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Old 24th Oct 2016, 7:47 pm   #21
PJL
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Default Re: Encapsulating wire wound resistors

I have a standee I would like to re-stuff too so would be interested in finding a fix. This stuff works to 200degC which might be enough if the standee tube can conduct heat away reliably.

http://www.ebay.com/itm/Halnziye-HY9...-/121174513266

Theoretically, the thermal conductivity is so much better than air there should be very little temperature drop across the material and the surface area of the standee will be significantly greater than the embedded resistor so the temperature should be lower. I would feel happier with something capable of 400degC.

Diamonds are excellent at thermal conduction and are non-conductive!

Last edited by PJL; 24th Oct 2016 at 8:01 pm.
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Old 24th Oct 2016, 8:25 pm   #22
PJL
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Default Re: Encapsulating wire wound resistors

Having looked at prices, diamond dust is not quite as stupid as it sounds! It might be a bit on the abrasive side though!
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Old 24th Oct 2016, 10:19 pm   #23
MotorBikeLes
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Default Re: Encapsulating wire wound resistors

I don't have the figures offhand, but silicon carbide (carborundum) has a high thermal conductivity, (I recall it was similar to cast iron A/C a manufacturers rep some forty years ago) but it does have some electrical conductivity, in fact it is also NTC, so probably unsuitable. But cheaper than diamond. Air may have low conductivity, but in free air, once it gets hot, it creates high convection velocities so speeding up the heat loss.
Les.
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Old 24th Oct 2016, 10:42 pm   #24
trobbins
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Default Re: Encapsulating wire wound resistors

If the resistors are covered in heatshrink, then you could even pour molten metal in if it were practical. Suggestions extend to using a high % of aluminium dust or aluminium oxide powder to load a void filling material.

Surprisingly, aluminium oxide (aka heater coating) was not too bad a thermal conductor, and the main thermal resistance between a heater filament and cathode tube was the air interface inside the tube.
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Old 25th Oct 2016, 2:07 am   #25
IanBland
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Default Re: Encapsulating wire wound resistors

Just a note that pyro powder absorbs moisture with great enthusiasm, resulting in plumetting resistance, hence the need to seal the ends of the cable properly. So the tube would need to be thoroughly sealed if you use that.

It's also probably not compatible with using a bucket of water as a heatsink.
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