"Ken Finney" wrote in message
...

"Ed Price" wrote in message
news:mqdud.3291$Af.1453@fed1read07...

"Bill Turner" wrote in message
...
On Tue, 7 Dec 2004 18:12:35 -0500, " Uncle Peter"
wrote:

I wish I could find carbon comps (decent values) for two cents
each! Those days are long gone around these parts.

__________________________________________________ _______

The days of carbon composition resistors are gone for good reason.

Unless you are an antique collector and like to restore equipment using
original parts, use metal film resistors instead. Their stability and
reliability are far superior.

--
Bill W6WRT


I agree that carbon comp resistors have better alternatives if you are
looking for accuracy and stability and low noise. OTOH, carbon comp
resistors are an excellent choice for pulse generators and shaping
networks.
A metal film resistor will exhibit inductance and will often arc across
its
trim line, usually with catastrophic results. You can hit a 2-watt carbon
comp resistor with repetitive pulses of hundreds of watts (peak, keeping
the
duty cycle low) many times without significant resistance shift and
certainly without catastrophic failure.

The carbon comps are not the choice for all circuits, but they still have
their merits.


1. Something that has to be kept in mind is that most "carbon" resistors
being sold today are "carbon film", not "carbon composition" and
won't stand repetitive surge pulses.

2. IIRC, a mil-spec "10%" carbon comp can vary 42.4% over its
life and still meet spec. Expect commercial parts to be worse.

3. There are ceramic composition resistors available that have all
the good properties of carbon comp (except, I'm sure, cost) with
none of the bad properties.



The ideal resistors for abusive, high-speed transient applications are the
silicon carbide (ceramic) resistors (think Carborundum / Cesewid or whatever
they call themselves now). But the carbon comp resistors are much cheaper,
more versatile (lots more resistance values) and easier to use.

I don't know what you mean by the "life" of a carbon comp resistor. My
experience is that they last forever, until you kill them somehow. Drift,
under low power, hasn't been a problem for me. I have never seen 40% drifts,
except for very abused parts, and if you are using the 2-watt carbon comps,
you can hear or smell when you are abusing them.

The silicon carbide resistors can be pushed so hot that you can use them as
room heaters, industrial furnace elements or infrared sources. Oxidation of
the plated silver terminals is a small problem. Also, they do have a
temperature dependent coefficient of resistance (not as bad as incandescent
light bulbs).

I agree about the carbon film resistors; they blow up about the same as the
metal film resistors.

Ed
wb6wsn