LED Thermal Management

[joj]

Hi everyone,
I am building a high lumen LED desk Lamp. I am having problems figuring out the calculations required to select the appropriate heat sink, Any help on the matter would be greatly appreciated.

 

[bychon]

There are 2 ways to go with this, as I see it. I can write down the thermal equasions and you can do the math or you can reveal something about the LEDs, like voltage, current, power, how many, and stuff like that.

 

[Dragon Tamer]

I'm wondering why you need a heat sink. If your LED's are getting that hot than there could be something wrong with your circuit.

 

[joj]

Thank you for responding, That would be great if you can walk me through with the formula as I will give you my LED specs.
voltage 10
current 700 ma
watts 6
power dissipation 9 watts
Please let me know what else you need to figure it out - Thanks again

 

[bychon]

I'm still a little stumped. I searched for a while and only find 6 watt LEDs mounted or in bulbs. However, I can tell from your specs that you read the label to me and you are really not an electronics person. Apparently you have a power supply and a LED that are seperate and you want to make sure the LED doesn't smoke.

So, I guessed a 25 degree centigrade rise for 6 watts and that comes out to about 4 centigrade degrees per watt. Then I looked in my catalogs for a heat sink that would do that, and I came up with 2 inches by 2 inches. That's how I did it and that's the answer. At least 2 inches by 2 inches of good, thick metal (at least 1/8th of an inch) is required to radiate the heat and give the LED a long and happy life. Good enough?

 

[joj]

I'm still a little stumped. I searched for a while and only find 6 watt LEDs mounted or in bulbs. However, I can tell from your specs that you read the label to me and you are really not an electronics person. Apparently you have a power supply and a LED that are seperate and you want to make sure the LED doesn't smoke.

So, I guessed a 25 degree centigrade rise for 6 watts and that comes out to about 4 centigrade degrees per watt. Then I looked in my catalogs for a heat sink that would do that, and I came up with 2 inches by 2 inches. That's how I did it and that's the answer. At least 2 inches by 2 inches of good, thick metal (at least 1/8th of an inch) is required to radiate the heat and give the LED a long and happy life. Good enough?

Yes, You are right about 2 things; I am not an electronics person and I am trying to build the led with a separate power supply. I am lighting designer, and trying to figure out how a heat sink that is rated to dissipate certain amount of heat. I will have different projects with LEDs, so I would need some sort of formula to be able to calculate heat dissipation and what to choose to mount it on.
Thanks

 

[bychon]

OK. Everything that makes heat gets warmer. Warmer than the air in the same room with it. What you do is decide how hot you will let it get. I guessed at 25 centigrade above ambient. That's 45 F. If the room is 75, the LED will be 120 degrees F. Warm, but you can hold your finger on it all day and not get burnt.

Then you calculate: 25 degrees increase caused by 6 watts is 25/6 = 4.166666 centigrade per watt. Plug your number into this formula. Now you have the standard ratings everybody else uses. I looked in a catalog from Mouser Electronics - Electronic Component Distributor and found a heat sink rated at 4.4 degrees per watt. It was 1.95" by 1.95". Round it off to 2x2.

You can also look at Wakefield web site to find comparable heat sinks. You can buy their heat sinks or fake it with a piece of aluminum if the job is smallish. There are "better" ways to do this, but it takes several pages to describe the official way to do it, and I'm too lazy to type all that in here. The Wakefield site has calculators you can use and pages of education.

 

[bychon]

ps, be careful about mounting LEDs in the ceiling. Attic air is a lot hotter than room air.

 

[joj]

ps, be careful about mounting LEDs in the ceiling. Attic air is a lot hotter than room air.

Ok, Great, that is pretty easy to figure out now, i have some basic way to start analyzing. Thank you.