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
r@somis.org
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