On Aug 17, 12:41*pm, "Martin Riddle" wrote:
"larry moe 'n curly" wrote in ...

When I come across a burnt resistor, I usually replace it with one
rated for twice the wattage as the original, but I was told it's
sometimes bad to do that. *Why? *I'm not referring to fuse resistors
but ordinary carbon composition resistors.

No, it shouldn't be a problem. Different wattage is usually associated
with size.
If the resistor was 'cooked' then it was running at or beyond it's
rating. Which means there was something else in the circuit that is
defective.

Most resistors are metal film nowadays, carbon comps are hard to come
by.
(But Carbon comps do handle peak currents much better than metal films.)

Carbon composition resistors have a negative temperature coefficient
of resistance - put enough current through them and you form a "hot
channel" with almost all the current being carried by thin filament
that is almost incandescent, and presents a very low resistance.

Back in 1974, my then boss amazed me - and several other engineers -
by persuading a nominally 10k carbon film resistor to pass something
like an amp with a voltage drop of about 100mV for about half an hour.
There was a very narrow stripe of discoloured paint above the hot
channel, but that was the only evidence of what was going on.

He been having trouble with an "intrinsic safety" standards committee,
and this demonstration (and a couple of repeat performances) managed
to persuade them that carbon film and carbon composition resistors had
to be banned in intrinsicly safe devices.

--
Bill Sloman, Nijmegen