straydog wrote:

Yep, I can vouch for this effect. I've seen it, too. Short the cap with
alligator clip-tipped wire for a few seconds if you want that voltage
down to microvolts. I think it is not "soak" but simple RC time constant
decay. Just like radioactive half-life. Residual voltage on a cap
_never_ goes to true zero, only according to the decay equation.
However, there is a so-called "electret" effect which really sounds like
your "soak" effect.

No, it's not a simple time constant -- the slope is much different. A
reasonably good model of it is a series R, shunt C, series R, shunt C,
etc. A large number of Rs and Cs with various time constants have to be
used to imitate it reasonably well.

I don't know about an "electret effect". An electret is a dielectric
with a permanently trapped charge, so it produces a static electric
field. It's the analog of a magnet, with permanently trapped flux and
producing a permanent magnetic field. A magnet that's not moving can't
produce a current (much to the dismay of the perpetual motion crowd),
and an electret that's not moving can't produce a voltage.


D'arsonval meters, especially sensitive ones, are also best stored with
a shorting wire accross the terminals.

That's for an entirely different reason -- to protect the meter from
damage, rather than the user.

Roy Lewallen, W7EL