To: mail11:;-at-msd26.enet.dec-dot-com (-at-teslatech)
Subject: haunted caps
From: "I am the NRA." <pierson-at-msd26.enet.dec-dot-com>
Date: Wed, 30 Aug 95 15:57:16 EDT
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>> I have just been reading an interesting discussion on
>> rec.pyrotechnics where they have been talking about high voltage
>> capacitors that seem to recharge themselves after being discharged.
>> Has anyone ever had this happen to them using Tesla coil
>I was one of the posters on rec.pyrotechnics, and yes, I have had it
>happen with my Tesla coil. I am running my system with caps on both
>sides of the spark gap, which virtually guarantees a charge will be left
>on the caps when you shut everything down since there is no direct
>path to ground. After discharging the caps, they will ocassionally
>recharge themselves enough to give a spark when I discharge them
>again before making any adjustments.
This is more or less expected behavior (as i expect Steve is aware.)
The dieletric can store a modest amount of charge IN ITSELF. (Look
up electret, like a magnet, only with an electric field stored).
A simple, one time, short circuit gets MOST of the charge. It leaves
a bit behind. If ther is LOTS of energy/voltage in the cap, the
bit left behind can be enough to damage/injure/kill. This leads
to the precaution of leaving a high value resistance across the cap
as this keeps bleeding until nothing is left.
The same thing happes with low voltages/small caps, but takes some
looking to see. When i was working on microvolt level a/ds it was
a noticeable problem & set the settling time/acquisition speed in
some designs. (in that case, with a volt or so on a cap, the
residual was on the order of hundreds of microvolts. A nusiance
if trying to MEASURE microvolts....)