On Thu, 14 Jul 2005, Roy Lewallen wrote:

Date: Thu, 14 Jul 2005 19:47:23 -0700
From: Roy Lewallen
Newsgroups: rec.radio.amateur.homebrew
Subject: QUESTION: Fun with Svetlanas or Staying alive with kV power
supplies

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.

Well, if you want to get technical about it, yes. All those layers of foil
all, individually, add up to all of those Cs and Rs.

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.

I doubt if there is very much in the universe with a real, practical
_permanence_. The second law of thermodynamics says that trapped charge is
going to try hard to become untrapped.

It's the
analog of a magnet, with permanently trapped flux and producing a permanent
magnetic field.

Most magnets that I've heard about will slowly lose their magnetism via
one of many mechanisms.

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.

In a laboratory, once, I made an electret out of, basically, frozen water.
The various pieces of these electrets, bathed in liquid nitrogen, behaved,
qualitatively, like little magnets. Bring one near another, and whatever
"pole" was at some point on one piece would spontaneously jump,
appropriately, to the piece I was holding.

An electret that is not moving can't produce a voltage? Semantics. There
_will_ be an electric field between one pole and the other and it _will_
be measureable and calculable and the results will be in volts regardless
of whether the electret is "moving". The units can be expressed in more
than one way depending on reference systems, definitions, and goal of the
measurement or calculation.


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.

For many people (since, in my life, it came up many times in
conversation), protecting the meter was more important.

Roy Lewallen, W7EL


Art, W4PON