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I am using this 10 W RGB LED, unfortunately DX does not provide a datasheet for it. I don't know how efficient these power LEDs are... so approximately how much applied power is converted into light and how much is dissipated as heat? Almost all 10W is dissipated or only small portion of it?

As there is no datasheet, I even cannot compute the required heat sink, so it seems I need basically to try :) Unfortunately I don't have any heat sinks at home, only one from old VGA card (which is 74 x 94 mm with 12mm high fins, but half of the surface has place for a fan, so no fins there). Do you think this is an overkill for my LED? I have no idea for how big heat sinks should I look. The LED itself is on a metal plate which is 14 x 22 mm, so the heat transfer from LED to heat sink should not be a problem.

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LEDs may be far more efficient than other light sources, but they still emit a lot of heat. Right now I can't find a good source, but a cree application note uses 100% of the electrical power as an estimate for the dissipated heat. –  0x6d64 Jun 10 '12 at 15:47
    
An interesting fact: traditional light bulbs are designed to dissipate heat. All the metal parts are potential heatsinks, including the cabling! The bulb screw is designed to transfer heat away from the filament better. LEDs cannot do that, because they're less direct. The tiny wires in the LEDs cannot conduct heat as well. That's why, even though LEDs dissipate less power, they alone are not equipped to dissipate a lot. –  Jonny B Good Sep 9 '12 at 20:24
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2 Answers 2

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I don't know how efficient these power LEDs are... so approximately how much applied power is converted into light and how much is dissipated as heat? Almost all 10W is dissipated or only small portion of it?

  • The large majority of energy in is turned to heat. Assume 100% for heatsink dimensioning. See below.

I cannot compute the required heat sink,
I have a 74 x 94 mm heatsink with 12mm high fins and half of the surface has place for a fan, so no fins there. (Summarised)

  • Run at "able to be touched comfortably" or lower temperature for longest life

  • Use of a fan with a modest heatsink will be required to achieve temperature under 50 C.


For practical purposes assume that all energy is dissipated as heat.
100% efficiency for white light is approach 400 lumen/Watt and an LED of that sort will be unlikely to be better than about 80 l/W (and may be quite a lot worse depending on who made the emitters and when) so MAY be 20% efficient, could quite possibly be 10% and may be worse still.

The difference between needing 100% and 80% heat sinking =
10W / 8W = 25% more
so insignificant in practice.

For longest life you want lowest heat sink rise.
So say 30C heatsink rise and the less the better.
30C rise for 10W = 3 C/W.
Without a fan this is a very substantial heatsink.
With a fan this is doable.
You can easily measure operating heatsink temperature or even finger test.
If you can hold the heatsink indefinitely (say 10's of seconds) when running and stable then it is OK (about 55C max).

With LEDs cooler is always better for longer lifetimes.

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"The large majority of energy in is turned to heat." Is that also true for LEDs? I know it's true for incandescent light bulbs. Can you give an average/minimum/maximum efficiency for those LED lamps? –  Federico Russo Jun 10 '12 at 18:10
    
@FedericoRusso - my answer was LED specific and addressed your question exactly. (That may not have been obvious). Best LEDs manage over 20% conversion to light. –  Russell McMahon Jun 10 '12 at 23:46
    
Oops, must have missed that. Thanks for answering. –  Federico Russo Jun 11 '12 at 5:05
    
Thanks for your answer. So I should be looking for heat sinks with 3 C/W and better? Does it make any significant difference if the heat sink will be oriented with its fins facing down (so blocking hot air from going up)? –  Marki555 Jun 11 '12 at 12:25
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I mounted a 10W LED on a cheap AMD CPU Heatsink with a 92mm PWM (4-wires) Fan. The 4th wire takes a PWM Signal on the 4th Wire. Connecting that to ground makes it run very slow, enough to keep the Heatsink Temperature for my 10W LED at around 32˚C (Room = 25˚C). The CPU Heatsink/Fan combination cost me less than 7€. (Actually it's a 20W LED running at 15-18W, so even better for you). Without the Fan I have to dim it down a bit to keep it around 35˚C, haven't measured the power to the LED, though. THe fan is very quiet, quieter than my idling Macbook pro on my desk.

CPU Heatsink/Fan Combinations can be a very cheap and effective cooling solution.

On the over Hand, a regular 100mm x 50mm x 40mm Heat sink (black, few large Fins) rose to over 60˚C in a very short time with that 10W LED.

Remember: LED Lifespan decreases badly with higher Temperatures.

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