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When is a 200W Power Resistor the Size of a 100W Power Resistor?
When is a 200W Power Resistor the Size of a 100W Power Resistor?
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Old 25th March 2020, 09:14 PM   #11
ampexperts is offline ampexperts  United States
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Join Date: Feb 2014
Location: New Milford, CT
Obviously, ambient conditions change the thermal capacity of a device, but in this world of constants, a smaller part is going to have less area to dissipate heat.

These dummy loads are heat sinked, but I don't trust these new resistors to be up to the capacity of the older ones. And their size is not in agreement with the seller's resistor size vs wattage rating chart. They sold me a 100W sized resistor.

Unless they changed their environmental conditions from, say 25C to 18C to get the smaller package to "meet spec", I don't see how these little parts will handle 200W for any duration.

My other dummy load can tolerate 1200W for 15 seconds without degradation or damage. I much doubt these will handle even short term over limit tests.

If the seller doesn't agree to correct the matter, I'll be filing a claim to get it refunded. But I have no intention of paying freight to ship these back to China!

Mouser has power resistors of this nature, but curiously, Ohmite makes them smaller still:

Length:89.7 mm
Width:72.5 mm
Height:41.8 mm

And $60 a piece, vs. $8 from eBay.

Starting to wonder if a change in materials is causing them to be smaller.
Mark A. Weiss, P.E.

Last edited by ampexperts; 25th March 2020 at 09:19 PM.
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Old 25th March 2020, 09:35 PM   #12
Mark Tillotson is offline Mark Tillotson
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Join Date: May 2018
Location: Cambridge UK
Originally Posted by ampexperts View Post
The only way that I can conceive of smaller resistors handling the same power is if the internal insulating materials have better thermal conductivity to the outside flange.

Well no, there's another way, allowing the resistor core to run at higher temperatures means the thermal conductivity can stay the same and the dissipation rating be higher, so long as the heatsink is rated for that power coming in. That does mean the shell of the resistor will be a lot hotter where its not in contact with the heatsink, affecting nearby components. This wouldn't make for a precision resistor value (!)
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Old 25th March 2020, 11:52 PM   #13
cliffforrest is online now cliffforrest  United Kingdom
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Join Date: Sep 2004
Location: South Coast, UK
" ..but curiously, Ohmite makes them smaller still"

Well, obviously you are an expert on this subject and don't need any "expert" advice.

Good luck!
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