On Sun, 15 May 2005 05:13:49 -0700 "Watson A.Name - \"Watt Sun, the
Dark Remover\"" <NOSPAM@dslextreme.com> wrote:
Quote:
I have fifty 910 ohm 1/4W carbon composition resistors, still in the
paper tape, which is labeled:
OHMITE
910 OHM
1/4 W 5%
RC07GF911J
OC9115
Judging from the last line, they're 14 years old. They've never been
used, AKA New Old Stock. Every resistor measures greater than 5%, some
are over 1k. Over 14 years, would drift from aging cause them to go
that far out of tolerance? I have carbon film resistore much older than
these and they still measure within 5% tolerance, usually within 2%.
|
If you could find an old Allen-Bradley catalog you'd be truly amazed
at the amount of detail they lay out on the moisture sensitivity of
these resistors.
The gist of it is as follows:
1) Carbon composition resistors will absorb moisture out of the air
and go up in resistance. If the resistance is important to the
manufacturer, they much keep them in a special dry engvironment, just
like flux coated welding rods.
2) If you have carbon composition resistors which have gotten damp,
they can be dryed out by heating them to temps below 100C for several
days. 2 Watters take longer than 1/4 Watters. Don't try to speed this
up by using higher temps.
3) If you solder them when they are "damp" the change in resistance
will be locked in, ie no longer reversible.
I did this test on a batch of 27k, 2W carbon comp resistors a number
of years ago. They were all high, and most of them were out of
tolerance, this was immediately as purchased from Newark. But they all
came back into tolerance after heating in my home-made oven for about
a week.
-
-----------------------------------------------
Jim Adney
jadney@vwtype3.org
Madison, WI 53711 USA
-----------------------------------------------