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To: "k7fm" <>
From: R L Measures <>
Date: Fri, 2 Jun 2006 14:29:07 -0700
List-post: <>
On Jun 2, 2006, at 5:47 AM, k7fm wrote:

> Rich said:
> "The carbon-comp R changes value by an amount not adequately explained
> by simple aging.  For example, a 300% to 500% increase in R in one
> day is not what I would call normal aging.  Also, such resistors show
> no outward indication of extreme age."
> Rich's statement implies that the changing of value is due to the  
> dreaded parasitic.  30 years ago, I wanted to use some 2 watt  
> carbon composition resistors in a commercial circuit.  I needed to  
> be able to dissipate 2 watts on occasion for brief periods of  
> time.  I built a test fixture and thought I could cycle the  
> resistor a few times and age it so that the resistor would then be  
> more stable.  What I learned was that if the 2 watt resistor was  
> heated to 1.5 watts, then cooled, it changed value - permanently.   
> What was more surprising was that cycling the resistor again would  
> change the value again - but not necessarily the same direction.  I  
> drew a graph of the changes in value and it was almost circular.   
> When I terminated the test, the resistor that started out at 300  
> ohms was at 475 ohms - and still changing.
> I suspect the resistor was acting as a hygrometer and probably  
> other things - but I did not really care as the results showed that  
> the 2 watt carbon composition resistor was not suitable for what I  
> needed.  I then tried a Corning Glass film resistor that was  
> repeatedly stable and worked fine at 450 MHz.
> I used dc to conduct the original tests.  I thought the results  
> were due to the nature of the carbon composition resistor - but it  
> now appears that the results may have been due to parasitics in the  
> power supply, which caused the change in value of the resistor.   
> Boy, how stupid I was when conducting those tests.  Never even  
> dawned on me that it could have been parasitics.

Me thinks he doeth protest too much.

> Colin  K7FM

R L MEASURES, AG6K. 805-386-3734

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