[R-390] Re: Parts Selection

[Tim Shoppa]

Mark wrote:

> I had a similar discussion with an Army Instructor.
> He claimed that a Carbon resistor acts as a fuse,
> and always fails open. That it will open under
> overcurrent(I.E. Heat) faster. Finally he told me
> to put my money where my mouth is. We took
> a handful of carbon comp. resistors, a power 
> supply, and an ammeter. Then put 1 Watt through
> the 1/2 watt resistors. Every time, current went
> up for about thirty seconds, climbing higher 
> and higher, until the carbon resistor was nearly a short, then it 
> opened.

Your experience mostly matches mine, Mark. I think there's
a good element of variability because carbon compositions
span 6 orders of magnitude in terms of resistance rating
(and therefore the "composition" part must be highly
variable depending on value and maybe maker)
but what you describe is the failure mode I often observed
(meaning, when I was a kid I would burn up resistors
this way on purpose!) for carbon comp resistors in
the 10 to a-few-hundred ohm range.

In the 2.2K range things seem to work different simply
because I find so many charred 2.2K's that measure
to be 5K or 7K or 10K at on the ohmmeter.

On the other hand, for metal films, the failure mode is
ALWAYS that exceeding the wattage rating by a factor
of a few will make the resistor so hot that the solder
on the leads melts and drips off. Measuring the resistance
then shows that the resistor is almost exactly the same
value as when you're started.

Now, exceeding the power rating by a factor of twenty
will make the metal film resistor glow red, and generally
it won't have the same value anymore if it remains
intact :-).

Others here obviously have a different attitude than me,
but I regard it as a FEATURE of a circuit if a failure results
in obviously burnt-until-inoperative-or-arcing-or-glowing
components. Choke fires are a common element that all
my transmitters have survived. Sometimes I finish the
QSO before I turn off the transmitter :-). Please
remember that I'm also the guy who advocates not
baby-stepping your electrolytics up but just turn
on all the volts and let 'em blow if they're weak!

Tim.