Having just switched off my test rig, here is the promised report on how the heatsink compound held up: After 1400 hours at 240°C with a thermal load set to dissipate about 20 watts there was no observed change in the resistance of the 18Ω resistor. There was some discoloration as seen in the photo below but no cracking or crazing of the insulation was found.



The resistor was a Vishay Sfernice RWM4X5 18Ω 3W resistor and the heatsink compound was Slice Engineering Mosquito Boron Nitride Paste purchased from Reprap World.

I think I will stay with using these resistors until something better comes along.

Mike

I thought it may be worth revisiting this thread as I have just replaced the 7W resistor on one of my printers. This resistor was changed with a head redesign at least 5 years ago and the printer has done thousands of hours of printing with the same resistor. Changing the heater voltage from 12V to 24V also necessitated changing the resistor.

The original resistor was a Vishay Sfernice RWM6X22 vitreous enameled resistor. Other than showing the same discoloration as on the photo in the last posting, there was no damage.

Other than resistors sacrificed to engineering studies, no vitreous enameled wire-wound resistor has died on my watch while several heater cartridges have died and/or fried mosfets on controllers.

btw, earlier in this thread( [reprap.org] ), DD said "Do you think a 3W resistor uses the same diameter nichrome wire that a 30W resistor (or a 30W heater cartridge) uses?"
In answer, it uses exactly the same size wire.

Mike

... with proper cooling (e.g. pressing the resistor body to/into a "thermal mass") you can "overdrive" this type of normally air-cooled resistors to maybe 5x to 10x the rated value - so a "5Watt-resistor" could excert up to 50Watts of thermal energy ... maybe even more

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Viktor
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