ac / dc rating for potentiometers ?

[Paul Perry]

At 03:05 PM 10/02/98 +0100, Juergen.Haible wrote:

>So are there really potentiometers that have problems with DC ? Some
>kind of
>particle migration or similar strange stuff ?
>Somebody must know this !!

well I have read that some pots want a 'small' current flowing from wiper to
resistive track, and some do not want any.... you can imagine, strange things
could be happening at the wiper interface.... I thought this was mentioned
in R A Pease
book on troubleshooting.... will look in a couple of days and post.....
I must say, I have enough trouble without worrying about this kind of thing!
but 
I do remember that in valve radios a common source of noise was when a cap
went leaky and put 
dc across the volume pot.... when you moved the wiper there was a god awful
noise!!!
but easy to fix..
I guess in a few years what we are doing will look as archaic as a spark gap
transmitter..!

paul perry melb aust

ps the Robert Pease book on analog troubleshooting is IMHO the second best
book ever written
on practical electronics.... it is in the EDN series, published by
Butterworth-Heinemann