[Boatanchors] Carbon Comp Resistors: The Darkness Gathers?

[Gary Peterson]

It all depends on the application.  There are RF circuits where carbon composition resistors work and some carbon film and metal film ones don’t perform as well.

I have read that some carbon film and metal film resistors are trimmed to value by cutting a spiral in the film.  I have measured and found some of these film-types to be inductive.  No big deal on 160 meters, but a potential problem at VHF.  Carbon composition resistors, that I have measured, tend to be very slightly capacitive.

I have hooked  older carbon composition resistors up to a power supply and applied enough voltage to cause the part to be hot to the touch and most returned to within their original tolerance within an hour, probably due to baking out absorbed moisture.  I would think that equipment that is used regularly would not suffer value drift anywhere near as much as the ones that have gathered dust.  Old radios, engines and human muscles and brains need to be used regularly or they atrophy.  

Once, I measured a huge pile of 10% carbon composition resistors.  Not a single one was 5% or better.  I had to assume that as these came off the assembly line, the better ones got the 5% gold bands and the ones between 5% and 10% got the silver bands.  In many cases, the tolerance isn’t all that critical.  

Gary, K0CX

“The majority of
these have started reading Hi-Z by 5% or more.
Many by much more.  I just threw a half dozen NOS
100K away because they read from 107K- to
122K-Ohms.  Other values are showing similar
drift.  Dave AB5S”