heatsink question?

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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
 
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.
 
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
 
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
 
I attached the image, I don't know why it didn't show, I'll try again, sorry. Oh to big I'll resive.
 

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I always use wooden small pieces, sometime thin MDF

This way i make the electrical isolation of the heat sink to the chassis or boards.
I do not like to complicate my life with insulators, screws, nuts and all those damn things.... and they are very bad to heat transference... i screw transistor mounted over aluminium heatsink, the transistor is checked if flat, because a lot are not, this way touch small part in heatsink, i check if heatsink is flat and i glued with epoxi directly in fiberglass or phenolic bord, and no difficult to remove if you need. I use alike everybody Silicone with dirty thermal compound and always a lot...must splash alike too much butter inside a bread... this way no air under the component.. air is not so good to heat transfer as metal contact... i sand, normally with the very thin sandpaper, here the number is 500 and done with watter to guarantee flat surface...have a metal block "guaranteed flatten", machinery made to be reference to me.

Sorry for the perfectionist, the one that try to make better than factories... and a lot of you do it better... this way i will ask you all permission to provocation.... to inflame you a little, to put some fire on this matter, to challenge someone.

Why?, if you spend so much time to make better or alike a factory... why not to buy?

Why?, doing the same appearance and quality assembling, why not to start produce it seriously?

Why? spend so much money to to something already done?


Why i answer this, because you boys, some young teenagers, and some new enginneers make better quality amplifiers than you find in brazilian shops (underdeveloped country i know), but here, theres a market to you.... why only be kidding when you can come here with 10K dollares and multiply it by 10000 in four years, our workman, unfortunattelly receive 80 dollares each month.... we have land, we like foreigners, sometimes we like you more than our own people, why to put in forum the picture, show to some friends, be proud and finish?

I do not want to tell you are crazy, but...... is not impossible that you can be.

Here, down in Brazil, our people sent money to foreign countries afraid of government sometimes stupid decisions, we need you!... lets go, move you ... and came hear to help you and help us.... sale this damn car and some other things and lets go to an adventure.

Carlos
 
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