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
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