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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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11th Sep 2018, 7:23 pm | #1 |
Octode
Join Date: Nov 2011
Location: Manchester, UK.
Posts: 1,875
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Washers - electrically insulating, thermally conductive
Hi folks
I'm working on an off-topic-for-this-forum device which includes a big diode and a transistor, which need to be in contact with a heat sink yet electrically insulated from it. I remember see-through washers on stuff I've played with before, sometimes smothered in a kind of white (thermally-conductive?) paste. I wonder if anyone can suggest any suitable materials to make the (thin) washers out of. It's (whisper it) automotive, so voltages involved are low. Thanks Mark |
11th Sep 2018, 7:38 pm | #2 |
Dekatron
Join Date: Nov 2011
Location: Surrey, UK.
Posts: 4,400
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Re: Washers - electrically insulating, thermally conductive
It's possible to buy thin, soft (i.e. compliant) thermally-conductive, highly electrically-insulating sheeting that doesn't need thermal grease, or bespoke patches for particular device package types. They're said to be more effective than olde-worlde mica washers anyway and less messy than white goo!
If it's a stud-mounting diode that needs insulating, I never trusted the small contact area- I've used stainless-steel penny washers to increase the area in contact with the soft material and additionally reduce the risk of bite-through, Colin |
11th Sep 2018, 7:56 pm | #3 |
Dekatron
Join Date: Jun 2009
Location: Staffordshire Moorlands, UK.
Posts: 5,276
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Re: Washers - electrically insulating, thermally conductive
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Kevin |
11th Sep 2018, 9:19 pm | #4 |
Octode
Join Date: Nov 2011
Location: Manchester, UK.
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Re: Washers - electrically insulating, thermally conductive
Reading elsewhere, I see Kapton tape suggested. What do folks think? I should have been more clear - a flat surface on the big diode, and the transistor, are pushed against a heatsink by the standoff on the pcb mounting - they don't screw to it directly as one might normally imagine, so normal fixing kits with correct hole spacing for the legs on a given package are not quite so necessary.
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11th Sep 2018, 9:53 pm | #5 |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,536
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Re: Washers - electrically insulating, thermally conductive
Stainless steel is a poor heat conductor- using even a larger area washer probably makes thermal performance worse. Copper, brass, aluminium or plain steel would be better choices.
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11th Sep 2018, 10:34 pm | #6 | |
Tetrode
Join Date: Oct 2016
Location: Manchester, UK.
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Re: Washers - electrically insulating, thermally conductive
Quote:
It is hard to find good thermal conductors that are electrical insulators because so much of the thermal conductivity is due to the electron mobility that gives the electrical conductivity. The best material that I know is sapphire which is an excellent electrical insulator and a genuinely good conductor of heat. It is not as good as a metal such as copper or aluminium of course, but a thermal conductivity of around 25 W/(m K) is pretty damn good for an electrical insulator. I have had made (for work) sapphire shims with bolt holes cut in them for cryostats. They were made by Crystran. You will probably find the cost prohibitive though.
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Mark |
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11th Sep 2018, 10:46 pm | #7 |
Dekatron
Join Date: Jun 2009
Location: Staffordshire Moorlands, UK.
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Re: Washers - electrically insulating, thermally conductive
Maybe some sil-pads might be better then? https://www.rapidonline.com/Bergquis...yABEgKlEfD_BwE
AFAIK mica is better than silicone impregnated pads but there you go.
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Kevin |
11th Sep 2018, 11:05 pm | #8 |
Octode
Join Date: Nov 2011
Location: Manchester, UK.
Posts: 1,875
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Re: Washers - electrically insulating, thermally conductive
Thanks Kevin, helpful suggestions. Maybe some mica shims will do me then - I can try to find some without holes in, and attach them to the heatsink with a dob of vaseline while I locate everything.
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11th Sep 2018, 11:25 pm | #9 |
Dekatron
Join Date: Nov 2011
Location: Surrey, UK.
Posts: 4,400
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Re: Washers - electrically insulating, thermally conductive
D'oh! Should have said "plated mild", I'm too stingey to use stainless too often for things. I've sometimes wondered about using "readily available copper alloy washers", i.e. those older 1p and 2p bits that don't respond to a magnet ground flat, but I worry that mis-using the currency of the realm might still be a capital offence....
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11th Sep 2018, 11:33 pm | #10 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
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Re: Washers - electrically insulating, thermally conductive
HP used to use heavily anodised aluminium washers to insulate stud rectifiers. The washers had a turned-up lip around their hole so that they would locate themselves and the diode stud in the hole in the heatsink.
There were also versions for TO3 etc transistors. Mica washers were quite good but not as good as the anodised ones. Thermal compound (the wakefield white stuff) is appreciably better than plain silicone grease. But it's messy. David
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12th Sep 2018, 3:42 am | #11 |
Heptode
Join Date: Oct 2009
Location: Melbourne Australia
Posts: 901
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Re: Washers - electrically insulating, thermally conductive
Imho, just buy a sheet of SilPad, or pads large enough to suit your devices.
I guess you have done some thermal analysis and identified the max junction temperature based on expected max dissipation and thermal resistances (of which the SilPad will now be identifiable, although you may need to do some calculation to work out what is achieved with a large diode flat base). If you haven't done an analysis, then that is really needed for starters, and will include your heatsinking performance and ambient environment. You can deduce diode dissipation for large devices by their turn on voltage and high current resistance parameters - most manufacturers have design tutorials/papers for that. |