On Fri, 9 Oct 2009 02:15:50 -0700 (PDT), "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.

Depends on what it's made of and how well it's cooled. We use 0603
surface-mount resistors at half a watt, because we heat sink them
well. Half-watt carbon resistors will die at half a watt in a confined
space.

Enameled wirewounds are very tough, up until the enamel melts.

Some resistors will die from temperture cycling stress.

But in general it's good to derate 0.5 maybe.

John