Calculate leds for a heatsink?

[shiftline]

I just ordered a bunch of 3w beidgelux LEDs. I want to mount them to an unused finnex aquarium LED heatsink

How would one figure out how many leds the heatsink could passively cool?

I believe it's about 18" long
I doubt I would run the LEDs past 50% but it's hard to say until I see them

 

[RetroTechie]

The math itself is fairly straightforward. Basically you add up thermal resistances (K/W value) for each element between LED die -> ambient air.

PER LED you have:
1) LED die -> LED case (or solder points). See the LED's datasheet.
2) LED case -> heatsink. Hard to say, depends on mounting method. You could try and find a K/W value for similar mounting method in some other application. Or use LED die -> LED case value as a ballpark figure. :thinking:

For all LEDs combined you have:
3) Heatsink -> ambient air. This K/W should be in that heatsink's datasheet, but would be valid for a specified mounting method only (horizontal / vertical / ...). And it assumes a uniform heatsink temperature. Which may be approximated with a large # of evenly spaced LEDs, but not when you've got just a few LEDs in different locations on the heatsink.

In each case, power (Watt) multiplied with K/W value will tell you how big a temperature raise you get from A to B. For example if LED is dissipating 2W and has 3.5 K/W from LED die -> case, then LED die will have 7 K (or o​C) higher temperature than LED case. And so on for the other steps towards ambient air. Note that LED dissipation is lower than the electric power (volts * ampere) since a significant portion of that power is radiated out as light. So 3W electric power -> maybe 2~2.5W heat dissipated. The remaining factor is max. allowed temperature of the LED die. This may be specified, but you may want to keep that at a lower temperature (say, 80~100 o​C where specified max is 120 o​C).

Personally I'd just mount enough LEDs on that heatsink that you're sure your lumens requirements are covered for the area the LEDs on that heatsink will cover. Then you can drive them at a current for which [each LED remains within specified limits] and [heatsink doesn't get hot enough to burn your hand when you keep your hand holding it]. Or any lower current.

The math may give you a good indication / ballpark figure, but semiconductor cooling isn't really exact science.

 

[Steve K]

I'm going to guess that a value for thermal resistance isn't available for this heatsink. If this is the case, you might start with the rough estimate that you need a couple of square inches of heatsink surface per watt. You can start with your best guess, and then add or remove LEDs in order to get the heatsink temperature that you want. 50C or 60C might be a good target temperature.

 

[shiftline]

So would a square inch count as in the circle around it or an inch on either side ? Ideally I can load it up and just dim it down if needed With the controller / power supply I have I would need to run 3 of each in series and I got 4 different colours. So hopefully I can squeeze 12 on it

 

[Steve K]

So would a square inch count as in the circle around it or an inch on either side ?

the heat can flow through the aluminum, so the location isn't critical. The aluminum does need to be exposed to air that is free to flow/circulate.

 

[DIWdiver]

It's hard to tell from the picture, is that a single roughly "D" shaped extrusion, or do the LEDs mount on a flat piece that slides into a "C" shaped extrusion?

If it's a single piece:

It's so thin that heat will not spread very effectively throughout it. It will be substantially hotter directly under the LED than it will be an inch or two away. Any temperature measurements (including a 'touch test") should be done directly behind the LED.

I wonder whether that heatsink can even keep one 3W device cool without help, so it's good you weren't planning on running them that hard. However, if you place LEDs 2-3 inches apart, there won't be a lot of interaction between them, so placing 6-8 of them on an 18 inch bar won't be much worse than only putting one on it.

As you put more on, you'll reach a point of diminishing returns where each additional LED makes you reduce the power in all the others so you don't get much additional light. I wouldn't guess it would be worth spacing them closer than 1 to 1.5 inches.

If it's a flat plate that slides in:

There will be little heat transfer from the plate to the extrusion because there's minimal metal-to-metal contact. The surface area of the plate is maybe 1.2" x 18" x 2 sides, one of which is exposed to open air, one to the inside of the extrusion? With a total of 43.2 square inches, you probably can't support more than 8-10 watts total. Maybe shoot for 15W and dim it until you get to a good temperature?

 

[shiftline]

As a ransom update I put 12 less on and it gets pretty hot. I'm now running it at 10% and it's warm but not too hot