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.
Colin K7FM
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|