[ARC5] Carbon Comp Drift (was: GRC-9 Update III)

[jeepp]

Well, these things will drift... period.  I have a stock of over 2500 1/4w, 1/2w  1w, and 2w comp. resistors in stock, here.  Most date back to probably 1970 to maybe 1990.  All are generally 5%.  As I pull one for use, I do a quick check.  Very... very rarely will I reject one for out of tolerance.  Granted, all are, or were, stored in a benign environment.  I have no qualms with them.  That said, run them hot and they will move, over time.Jeep K3HVGSent from my Verizon 4G LTE smartphone

-------- Original message --------
From: David Stinson <arc5 at ix.netcom.com> 
Date: 3/1/19  17:57  (GMT-05:00) 
To: arc5 at mailman.qth.net, milsurplus at mailman.qth.net 
Subject: [ARC5] Carbon Comp Drift (was: GRC-9 Update III) 

Ages ago, I was told to always look closely at any carbon comp witheither a green or blue multiplier band, as these drifted high moreoften than others.  And that seemed true at the time. Strangely, in the last few years, when refurbing sets, have begunseeing far more with a yellow or orange multiplier band out-of-boundsthan the green/blue bunch.  Most common resistors to find badare 33K, 47K, 100K, 330K and 470K.    Maybe it’s just because thoseare more common in sets than 1 Meg or 10 Meg, but I don’t think so.Rarely see a carbon comp  in a set below 10K that’s out of spec.Black, brown and red multiplier bands are 90% or better OK.“NOS” carbon comps that have been sitting in boxeswithout climate control are a different story.  Just thinkingof ones I’ve seen actually in sets. Have a theory about this:Inside a carbon-comp resistor is a carbon-based resistance“filling” or “matrix” with conductive plugs on each end.  One can visualize this matrix as a bunch of resistors in series and parallel;   think of a series string of resistors, then manystrings layered like an onion.  Taken all-together, theymake one equivalent value. As the matrix ages, the outer “onion layers” fracture.Fracturing one of the series resistors does not open that string, because the bottom of the fracture is stillin the matrix.  It makes that string longer than the others,making two resistors in series where there was oneand thus, increasing the overall matrix resistance.As aging continues and the number of these fractures grow, the overall resistance of the part climbs everhigher. The effect is less pronounced in lower-value resistors,because the value of each of the imagined individualresistors in the matrix is lower, so the value ofthe effectively “extra” resistors added by each fracture is  also smaller , i.e. the same physical fracture rate that would move a 1-Meg resistor  up 500K  would move a 1-Ohm resistor only 5 tenths of an Ohm. Thoughts on drifty resistors?73 Dave S. 
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