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Step 5Heatsink selection
a lot of power goes into those LED's when they are running - 40 or 50 watts for my 15-LED lamp. unlike an incandescent bulb, we can't let the LED's glow red hot or they will burn out. in fact, the core of the LED can't get much above 100 degrees C before they will break! on top of that, LED's are less efficient the hotter they get. all this means we need a way to keep them cool. this is where a heatsink comes in. the heatsink is just a big piece of metal that conducts heat away from the LED's and into the air, by convection.
all the LED's will need to be attached to a heatsink in order to run at full brightness. in fact, without any heatsink at all the star-LED's can only run at about 2/3-watt each!
so a heatsink is just a piece of metal? YES! this is your opportunity for creativity and recycling. the heatsink will be large (much larger than you expect), so plan on it being the main structural element of your lamp. if your metal frame will be visible, you can use a piece of decoratively shaped metal.
- use a highly-conductive metal for your heatsink: aluminum, brass or copper. steel does not conduct heat very well - you can use it but you'll need a thick piece. scrap aluminum is easy to come by, and there are also many inexpensive sources for it at your local hardware store if you know where to look - rain gutters, various metal trims.
- how big a heatsink do you need? use this rule of thumb: minimum 10 square inches of exposed metal surface per watt of LED power. you can see in the photo that i used a piece of angle-bracket aluminum. my piece is 45 inches long, 1/8" thick, with 2" width on each side of the angle. that gives me an exposed surface of 45 x 2 x 2 x 2, or 360 square inches. count both sides of a sheet of metal only if both sides are exposed to the air. using this rule of thumb, your heatsink will run about 30 degrees C hotter than the surrounding air temperature at full power. keep in mind that the LED's internal temperature will be another 40-50C hotter than that. if you need a smaller heatsink, the only options are to use fins (which fit the same surface area into a smaller volume), or to use a fan. the rule of thumb here assumes "free air convection". if your heatsink will be in an enclosed space where the surrounding air will get hotter than 25C, then you need a bigger heatsink or a fan. advanced: try this handy calculator: http://www.frigprim.com/online/natconv_heatsink.html
with this calculator you can approximate most odd chunks of aluminum as a 2-fin heatsink with equal total surface area.
- how much is your LED power total? multiply: 3.8V per LED x LED current x number of LED's. for my lamp that's 3.8V x 0.8A x 15 = 46 watts, so i'm a little on the hot side.
- you need enough FLAT area on the heatsink to mount all the LED's. the mounting area must be perfectly flat to ensure good heat transfer between the LED and the heatsink. if your piece of metal is so thin that it can flex, then it will need some support or protection - you don't want an LED peeling off the heatsink under full power, it will burn out immediately.
all the LED's will need to be attached to a heatsink in order to run at full brightness. in fact, without any heatsink at all the star-LED's can only run at about 2/3-watt each!
so a heatsink is just a piece of metal? YES! this is your opportunity for creativity and recycling. the heatsink will be large (much larger than you expect), so plan on it being the main structural element of your lamp. if your metal frame will be visible, you can use a piece of decoratively shaped metal.
- use a highly-conductive metal for your heatsink: aluminum, brass or copper. steel does not conduct heat very well - you can use it but you'll need a thick piece. scrap aluminum is easy to come by, and there are also many inexpensive sources for it at your local hardware store if you know where to look - rain gutters, various metal trims.
- how big a heatsink do you need? use this rule of thumb: minimum 10 square inches of exposed metal surface per watt of LED power. you can see in the photo that i used a piece of angle-bracket aluminum. my piece is 45 inches long, 1/8" thick, with 2" width on each side of the angle. that gives me an exposed surface of 45 x 2 x 2 x 2, or 360 square inches. count both sides of a sheet of metal only if both sides are exposed to the air. using this rule of thumb, your heatsink will run about 30 degrees C hotter than the surrounding air temperature at full power. keep in mind that the LED's internal temperature will be another 40-50C hotter than that. if you need a smaller heatsink, the only options are to use fins (which fit the same surface area into a smaller volume), or to use a fan. the rule of thumb here assumes "free air convection". if your heatsink will be in an enclosed space where the surrounding air will get hotter than 25C, then you need a bigger heatsink or a fan. advanced: try this handy calculator: http://www.frigprim.com/online/natconv_heatsink.html
with this calculator you can approximate most odd chunks of aluminum as a 2-fin heatsink with equal total surface area.
- how much is your LED power total? multiply: 3.8V per LED x LED current x number of LED's. for my lamp that's 3.8V x 0.8A x 15 = 46 watts, so i'm a little on the hot side.
- you need enough FLAT area on the heatsink to mount all the LED's. the mounting area must be perfectly flat to ensure good heat transfer between the LED and the heatsink. if your piece of metal is so thin that it can flex, then it will need some support or protection - you don't want an LED peeling off the heatsink under full power, it will burn out immediately.
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Oct 3, 2009. 3:23 PMrawdog79
says:
Thank you for all the responses!
Reply
May 19, 2008. 8:57 PMrawdog79
says:
Does it matter is the aluminum is galvanized or not?
Reply
Nov 29, 2008. 8:14 PMGordieGii
says:
There is no galvanized aluminium. There is galvanized iron and steel. Don't use, it's not a very good heat conductor. There is anodized aluminium. This is good.
Reply
7
Jul 19, 2008. 1:36 AMDerin
says:
IT DOES-DO NOT USE GALVANIZED METAL UNLESS YOU WANT TO GET ILL
Reply
Nov 29, 2008. 8:15 PMGordieGii
says:
Why would galvanized metal make you ill?
Reply
7
Jan 11, 2009. 5:40 AMDerin
says:
It gives off a poisonous gas when heated.Don't use galvanized metal where there will be high temperatures,like this project.
Reply
Jan 12, 2009. 3:57 AMGordieGii
says:
What gas does it give off and at what temperature? The only gas I can think of that would come from galvanized metal would be zinc oxide and that would only happen at several hundred degrees Celsius. By that point the LEDs would have turned to ash (having given off much more toxic fumes than zinc oxide) and either blown a fuse or stopped conducting. You should be careful when welding galvanized metal or using it in a fire or furnace. (But it's OK to put zinc oxide on your skin to protect it from the sun.) If for some reason galvanized metal seemed the optimal solution (existing infrastructure or unavailability of alternatives) I don't see any reason it shouldn't be used. I don't think you need to worry about 'metal fume fever' at these temperatures.
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7
Oct 22, 2008. 10:00 AMDerin
says:
that only goes for when you use high temp things like this
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