Ideas on heatsink/chassis insulation

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bokbaard

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Chaps,

I have found a problem with my heatsink recently, and fixing this problem, introduced another

I am using two NPN's as pass transistors on a heatsink, which is fine, as the collectors are tied together anyway.
My problem is, I need to insulate the heatsink from the chassis, electrically and I need ideas.

I can always revert to mica insulators under each transistor, to isolate them from the heatsink, which should be ok then, but I am not sure.

I have read a piece on mica insulation, and heard that they retard the thermal transfer.

What are you ideas/opinions on using paste+mica washers?

Should I use the above mentioned solution instead of trying to electrically insulating the heatsink from the chassis?

Thanks and regards
MarloN
 

hi,
Mica washers and bushes are fine, just smear a little 'heatsink' compound on the washer before fitting... gets all the trapped air out.
 
I guess it all depends on how much temperature rise the transistors will see with the power dissipated in your application.

Using mica washers etc has the advantage that the heatsink will not be live, accidental contact could be catastrophic.

If accidental contact is not a problem, mount the heatsink on insulators.
Use nylon screws and washers, or use insulating pillars.

JimB
 
Last edited:
It depends on the thermal resistance of the heatsink compared to what you require.

It also depends on the voltages used purely for safety's sake. If it's <60VDC then just isolate the heat sink from the other live parts. If it's >60VDC then insulate the transistors.
 
I always use sil-pads. They're inferior as far as thermal transfer compared to mica and grease, but usually good enough and sooo much nicer than fooling around with grease.
 
Last edited:
speakerguy, Ive got some, but Im not sure if they will work with my application.
Im not sure what the TR of my heatsink is, but it seems Im going with isolating the heatsink from the chassis. The voltage won't exceed 40VDC and around 7A.
Seems my favourite heatsink compound comes from the UK, Electrolube
 
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