[ARC5] Resistor Excursions

[scottjohnson1 at cox.net]

Heating drives off moisture, which, depending on the admixture of the composition, will either move the resistance up or down.  The problem is that once it has absorbed moisture, the composition is more absorbent, and will take up the moisture readily.  This phenomenon is explained in great detail in many failure analysis and materials texts. Father time sucks.

Scott W7SVJ

-----Original Message-----
From: arc5-bounces at mailman.qth.net <arc5-bounces at mailman.qth.net> On Behalf Of Richard Knoppow
Sent: Friday, April 19, 2024 16:15
To: arc5 at mailman.qth.net
Subject: Re: [ARC5] Resistor Excursions

     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/> 
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--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
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