Heatsink…LED Question

[Spidey]

Do you think I could keep 5 to 6 20 watt LEDs cool enough with a 12” x 12” x .25” aluminum plate.

I would like to stick with passive cooling if at all possible. They would have aprox 4" space between them.

One side will have the LEDs mounted (bottom), and the top will be completely exposed.

The aluminum is 6061, so I would need a worst case guess considering that is not the best for heat sinking.

I’m really only looking for a gut guess here…

[Krom]

The million dollar question is: How much of that 20w per LED is heat?

A plate of aluminum that big could dissipate a fair chunk of heat, but if that 100-120w worth was ALL heat the plate will most definitely get very hot. The performance could be much better if it was a proper heatsink with fin stacks on it. Then you have to ask how efficiently you are transferring the heat from the LEDs to the aluminum, poor contact won't cool them well at all (which is why computer chips need thermal compound).

[Tunnelcat]

Aluminum sucks at heat transfer, it's better when bonded to another type of metal like steel or copper, but I'd think having some type of cooling fins on the backside of that aluminum plate would improve the cooling efficiency immensely, especially if there's air moving across those fins. The more fins the better too. Maybe riveting some L or H pieces of aluminum stock on the backside of your plate and then make sure you use some sort of thermal paste between the pieces to aid in the heat transfer? You may want to read this:

https://electronics.stackexchange.com/q ... -with-leds

[Krom]

Grand scheme of thermal conductivity of common bulk materials, aluminum is second best to copper. Adequate without being as heavy or soft as copper and it is easy to work with, so aluminum heat sinks are quite common actually.

Now if only someone could get Diamond into the shape of a heat sink, its thermal conductivity is 5 times better than even copper.

[Spidey]

Heh, yea the point is to try and get by with the plate only, I don’t really want to start attaching fins or a fan if I don’t need to…of course when I attach the LEDs and light them up, I will know for sure.

I also don’t want to buy a “proper” heat sink if I don’t need to, this project is to replace a failing 4’ 80 watt fluorescent fixture, and if the budget is too high, I could just buy an off the shelf replacement.

I also know that about 20% or so of the power used by this type of LED is converted into light. So that does reduce the heat wattage some.

I also plan to run the LEDs at line voltage using a bridge and cap, but I plan to use a smaller cap to add a little ripple so the lights go from full bright to a dimmer level about 120 times a second, and save some wattage there. (also 6 LEDs in series won't get full voltage anyway about 28 of the 30 or so needed to go to full)

See I don’t want to build this thing, and then have to start adding fins and fans after the fact…so that’s why I asked this question.

I do have a piece of scrap 4 x 4 so I guess I could mount one of the LEDs to that and see what happens. (each LED on the 12 x 12 plate will have the same area)

[Krom]

I think its worth a try at least, because that is still a pretty sizable block of aluminum. Someone with the exact specs of the LEDs and the exact specs of the aluminum plate could probably calculate how well this would work. But again it all depends on how well you can dump the heat from the LED into the aluminum. Once its in there a plate that size should be able to soak up quite a bit of heat on its own, but just getting it into the plate may be the hardest part.

[Spidey]

Yea, these LEDs are set on a nice copper alloy heat spreader, and do a pretty nice job.

I will let you guys know how the 4x4 test goes...

[Tunnelcat]

Grand scheme of thermal conductivity of common bulk materials, aluminum is second best to copper. Adequate without being as heavy or soft as copper and it is easy to work with, so aluminum heat sinks are quite common actually.

Now if only someone could get Diamond into the shape of a heat sink, its thermal conductivity is 5 times better than even copper.

Yeah, you're right about copper being twice as good as aluminum. My best aluminum cooking pans have thick copper bonded to the aluminum on the bottoms. I've also scavenged those aluminum heat sinks from old computer components, but they always have loads of fins molded in them, e.i, lots of surface area to radiate heat requiring airflow over those fins to work efficiently.

You'll have to show us what you've created Spidey. I've got a 4' 4 tube T5 HO plant light fixture that I converted to run red and blue LED tube's using a new special electronic ballast. I started this project because I wanted to eliminate the UV and heat from florescent HO tubes since the plants don't use that part of the light spectrum and don't need the heat anyway. So I obtained 4 LED tubes which are of specific wavelengths. 430 to 450nm in the blue range and 640 to 680nm in the red range. It turns out that 3 red to one blue is the right proportions for most plants in a 4 tube fixture. Neither the new LED tubes nor the ballast run anywhere near as hot as the old florescent tubes and ballast did and the light output is great. In fact, the temperatures in my plant stand used to get up into the 80 degrees F range. Now, it stays pretty much at room temperature, the energy draw has been cut by a third and the plants seem to love the new light spectrum. Now if I could only get used to the purple pink light. You'll know what I'm talking about if you've ever driven by a T-Mobile store. THAT color.

[Top Gun]

You growing some, uh, good stuff?

[Tunnelcat]

You growing some, uh, good stuff?

Naw. I wouldn't want my house to have that pot "perfume" smell. The lights are for my cactus and air plant collection. The cactus go outside in the summer and overwinter in my basement. The air plants stay inside all the time.

[Spidey]

So I wasn’t able to do the 4 x 4” single test to determine the suitability of the 12 x 12”, because I just couldn’t get enough juice out of either of my bench supplies to get it to full brightness. The website said 30-32 volts, but when I got the LEDs the packaging said 32-36 volts…

But I did manage to find the perfect heat sinks for the job, from some shop equipment that was decommissioned years ago, and I have 4 pieces…so there is no sense trying to use the 12 x 12” plate when I know for sure two of these will do the job.

And I decided to go with just 80 watts, so it will be 2 LEDs on each sink, and the light will be spread out much better as well.

I also decided to go ahead and buy a proper LED driver, instead of just using rectified line voltage, especially with the unexpected voltages and no way to do a proper test. Plus using less LEDs means a lower voltage which would mean having to waste wattage with a resistor. (not that a proper driver won’t waste some…just less)

[Tunnelcat]

What the heck is this project you're building anyway? Pics?

[Spidey]

Nothing special...just a kitchen light to replace an old failing fluorescent fixture.