On Tue, 16 Aug 2011 19:11:18 -0700 (PDT), the renowned "larry moe 'n
curly" wrote:

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.

The safest thing is to use an exact replacement unless you understand
the circuit more than well enough to have designed it in the first
place. Or get an ECO that allows the use of the substitute part.

Replacing a current-sense resistor with the wrong type (too inductive)
could cause a whole lot of pain, for example.

If you actually are talking about carbon composition resistors (fairly
uncommon and relatively expensive these days, as others have said),
then there isn't much problem with a higher wattage other than getting
it to fit, but most resistors these days are metal film (with some
carbon film still around in through-hole applications, and wirewound
used in power applications). There's also metal foil, fusible, high
voltage rating, metal-oxide-film (MOF), tight tolerance, low tempco,
positive TCR, low inductance, and lots of other types of resistors.

Wattage rating is only one parameter (and it really isn't a simple
parameter- it might be lower in a given application than the big print
value if you limit how hot the body of the resistor can get or limit
the ambient temperature or have pulses of current that yield high peak
power).


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
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