Quote:
Originally Posted by GMGQ
Even with a massive heatsink, I dont think you can escape overheating issues. I think of heatsinks like batteries/capacitors. Once you fill it up with heat energy, it will be full, and it will not be capable of drawing any more heat from the LEDs.
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see and hence the problem, people don't know how heatsinks work or how they are rated.
here is a quick and dirty explanation.
heat sinks are rated by there ability to disapate heat and it is measured in watts/square inch. so when you build a set up you are putting a heat source (the LED) in direct contact to the heat sink. by doing this tyou are relying on the ability of the heat sink to release that heat to the air.
in theory a heat sink that is rated higher than 3watts/square inch will not need any fans using 3 watt LEDs but that is under ideal conditions. humidity ambiant temp ect, all affect the rate at which a heat sink will disapate heat.
part of the rating is ambiant temp, humidity and temp differential between the heat sink and ambiant air. generaly you will see a heat sink rating at watt/square inch with the max efficiency at a 10 degree diff or some number like that. which means that to get the full watt/sq in the heat sink has to warm up to about 10 degrees above ambiant, so that is another thing to concider, as if you have a hood temp of 90 degrees the die temp will run about 100 degrees so lowering the ambiant temp will lower the die temp.
so to increase your ability to keep your LED die temp cooler you can do a few things, the easiest is to build a fan set up to force air across the heat sink surface increasing the amount of heat in can disapate.
another way is to use a hollow heat sink which you can run liquid through to adsorbe the heat. theroeticly you could run sump water though it and keep your heat sink at 80 degrees, but this could add to the heat in your tank so you may or may not get away with this unless you use a chiller or another radiator to disapate the heat somewhere else.
you could also run air through a hollow heat sink, this is what "steve's LEDs" does with his 1.25" square tube LED heatsinks.
but with out some sort of safety shutdown I would be hesident with this style as if the fan fails the LEDs have a very good chance of overheating.
the third main way is to increase the size of the heat sink to a higher watt/square inch rating. if you increase this enough you may not need fans as they heat sink will be capable of a higher rate of heat disapation than the LEDs can put in to it.
now us being over cautious we tend to do both, high rate heat sinks and fans. for instance I am using a high rate heat sink with fans and a way to monitor the temp of the heat sink, I am setting it up with a small quiet fan just to ensure a slight air flow over the fins to prevent any stagnent areas which could cause local hot spots, the second fan will kick on with a higher flow if the temp gets over a set point, and at another set temp 1/2 the lights will shut down and if temp still goes up the rest of the lights will shut down.
this seams like a lot of over kill and yes it is, but my intent is to run the LEDs at full current and keep them subcooled and prevent any premature LED faliures. could I do it with a cheep chunk of alumium flat bar.. hell ya, it would need a lot more fan cooling hence more power usage, more noise, ect but ya would be a little lighter and little cheeper.
I personaly don't care what people use to make there setups. could use tinfoil for all I care
, but what I do care about is when people critisize others for what they see as overkill because they don't understand how something works. So my hope is that this clears up some of the theory of how a heat sink works and maybe helps people to understand why some people chose to go a little over kill.
there is also they other factor.. finned aluminum can just look cool also
Steve