Carbon Comp input resistors

[Harry Bissell]

I have to stick up for Carbon Comps !!!

They were once the most common and most affordable resistor. They were not too
temperature stable (but neither are carbon films), they had some long term drift
(carbon films also).

But they have some good points. They are a "bulk" resistance... they have almost
no inductance at all... similar to a peice of wire the same length. Metal film
and Carbon film...
even those designed to be "non-inductive" almost ALWAYS have higher inductance.
Can be mighty important in some circuits... especially things like current
sensing in high
frequency applications... where you want a Resistor, not an Inductor...

Carbon Comps have incredible pulse power ratings. The power is distributed
throughout
the entire material, not just a "thin film".  This again has its place. I like
the carbon comps for IGBT gate drive where peak power and no inductance are
handy to have....

OTOH they just aren't readily available anymore. They are making "Bulk Cermet"
resistors to fill the gap in these applications... also Silicon Carbide
("globar") resistors.

For the Moog input, sure... change 'em. The higher stability of the metal film,
precision, low tempco, all add up to better. But in some (gasp) TUBE apps I'd go
for a Carbon Comp if that was the original design part. Might get you through
some tough times (electrically) while things warm up...

As we sub parts we must keep in mind "What did the original designer have in
mind when he did it THIS way ???"  I find that either the original designer was
an IDIOT (not often) or that he was SO CLEVER that I can barely appreciate the
elegence of his design...

OTOH I have been burned exactly one time by using Metal film in place of Carbon
Comp... the odds are in your favor...

and that's my $.02 (what I paid for my last 200 metal film 1/4w 1% 50ppm....)

H^)  harry

danial stocks wrote:

> avoid carbon comp res they are unreliable, tho carbon or metal film is fine,
> dissipating 1/4 watt into 100k res amounts to 158 volts, so I dont think you
> will have a problem there.
> Cheers,
> Dan
> >I am replacing some input resistors on the control inputs on a Moog 900
> >series VCA.  It looks like they use 1/2 watt 100k 5% carbon comp
> >resistors.  Should I replace with same or can I use 1/4 watt 1% metal
> >film replacements.
> >
> >Thanks
> >
> >G. Wong (fivema)
>
> ______________________________________________________
> Get Your Private, Free Email at http://www.hotmail.com