On Wed, 17 Aug 2011 08:13:16 -0700, Don Lancaster
wrote:

On 8/16/2011 7:21 PM, Jon Kirwan wrote:
On Tue, 16 Aug 2011 19:11:18 -0700 (PDT), "larry moe 'n
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.

Hmm. Not obvious why there would be a worry. The
differences amount to the following:

1) Longer body, and
2) Larger body diameter, and
3) More weight, and
3) Lower temperature at given power dissipation.

Other than the thought that it does have a fuse function,
which you dispute above, there can be:

A) The longer body or larger body diameter causes the wiring
to be too close to nearby parts, mechanically stresses
something, or blocks some hole that needs to be clear, or
B) The greater weight causes some problem (such as if it
were at the end of a propeller, for example), or
C) The lower temperature affects something else that depends
on the earlier higher temperature.

In short, I can't think of a problem that would not be pretty
obvious when you were replacing it. But then my imagination
ain't what it used to be, either.

Jon

If the original resistor burnt, something is wrong and needs
fixed.

That _may_ be the case. You don't know and neither do I. But
the OP is asking about whether or not it could be bad, so I
think it's good you point this out -- others already have,
though, so there's no need to do it twice. And I already
know, so it doesn't help telling me.

Replacing the resistor with a bigger one is not in any
manner a solution.

Designers aren't always perfect. Some of them are even
hobbyists or worse. These are carbon and the devices may be
old and just gradually failed -- I've seen that happen. They
do develop fissures and fail, sometimes just from vibration
sometimes from just aging or part variability that wasn't
accounted for. It _may_ be a problem elsewhere in the
circuit. It _may_ just be a failed part, itself.

I'm just glad the OP is getting all the options laid out.

Jon