Re: One Terminal Capacitor

Jerry Wayne Decker ( )
Thu, 8 Jul 1999 10:22:36 -0700 (PDT)

Hi Folks!

I found this interesting comment about capacitors
'taking their power from the air' to produce as much
as a 3 foot spark quite interesting.

It is well known that capacitors will charge just
sitting by themselves OR will charge instantly if
dropped any distance but I wonder how long it takes
for this multi-farad pulse power caps to recharge???

Getting any closer?;

In article
<>, wrote:
> I suppose capacitors in the '50s were "different".
> In civilian radio construction and servicing we
> discharge them by shorting with a screwdriver blade.

> Then I was called up into Military radio and radar;
> and we were not supposed to do that - alleged to
> shorten their life.
> We had high resistance carbon sticks with insulating
> handles for the purpose.

Did that make as big a "pop"?

Shorting out a high-farad capacitor with a screwdriver
still makes an impressive demonstration for beginning
electronics students; so does attaching a much higher
voltage supply than a capacitor is rated (use a
low-farad cap when doing the latter).

> We noticed that a discharged capacitor would
> "recharge" over a few hours, enough to make you jump

> and throw something expensive at something else
> expensive. Maybe charges in the "insulator" were
> able to migrate, very slowly.

I mentioned a few months ago on this forum that my
instructors in Pulse Power warned my class that the
large (multi-farad) capacitors used in pulse power
will acquire a charge on their own, "from out of the

The discharge from such a capacitor may form a 3 foot
long spark, and is lethal to human life. There are
shorting rods on the large caps; one end is
permanently attached to a hinge on one terminal, the
other end is supposed to be clamped to the other
terminal when the cap is in storage.

Several people informed me that it is impossible for a
capacitor to charge "from out of the air," and I'm
sure they will reply to my comments here, too. My only
knowledge on this issue is what we were told by our

--Richard AlexanderRichard's Electronic Kingdom


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