Power dissipation question

Note the following photograph of the two power resistors from my power supply.

The large resistor is 27Ω at 10 watts.
The smaller one is 10Ω at 5 watts.

They are the static loads for the 12 and 5 volt supplies respectively, and they should each draw about a half-amp.  This should give an approximate power dissipation of 5 and 2.5 watts - say a total of 7.5 watts.

Despite the very low power draw, both resistors individually get quite warm, to the point I used high-temperature heat-sink glue to attach them to a metal heat sink.

7.5 watts isn’t that much.

Any idea why they’d get so hot?

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How hot is “so hot”?

For comparison - when Dave’s RaspberryPi 3B+ runs at 7W it reaches 68degC.

I saw one answer to “How hot does a 2W resister get?” thus:
The rating 2W at 155degC in a 25degC room means ~130degC rise for 2W load. If you run it at half load of 1W, the temperature rise should be half or 65degC rise above the 25degC room. That is pretty hot! (Can’t vouch for the truth of it, just relaying possibly relevant data.)

Also: datasheet - Why is 10W resistor getting hot with only 6.5W running through it? - Electrical Engineering Stack Exchange


The load resistors don’t necessarily get to “fry an egg” hot, but way too hot to comfortably hold or touch, and hot enough to make me worry about the heat-sink being attached directly to the plastic of the case.  (The fan is located directly above the load resistors so that should help to keep them happy.)

By comparison, a 7w night light and/or Christmas tree light does not feel that hot.

Once I get things back together again, I will try to get a good temperature measurement.

I’m curious.   I’m thinking that 7w shouldn’t generate that much heat.


Guess it’s gonna get hot.

Maybe I should consider a more gnarly heat sink? :wink: