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Old 16th June 2004, 03:47 PM   #1
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Question heatsink question?

ok say i have a heatsink (i do) for 2 to-220 devices. the collectors (middle pins and thus the tab as wells) are tied to very different independent voltages. i can use a mica insulator and thermal grease to attach them, but what kind of screw/nut/bolt setup can i use to be sure they're electrically isolated from each other? is that a generally bad idea? i've only ever used single heatsinks with clips instead of screws so i have no idea what im doing with these. but the heatsink im referring too is really cute and i want to use it =)

thanks for suggestions
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Old 16th June 2004, 04:59 PM   #2
johnnyx is offline johnnyx  United Kingdom
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If you buy an "insulating kit", you will get the mica or silicone rubber insulator and some washers. The washers have a short tube which goes between the screw and the metal of the heatsink or the tab of the device. The special washers prevent shorts between the screw and the metal. I would use a rubber insulator if possible, it's less messy. You can get nylon screws, but they aren't so strong, and can break when tightening. The size of screw varies, M3 is probably ok.
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Old 16th June 2004, 05:15 PM   #3
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ok what if they were back to back ? i guess i'd need a nylon nut and bolt or similar right? the washer with the insert sounds good, but i think i'll just try to find nylon nuts and bolts and go easy on them. the transistors arent going to get SUPER hot by any means - only passing about an amp at peak and they're tip121's
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Old 16th June 2004, 07:54 PM   #4
ekaerin is offline ekaerin  Sweden
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Hi,
I believe you can use a metal screw since the washer will prevent
the screw head to get in contact with the transistor flanges.

OR

Use clips that grabs the insulated epoxi body of the device.

I agree mica and grease tends to get messy but works well.

Heck, once I was out of solvent, had to grab my 12yrs Bowmore
to clean up the grease on the cooler, that hurt.

/ Mattias
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Old 16th June 2004, 08:21 PM   #5
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I wouldn't feel safe with nylon screw.

Use a plain metal (steel) M3 screw TOGETHER with a bushing.

http://www.elfa.se/elfa/produkter/se/2008964.htm

Clips are also good but the heatsink must be made for this.
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Old 16th June 2004, 08:31 PM   #6
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What if I use a black anodized aluminum heatsink. Since anodizing can produce nonconductive aluminum surface, would I even need mica washers and such? I want to use heat conductive epoxy to glue the output devices right onto the heatsink. What would be the problem with this sort of approach? I measured my heatsink with a meter, and get infinite resistance.

Thanks
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Old 16th June 2004, 09:09 PM   #7
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I've used nylon screws with no problems what so ever. These are on a power amp holding the predrivers and bias trannies to the heatsink.
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Old 16th June 2004, 11:25 PM   #8
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yeah i think im going to try them and see what happens. im sure it wont' be bad. like i said its not a very powerful application, shouldnt get too hot so thermal resistance isnt a huge problem

btw, i thought i was the only one with a strange 5th element obsession. guess im not alone!
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Old 17th June 2004, 03:12 AM   #9
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Quote:
What if I use a black anodized aluminum heatsink.
This is exactly what bose uses on their acoustimass 5 sub modules and I assume others.

Their is a metal spring clip that attaches to the top of the transistors/bridges that holds them down, no screws.

So anodized heat sink are safe mating surfaces but I think only if you dont use screws.
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Old 17th June 2004, 03:14 AM   #10
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Here's a pic of the Bose amp module.
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