# Surface temperature of a load resistor

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?

• A 2 or 5 watt resistor will get a lot hotter than a 10 watts resistor for the same power electrical dissipation. – Andy aka Mar 2 at 14:48
• 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. – Wesley Lee Mar 2 at 14:51
• 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. – Mattman944 Mar 2 at 16:18