Aluminum Channel Heatsink Question

Holokai · Jul 5, 2014

Going to be starting my first DIY LED build when my parts get here this week. But in the meantime I want to start building the Heatsink out of aluminum U channel from Lowes.

I've seen many many builds here, and they all seemed to have screwed the channel pieces to each other. I'd prefer a 'cleaner' look with no overlapping aluminum. i was planning to cut 45 degree angles to build the frame and am looking for an acceptable way to bond them together. Some sort of high temperature glue or bond of some sort.

It's worth pointing out that this is primarily a test or trial run, and will only need to hold 12 LEDs. So not much weight. It's also only going to be about 10 inches by 10 inches.

Any ideas? Thanks!

Holokai · Jul 5, 2014

Forgot to mention that this will be a free hanging passive cooling system. So the channel itself will be very visible, hence the concern with looks.

uncleof6 · Jul 5, 2014

Heatsinks are a science unto themselves, and involve conductivity, radiation, blah blah, as well as thermal resistance. Gluing pieces together will increase the thermal resistance (thermal resistance of the 'glue' is higher than that of the aluminum,) therefore gluing the heat sink together will decrease the efficiency of the heatsink a great deal. Even screwing them together is far from ideal, without perfectly flat (lapped, smoothed, non-pitted) mating surfaces; a 1 piece heatsink is by far the best.

Heatsinks and thermal resistance are interesting fields to look into, one of the first things you learn is that a heatsink that is not warm to the touch is not working...

I use heatsinks in higher output applications, power amps, but the principles are the same.

Holokai · Jul 6, 2014

H Uncle, thanks for the info. What I may try then is cutting the channel at 45 degree angles and using aluminum flat L brackets and two screws in each corner. I'll put thermal paste between the bracket and each piece. This should be better heat transfer than just gluing the Channel together then?

This is like I said a trial run. I have selected a heat sink from heat sink USA that I will use if I build a permanent fixture. But for now, $15 of parts is better for the wallet

Since you seem to know what you're talking about is it better to have the fins up or down in this sort of application?

Thanks!

karimwassef · Jul 6, 2014

I wanted the same and I used thermal epoxy (white). It's the same material I used to attach the LED stars to the aluminum.

In my case, I used a 3" wide x 1" high x 8ft long channel and attached two 3/4" square x 8ft channels. This makes a giant 8ft heatsink with 6 fins @ 3/4". I squeezed the metal plates together and clamped them down to keep the epoxy layer to a very thin film between the layers.

With 400W of LEDs, I checked the temperature gradient and I couldn't measure more than a couple of degrees from the metal on the 3/4" channel inside and the 3" wide channel top.

I attached 7 x 3" fans to blow air into the channels and used metallic tape to create ducts in and out of the channels.

This was inexpensive and effective.

rwb500 · Jul 6, 2014

i assume each heat sink section would be fine on it's own (conductivity down the length of u-channel is bad anyway), so you don't need to worry about thermal conductivity between each section.

as far as bonding goes, I would not trust anything except for welding or bolts. I have used lots of JB weld and other epoxies with metal and for an application like this (really bad if it fails) I just wouldn't trust it. perhaps you could get some 90 degree brackets on the inside of your miter corners (hard to describe) so that the screws would not be visible from the outside.

uncleof6 · Jul 6, 2014

Holokai said:
H Uncle, thanks for the info. What I may try then is cutting the channel at 45 degree angles and using aluminum flat L brackets and two screws in each corner. I'll put thermal paste between the bracket and each piece. This should be better heat transfer than just gluing the Channel together then?

This is like I said a trial run. I have selected a heat sink from heat sink USA that I will use if I build a permanent fixture. But for now, $15 of parts is better for the wallet

Since you seem to know what you're talking about is it better to have the fins up or down in this sort of application?

Thanks!

For a passive system (no forced 'air flow') the best heatsink orientation is with the fins/channels vertical rather than horizontal. Since heat rises, this naturally pulls cool air up through the channels, giving maximum 'cooling.' It is complicated, but that should suffice. For LED setups this is rather inconvenient to say the least, but it is how a passive system should be set up.

Forced air flow is better for a horizontally oriented heatsink, with air flow along the fins and channels, not perpendicular. Since the LEDS face down, naturally the fins would point up.

This has nothing to do with 'hobby' recommendations/implementations/opinions, rather the basics of heatsinking semi-conductors (an LED is a semi-conductor and is capable of exceeding the T<sub>j(max)</sub>, e.g. maximum junction temperature, for a variety of reasons.)

sfsuphysics · Jul 6, 2014

I wouldn't worry about thermal conductivity as an issue even if you had weld ends together if you pack too many into a small space you're not going to get the desired cooling effect either way. If anything I would worry about having enough surface area to have any glue connect... that said I would not use glue to connect ends together.

Welding would be the only method I would trust not to fall into the tank, and have the potential of being "clean"... otherwise bolt them.

karimwassef · Jul 6, 2014

Aluminum Channel Heatsink Question

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