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Mike Sweeney

Mike Sweeney

AK Subscriber
Subscriber
After watching my 100 watt 8 ohm resistors get hot enough to melt a spray wand by mistake with only 30 watts going into them, I went big.. real big :D

I bring you 400 watt wire wound resistors at 8 ohm with 4 ohm center taps. A pair cost me 14 bucks a piece plus 7 to ship. Ugly as hell but they will get the job done since the next amp is rated at 85 ish watts per channel.
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awillia6 said:
Do you have a ready source for those ugly suckers? I think I'm falling in love...
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owlmills on the "auction site".. he has several more. Drop him an email before you buy them if you are getting multiples. He will work with you on the shipping costs. They are 22 each now.. I was off on what I paid :/ but still a bargain compared to the imported metal can resistors.
 
I am not surprised. Those smaller resistors should have been mounted on a heat sink instead of a wooden board. And I would mount load resisters in a shield or enclosure to avoid inadvertent contact. A small fan may not be a bad idea, particularly if you plan on having 84 watts go through each one. Heaters get hot when they are small and have to dissipate a lot of energy. Light up a 30 watt incandescent bulb and touch it. Those resistors are no different.
 
RTally said:
I am not surprised. Those smaller resistors should have been mounted on a heat sink instead of a wooden board. And I would mount load resisters in a shield or enclosure to avoid inadvertent contact. A small fan may not be a bad idea, particularly if you plan on having 84 watts go through each one. Heaters get hot when they are small and have to dissipate a lot of energy. Light up a 30 watt incandescent bulb and touch it. Those resistors are no different.
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Normally I wold agree. But, these were 100 watt units with 30 watts being dissipated. At the time, they were just on the bench. There is no way a 100 watt rated resistor with 30 watts max of power hitting should have gotten hot enough to melt plastic. I'm guessing that the Chinese manufacturer was loose with the ratings. I have some old school ceramic power resistors that I made into a 50 ohm load for my ham gear and they do not get even warm at 100 watts with an overall rating of 200 watts. The ones mounted to the board was for a different project that I didn't end up using after all. i just wanted to show the size difference. Which is substantial :)
 
Mike Sweeney said:
There is no way a 100 watt rated resistor with 30 watts max of power hitting should have gotten hot enough to melt plastic.
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That really depends on the size of heat sink specified to facilitate the 100W rating, rise over ambient, max temp, etc. If achieving the 100W rating requires a massive heat sink then certainly the resistor could become blazing hot at 30W just mounted on board.

Out of curiosity I looked at a TE Connectivity 100W chassis mount. With a heat sink of the size they specify the resistor appears to operate with surface temp rise of around 115C @ 100W. So, even at reduced power operation but without heat sink it's going to be hot, hot, hot.
 
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You'd be surprised. Those things have massive de-rating curves depending on heat sink size. No heat sink they're probably only 15 watts or so.

If you find a junked car audio amp, they make excellent heat sink donors. The whole case is finned aluminum and nobody wants broken car amps.
 
Good point about the heat sinks.. but I had really thought that at 30 watts, a "100" watt rated resistor should be able to handle it.. perhaps I'm optimistic :/ I like the idea of a dead car amp for parts.. I have a shop just a mile away that specializes in car systems like that.
 
Just because, I sent the vendor a request for the spec sheet on the orange "100" watters to see what is up with them. Assuming they have a spec sheet..
 
The spec sheets for a Mouser supplied 100w 8Ω resistor like yours show it will need to be attached to the proper heat sink which is 995cm²/154 in² of 3mm thick aluminum with heat sink compound used correctly. The finned heat sink would need to be positioned to allow air flow, fins vertical as in amps and such. Without that heat sink you can easily burn up that resistor using just 30 watts. No wonder it got hot. Now you know why speakers can burn up when way overdriven.

The new resistors you have should be air cooled for proper power handling no matter how much power you dump into them.
 
Mike Sweeney said:
Good point about the heat sinks.. but I had really thought that at 30 watts, a "100" watt rated resistor should be able to handle it.. perhaps I'm optimistic :/ I like the idea of a dead car amp for parts.. I have a shop just a mile away that specializes in car systems like that.
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The resistor did "handle" it. It was just hotter than you expected.

A watt is a unit of power. The more power (watts) in a small package, the hotter it gets. And the smaller the package for a certain amount of power, the hotter it gets.

I have a small, 30 watt soldering iron. That is the same amount of power that each of your resistors saw. That soldering iron is HOT. I would expect those resistors to be hot, too, although the temperature may be slightly lower than the soldering iron because the surface area of the resistors is larger, allowing more heat dissipation.
 
It really is all about the heat sink. I made my dummy loads from 100 watt 8 ohm Parts Express dummy load resistors. I mounted them on an old industrial rectifier heat sink. Each one is about the size of a box of tissues, and has taken over 400 watts without getting hot. Be advised that you can run them over their rated specs like that, but not for long. I did just long enough to take measurements, probably around 30 seconds. Also, when they heat up their resistance usually rises as well.

Food for thought...
 
RTally said:
I remember way back when. We would put a 50 ohm resistor in bucket of oil to load test our ham transmitters.
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It would be my luck to kick the damn thing over ;) Or my kids would toss a basketball into it by mistake.. things seem to happen around here like that :eek:
 
Two 16's in parallel = 8 Ω @ 600 watt / two 4,s in series = 8 Ω @ 600 watt .
 
avionic said:
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Gotta keep the coffee hot!!
 
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