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How can I calculate the surface temperature of a load resistor?

I'm given requirements that I may need a 1W 500 ohm resistor.

During test fixture or product debugging the resistor is almost always is too hot to touch.

In this case I would get a 10W resistor for safety.

Would getting a 2 or 5 watt resistor be just as safe?

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  • \$\begingroup\$ A 2 or 5 watt resistor will get a lot hotter than a 10 watts resistor for the same power electrical dissipation. \$\endgroup\$ – Andy aka Mar 2 at 14:48
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    \$\begingroup\$ Normal way would be getting power dissipation (which you have) and do the math with thermal resistance to ambient (which isn't necessarily easy to find for low power resistors). If you are worried about that get a nice resistor with a heatsink. Or add parallel/series resistors. \$\endgroup\$ – Wesley Lee Mar 2 at 14:51
  • \$\begingroup\$ One of the first things that my father taught me regarding electronics, almost 50 years ago: when you run a part at its rated power, that is just below the level at which it catches on fire. He was exaggerating, but it's not a bad thought to have in your head. My saying: when margin is cheap, put in a lot. If I was just building a few, I would use a 5W. \$\endgroup\$ – Mattman944 Mar 2 at 16:18
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You don't need to know the surface temperature to determine the appropriate power rating. Calculate the maximum power that will be dissipated in the resistor for your specific application.

Then, select a resistor with a power rating significantly greater than your calculated maximum power. If you really care about reliability, make sure the rated power is, say, 30% higher than the calculated actual power. If your ambient temperature is higher than normal room temperature, check the resistor's datasheet to see if it needs to be derated.

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For a reasonable estimate, if you look at the resistor datasheet, the ambient temperature at which the resistor allowable dissipation drops to zero will be the resistor temperature at maximum dissipation and ambient for that dissipation. You can adjust that to account for actual dissipation and ambient range.

For example, this Royalohm datasheet:

enter image description here

Shows a 155 degrees C temperature at full rated power and 70 degrees C ambient. So if your ambient is 60 degrees and the power half of of rated the temperature should be around 100 to 105 degrees C.

Note that the thermal resistance to ambient of surface-mount parts is quite sensitive to the layout and amounts of copper around then so this applies when you are using the ‘standard’ test layout. A well-heat sunk 0603 might be able to run cooler than a poorly heat-sunk 1206.

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