9 Jun
2009
9 Jun
'09
3:37 p.m.
Dave McGuire wrote:
On Jun 9, 2009, at 2:26 PM, Chuck Guzis wrote:
Yes. I used to work on TVs a lot when I was a kid; I'd dig dead
ones out of dumpsters and (sometimes) fix and sell them. I got two
or three good jolts from CRTs before I figured out what was going
on. I never did quite understand how it happened, though...do you
know the physics behind it?
I don't claim to have a full physics understanding of the issue but
I think a rough hand-waving explanation is that with the capacitor charged and
the dielectric under potential stress, some charge creeps into and is held by
the dielectric itself. It takes time for the charge to creep into the
dielectric. Once the potential is removed by shorting/loading the cap, the
dielectric charge will redistribute itself to reflect the new potential state,
i.e. creep back out, but this too takes time. If the discharge path has been
removed the charge will accumulate on the plates.
Search for "dielectric absorption", or look up "permittivity" in
Wikipedia to
get an idea of just how complex capacitors are (!)
If my understanding is correct, the phenomenom will show up if the dielectric
is a material, as it involves the atoms of the material, but will not if the
dielectric is a vacuum.
I've run into this! Except one doesn't need a high-impedance
meter for it...I've seen them somehow recover enough charge to
produce a spark when re-shorted. It's very odd.
Anyone who has done a lot of work on CRTs has experienced this.
Disconnect the anode lead, short the anode cap, come back in a few
minutes only to get another spark.