On Oct 9, 2:15*am, "larry moe 'n curly"
wrote:
When run at its rated power?

I'm referring to 1/4 - 10 watt resistors in TVs and PC power supplies,
probably in 30-50 Celcius surrounding air.

I thought that resistors were supposed to be chosen for twice the
actual power, but I've seen many 0.5W to 1.0W resistors run at almost
exactly their rated power continuously.

The life at rated wattage will depend on many factors. The legs of
the resistor will conduct the heat away. How much copper they hook to
will control how well this works. The brand of the resistor also
matters. One from Uncle Chin's Resistor Company will last much longer
than those from the Shaw Dee Resistor Factory because Uncle Chin uses
better ceramic.

If you use that urethane conformal coating that the US navy seems to
like so much, it will turn dark and look very ugly long before the
resistor fails. The ceramic materials will do their best when clean.