In article g,
straydog wrote:

Don't forget about "soak", also known as dielectric absorption or dielectric
hysteresis. In some capacitors, particularly electrolytics, charge "soaks"
into the dielectric and can take from seconds to days to work its way out.
The result is a "discharged" capacitor that seems to spontaneously recharge
itself. (One story I heard during my broadcast days was someone who put a
screwdriver across a "discharged" capacitor.

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.

Yup. Formally "dielectric absorbtion". In a lot of types of
capacitor, enough stored charge can come back out, over the course of
a few minutes, to raise the open-circuit terminal voltage of the cap
to a significant percentage of its fully-charged level, even if you
had previously discharged the cap all the way to a zero reading.

It can be enough to give you a nasty bite.

--
Dave Platt AE6EO
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