Hal Murray wrote:
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.

There is another dimension to consider: duty cycle. I've been told
(but don't have first hand experience) that old fashioned carbon
composition resistors are much better at pulse loads that newer
carbon film resistors.


Proven! I was using some 100 Ohm resistors to limit current in a
spark-gap ignitor for mercury short-arc lamps. The film resistor just
BLEW off its resistance film on the first shot. I mumbled "oh yeah, I
KNEW it was going to do that" and replaced with a bulk carbon resistor,
and that held up for the life of the unit. You can guess, the pulse
current was 20,000/100 or about 100 A. 200 A ^ 2 * 100 = 4 mega-watts
for a couple microseconds.

Jon