Author Topic: Custom heatsink mounting  (Read 838 times)

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Offline PeacefrogTopic starter

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Custom heatsink mounting
« on: January 07, 2020, 03:56:31 pm »
I've mounted transistors to heatsinks before and used off the shelf kits (both for the heatsinks themselves and insulation), but I'm now creating a custom solution for a project, based on a single piece of aluminum that I'm cutting and drilling (and need to mount four transistors to).

This got me thinking: the plastic insulator I'm using (alongside the mica pad, to ensure the bolt and the transistor don't short) isn't as deep as the aluminum. The bolt should therefore still be insulated, but there will be some part that *isn't* shielded by the plastic internally, with an air-gap between it and the metal.

This is probably 'normal', based on some Googling, but should I be looking for a deeper plastic insulator instead that lies flush either side?
 

Offline schmitt trigger

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Re: Custom heatsink mounting
« Reply #1 on: January 07, 2020, 04:15:35 pm »
By "plastic insulator" you mean the flanged nylon washer that inserts between the transistor's mounting hole and the screw?
 

Offline PeacefrogTopic starter

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Re: Custom heatsink mounting
« Reply #2 on: January 07, 2020, 04:20:01 pm »
Yup - sorry, I guess the correct term is shoulder washer.
 

Offline andy3055

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Re: Custom heatsink mounting
« Reply #3 on: January 07, 2020, 05:54:21 pm »
I don't see any issue with that unless you have high voltages that could arc over from the screw stem to the heat sink.
 

Offline fourtytwo42

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Re: Custom heatsink mounting
« Reply #4 on: January 07, 2020, 06:14:49 pm »
This got me thinking: the plastic insulator I'm using (alongside the mica pad, to ensure the bolt and the transistor don't short) isn't as deep as the aluminum. The bolt should therefore still be insulated, but there will be some part that *isn't* shielded by the plastic internally, with an air-gap between it and the metal.
This makes me think your using the shoulder insulator in the wrong place maybe, every time I use them they are in the TO220 or whatever device flange so the screw is at heatsink potential NOT power device tab potential. I think this explains why you think they are to short :) Of course high voltage is a whole other ballpark!
 

Offline schmitt trigger

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Re: Custom heatsink mounting
« Reply #5 on: January 07, 2020, 10:33:38 pm »
 :-+ What fortytwo said.  :-+

The shoulder washer's flange is only as long as the transistor's mounting tab is thick.
 

Offline PeacefrogTopic starter

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Re: Custom heatsink mounting
« Reply #6 on: January 08, 2020, 01:59:50 am »
That makes sense, now - thanks all!
 


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