[ARC5] Resistor Excursions

[Richard Knoppow]

     FWIW, carbon composition resistors were not made in tolerances 
closer than 5%. I think the reason is that the heat of soldering could 
move them at least that much.
     My understanding of the cause of the drift is that over time, and 
with the influence of moisture and heat, the tiny carbon particles, 
which make up the conductor, lose contact with each other. This is also 
the reason they are noisier than deposited film resistors and noisier 
than the Johnson noise predicted by the value.
     If moisture the the main cause its possible that heat of soldering 
might move them back by increasing the particle contact. I suspect that 
otherwise heat or baking will cause them to move further away from the 
specified value. It would take some formal experimentation to determine 
this.
    My experience is that higher value CC resistors are almost always 
out of spec, anything of above 100K and probably less. At 1 meg and more 
the drift is severe.
    CC resistors seem to have no paricular virtues although I have seen 
pulse handling mentioned. Since deposited carbon or metal film resistors 
are now cheap and easy to obtain it seems to me there is no good reason 
to use CC resistors for anything.   BTW, there are resistors known as 
oxide composition. I think these are rather different from the old 
carbon comp resistors and seem to have advantages for moderate power 
levels.

On 4/19/2024 3:14 PM, sbjohnston--- via ARC5 wrote:
> Over the years I've noticed that an off-value resistor can be 
> significantly impacted by the heat of the unsoldering to take it out of 
> the circuit.
> 
> Usually the heat makes the out-of-tolerance resistor get better for a 
> while, but in a few cases the warmth made it worse.
> 
> For me it didn't matter - I could not trust that resistor and it was 
> headed to the trash either way.  But this might give us some insight 
> into the problem.
> 
> 
> Steve WD8DAS
> 
> sbjohnston at aol.com
> http://www.wd8das.net/ <http://www.wd8das.net/>
> http://af4k-crystals.com <http://af4k-crystals.com>/
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-- 
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998