LED's other than the usual heat-sink method? On computers they use fans and watercooling but obviously this is not practical for hand sized lights so just wondering if there's anything else out there.


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## Ken_McE (Aug 9, 2007)

CTR said:


> Has anyone come across any other means of cooling LED's other than the usual heat-sink method?



You could look into heat pipes. If you're building it you might consider using more emitters and under driving them so they will run more eficiently. Some people will take a stock torch body and cut decorative scallops and grooves into it to increase the surface area. You can also add fins or look into using the batteries as an additional heat sink


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## frenzee (Aug 10, 2007)

This thermionic vacuum diode technology was discussed here some time ago, but I don't think it is on the market yet.

Also, you might consider using a miniature fan coupled to a miniature heat sink. I had some succuess with a 15x15x6mm fan from ADDA (which you can buy from Mouser) attatched to an off-the-shelf copper memory chip heat sink in the same configuration you see in CPU coolers. It worked well, but reliability was a concern, and obvously the thing wasn't very water proof.


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## KreAture (Nov 15, 2007)

How about using a TEC (Thermo Electric Coupling)?
Basiclly known as a Peltier. It removes efficiency from the system though as you use energy to transport heat. What you'd end up with is a torch with a cold nose and a hot body.

It would extend the life of overdriven LEDs I am sure.
You need to use about 1w to move 0.6w or so leading to a heat of 1.6w to be dissapated on the hot side. The genious thing is there is no moving parts so it will not make noise. The increased temp allowed on the interface to the hot side could allow very high overdrive.

If you created a temp difference of 40°C you could have your LED thermal plate at 20°C and the hot side at 60°C. This high hot-side temperature will make heat flow easier too. I don't think the actual torch should reach 60°C where you are holding it but it would be interesting to investigate how this heat would dissapate for a given torch design. The LED wouldn't mind a constant 20°C on the sink, that's for sure.


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## jtr1962 (Nov 15, 2007)

Regarding using TECs, if you use an oversized one relative to the amount of heat you're pumping (say one capable of pumping 50 watts to cool a 3 watt LED), then you can get COPs well in excess of 1, often up to 3 or 4, at moderate temperature differentials of 15° to 25°C. So for a little extra power, you might be able to use a smaller heat sink and/or run the junction at much cooler temperature. For certain colors which are very output sensitive to temperature, such as red-orange or amber, the extra power used might result in enough extra output to make it worthwhile.

I tried something like this once with an amber Luxeon. See here.


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## TorchBoy (Nov 15, 2007)

From memory the Peltier devices available locally are quite expensive and draw 4-5A. Are you guys talking about lower spec ones or just running them on less than full bore?


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## Gomer (Nov 15, 2007)

Peltier coolers are also pretty inefficient. You might be drawing 10watts from the emitter side and dissipating 20 watts on the hot side.


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## jtr1962 (Nov 15, 2007)

TorchBoy said:


> From memory the Peltier devices available locally are quite expensive and draw 4-5A. Are you guys talking about lower spec ones or just running them on less than full bore?


No, just running them less than full bore. A 5 amp device can be very efficient when run at only 1 amp. You obviously can't get as large temperature differentials, but you can pump perhaps 5 times the heat per watt as you can running them at full tilt. Actually, in general running at full power is a game of diminishing returns. Whenever I do anything with peltiers, I'll increase the current in small steps and wait until my cold plate temperature stabilizes. Once it stops decreasing, I'll use that current for my maximum current. It's always less than the peltier's rating. I typically find that I can obtain the coolest temperatures with a decent air-cooled setup at perhaps 65% to 75% of maximum current. A liquid-cooled setup might be optimized at 90% of maximum.

Lots of peltier performance charts at these sites:

http://www.marlow.com/

http://www.ferrotec.com/technology/thermoelectric/

http://melcor.com/index_melcor.html

http://tellurex.com/

http://www.sctbnord.com/

http://www.tetech.com/


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## KreAture (Nov 15, 2007)

Yeah. I saw (when the search function started working again) that some had tested cooling with TEC's and found amonghst other things that efficiency increased a lot.

I happen to have quite some experience with TEC's and from what I saw several things were done "wrong" in the tests. Great results, but using much more power than necessary.

To prevent condensation inside the elements one ofteh pots the element. This is sealing with silicone around the edges of the element in a dry-air environment. This also insulates the junctions a bit.

Next step is to insulate the path around the sides. This is a major source of energy-leaking. Heat will simply leak back to the hot side around the edges of the TEC.

Finally one insulates as much as possible around the cold side. Any area of the cold side not being covered by the object to be cooled should be insulated. 3-6mm depron works well. The thicker the better.
Energy really doesn't like being removed from somewhere and it will try sneaking back any way it can!

When it comes to driving one should note that there is a maximum temp differential obtainable with a TEC. It will range betweeen 70 and 80°C and is only obtainable when transporting zero energy.
There is no reason to drive the TEC at all if the max difference is reached. Likewise, one might want to drive the TEC for a fixed cold-side temperature. As long as this temperature is within 40-50°C of the ambient side and one has ok cooling this is possible with a large enough TEC. Remember that the maximum heat pumping capacity is when the difference is 0°C and thus no cooling is performed, only heat transport through the TEC.

If theese tips are folowed, only approximately 2-3 watts will be needed to move 1 watt through the TEC for a total of 3-4w output to be dissapated. Using a TEC rated for 2x the max power that will be moved would allow around 1/2 the max temp difference and close to max efficiency in the TEC.

A very good page to read is: http://www.tellurex.com/cthermo.html
I also recommend: http://www.tellurex.com/12most.html

I have a small 8w TEC that I think will be perfect for abusing a LED at 5w. "Can't resist... MUST play... Need to be geeky! GNAAAAH!"


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## TorchBoy (Nov 15, 2007)

It still doesn't sound a very efficient way to cool an LED - you halve your total efficiency straight away. Why not just use an incan light bulb and radiate the heat away without the extra cost and weight?

For a hand sized (not pocket sized) light I think I'd go with a fan-cooled heatsink (or just abuse the LEDs and let them get hot). I've got some 5V 40mm fans waiting for the purpose.


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## KreAture (Nov 16, 2007)

Where's the fun in that ?


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## TorchBoy (Nov 16, 2007)

Hmmm, now that I think about it, some people think it's fun putting a regulator on an incandescent. I might leave them to it, while admitting if I had more time and maybe government funding I'd probably join them. :sigh:


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## KreAture (Nov 16, 2007)

It's also fun seeing how far you can push the envelope.
If done right, how many lm/w can you get from a supercooled LED setup...
It reeks of a challenge if you ask me... Maby even a contest?

Come on ppl! Post images of your setups together with performance numbers and we'll see how poorly/well this can work


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## Gomer (Nov 16, 2007)

...I do have access to liquid nitrogen....


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## evan9162 (Nov 16, 2007)

Supercooling does very little to nothing for white LEDs. You're looking at getting a 10% improvement by cooling them down by 40-50C.


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## Gomer (Nov 16, 2007)

evan9162 said:


> Supercooling does very little to nothing for white LEDs. You're looking at getting a 10% improvement by cooling them down by 40-50C.




well, it will change the bandgap


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## KreAture (Nov 16, 2007)

Don't forget the power capability  I wonder if I can get a single LED to emit 300lumen?


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