Heat Sink Design

I'm thinking about heat sinks for my next design. I've been looking at plumbing supplies. I think I like 1-1/2" sink drain pipe. It's thin wall and comes in plastic and chrome plated brass. It has a shoulder in it that would be a good mounting spot for a 1-1/2" aluminum disk. Here's my question.

Which is better (both with thermal goop):

1. An aluminum disk mounted in an easy to cut plastic pipe on a shoulder with an LS/O on one side and maybe some fins on the other side of the disk and then a couple of air holes in the pipe near the fins.

2. An aluminum disk mounted in a more difficult to cut and join, heavier metal pipe on a shoulder with an LS/O on one side and smooth on the other side of the disk.

For (1) the heat has a wide path to the air close to the center of the disk.

For (2) the heat has a long thin path to the outside of the metal pipe.

(1) is like a plastic head lamp conversion. How hot does the LS get in those sealed lights?

If I just put a 1-1/2" flat aluminum heat sink disk inside a sealed plastic box, how much current can I pass through an LS/O mounted on the disk and still stay in the warm zone?

If you're using the pipe as the flashlight body, I think it would cool better with the metal pipe than with the plastic pipe with enclosed heat sink.

You might want to compare the two designs by comparing the total surface areas of all heat sinked metal parts. Then also consider that any surface area that's exposed to outside air will allow for convection, and is better utilized for heatsinking.

I've been wondering if anyone else was considering chrome plated brass! The idea first hit me at Ace Hardware, when I saw a 1.5 inch slip coupling: both ends have threads and screw-on rings (bezels!?), and the center of the coupling is reduced in size slightly. I went to Lowe's and found not only 1.5 but 1.25 inch slip couplings. I decided to go with the 1.25 inch (less bulky). I did not figure one would be long enough so I bought two. I cut about a third off one coupling, did some creative cutting with side cutters, then made the screw-on ring and plastic compression ring captive, then bent back the tabs I created (12), then shoved it into the other coupling. It made a good solid fit plus thermally connected the two couplings (so I figured). With a jig and a drill motor and Dremel tool I cut down a heatsink to make a disc to fit into the cutdown coupling. Worked great (the jig)! Mounted my LS/o on the disc and ran wires out to a battery pack. I had a glass lens that fit perfectly into the "bezel" and screwed it onto the coupling. Using 1.5 ohms and 3AA's, the current (if I recall correctly) was 500 ma but the light output was no where near what the LS/o would do on a large heatsink from an old TV. The coupling also warmed up a bit. Obviously the project would call for a custom battery holder, but since the full coupling had the reduced in size center section, things are crowded.

I'm convinced the way to go would be 1.25x4 inch aluminum pipe nipple, standard wall (schedule 40). According to standard pipe dimensions, ID would be 1.380 inch, OD is 1.660 inch. That would be plenty of room for the LS/o, 3AA's, and plenty of heatsink, plus you can buy the slip coupling nuts ("bezels") by themselves!

To answer your question jeff1500, I would definitely go with a metal pipe. If you want to stay with 1.5 inch pipe (aluminum), standard pipe size, ID would be 1.610 inch, OD is 1.900 inch, wall thickness 0.145 inch. You could then use a regular square 4AA battery pack. Also, you can buy the 1.5 inch slip coupling nuts ("bezels") separately.

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Xcandescent:
what parts of the LS need to be heat-sinked?

I'm guessing that the wires are generally kept either away from the heat sink, or passed through it with insulation?

-XCN-<HR></BLOCKQUOTE>

All the heat is generated at the led junction. That's at the center of the aluminum plate. The aluminum plate is electrically insulated from the led electrodes by a thin green film material. That means the heat sink that comes with an LS is electrically isolated, neither + nor -. If it gets hooked to the grounded body of a metal flashlight, it's still isolated from the + terminal by the film.

The heat sink has to move the heat away from that center point to, ultimately, the surrounding air.

There are three issues:

1. Degree of thermal contact with the LS. Heat sink compound helps.
2. The heat conduction path.
3. The heat convection surface.

It's a chain link effect. The weakest link blocks the heat flow.

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by NewsFlash:
I've been wondering if anyone else was considering chrome plated brass. I went to Lowe's and found not only 1.5 but 1.25 inch slip couplings. I decided to go with the 1.25 inch (less bulky). <HR></BLOCKQUOTE>

How much clearance did you end up with between the positive connection on the LS and the inside of the 1-1/4" pipe?

If you mean the corners of the thin aluminum heatsink of the LS/o, the answer is every corner is ever so slightly less than 1/16" away from the inside of the 1-1/4" pipe. BTW, originally I had trouble with 1.5 ohms and 3AA's warming things up a bit. I went to 2 ohms and things don't seem to heat up. In fact, 2 ohms seems to be very nice. I've never seen an Arc LS in person, but judging by the pictures I've seen at the LED MUSEUM and elsewhere, performance of my chromed brass flashlight at 2 ohms is right on par with the Arc LS. Not super bright but nice.

The aluminum disc I "turned" is 1/8". I was careful not to make it too small, then popped it into the slip coupling with a large 1/2" drive socket and a rubber mallet. It's not super tight and it will need to be epoxied in (that's if I stay with brass and not go to an aluminum pipe nipple).

All in all, I could live with this performance (similar to the Arc LS). More than likely I will try the aluminum pipe nipple to find a balance between good and bright and keeping the LS/o cool. When I had my LS/o on a large heatsink from a TV, it didn't feel warm (obviously not cool either). Shining it around in my basement the light was pretty impressive. No matter what package you use (metal), I'm sure there's a balance between brightness, cool running and longevity.

Something's been bugging me about the whole heat-sinking issue. If the flashlight barrel is part of the circuit (which it is in MagLites), how do you heat sink the LS without causing a short? Or to be more specific -- what parts of the LS need to be heat-sinked?

I'm guessing that the wires are generally kept either away from the heat sink, or passed through it with insulation? I can never tell, because I've never seen good pictures of how people actually do it.

-XCN-