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Power resistor strange rising in temp

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RoGeorge:
I was probing a 15\$\Omega\$ resistor at 5Vdc (0.336A measured, Pd = 1.68W).  It's been sitting on the bench for an hour now, in free air, with nothing but two alligators from the power supply at 5V, and its temp keep rising...  :o

It measures 64*C already (147*F), and still increasing at about 0.5...1 degree per minute.

The resistor is from a former 8'' floppy disk.  It has a body of aluminium 3.2x1.5x1.5 mm (in inch 1.2x0.6x0.6), with mounting ears.  Marked as REMIX-R6192 15RK TN, most probably 8W nominal power by this page:  https://vikiwat.com/en/resistor-68ohm-8w-5-remix-r6192.html



I don't understand how such a big resistor can get so hot in open air?  And how that after an hour of constant power and constant conditions, sitting on the bench in open air, the temperature didn't settle yet?  :-//

james_s:
Why is that surprising? It's dissipating power which is constantly adding thermal energy to it. It's losing thermal energy to the room via conduction, convection and radiation. At some point the temperature will rise to the point of equilibrium where the heat losses equal the electrical energy going into the resistor but you are not there yet. With almost 2 watts being dissipated I would expect it will reach 80-100C before that happens.

The 8W nominal power is probably assuming it is bolted to some kind of heatsink. It may also be specified at an operating temperature of 125C or more, if you can find a datasheet that will tell you.

Siwastaja:
Sounds unsurprising as you have not mounted it to any heatsink, and it can't radiate due to the bare aluminum finish. That only leaves convection cooling with not much surface area.

If you measure constant power in, then surely it can't heat up more than that. Or maybe you are soon being nominated for Nobel prize on going overunity.

RoGeorge:

--- Quote from: james_s on March 30, 2023, 05:57:21 pm ---Why is that surprising?
--- End quote ---

I guess it's my limited experience with power electronics, then.  I was expecting an Al block the size of a thumb to dissipate 1.7W in convection without much heating (my guess was 30-40*C).  Also I was expecting the temp to settle in a few minutes, not in longer than 1 hour.  Seems I was wrong.

It's a vintage resistor, couldn't find any datasheet.
It was used as a current limiter in series with the windings of a motor in 8'' floppy disk drives.

Siwastaja:
Note that natural convection works by warm air rising and cold air replacing it. Therefore the cooling power (amount of thermal energy being removed per time unit) is proportional to temperature difference. The hotter it gets, the more watts can be removed; and by more than linearly.

You can try how forced cooling works by using a fan. You may be surprised how much more effective it is even when you have the exact same surface area available.

But if you can allow the resistor to get hot, then the difference to fan cooling is not that much anymore, because the heat removed with fan goes up only linearly with temperature. Hotter air will transfer more heat out, yes, but you only have the temperature to work with; air flow keeps the same, as defined by the fan.

Natural convection starts at zero cooling power because there is no airflow, and climbs up slowly, but starts to work at higher temperatures. Heat something up to a few hundred degrees and you can start feeling the air flow like a fan was running.

With radiation, the exponent is even bigger.

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