# Re: Ganging Caps

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---------- Forwarded message ----------
Date: Fri, 26 Sep 1997 09:24:32 +0300 (EET DST)
From: Harri Suomalainen <haba-at-cc.hut.fi>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Ganging Caps

On Thu, 25 Sep 1997, Tesla List wrote:
> of these apart, I found 5 cap-rolls paralleled. Each roll of course is
> it's own cap measuring 20uF. I did the math and found I had enough to
> get down to 0.001uF if series'd up (yes, I have a bunch). I could even

It most certainly can be done. However, there are few points of consern.
Do not assume equal voltage distribution! If you can estimate the
tolerance for each roll calculate the worst case. Assume all other caps
in the series string are at high limit and only one at low limit. Make
sure the voltage calculated that way is ok for the worst case smallest
cap.

Do not assume equal electrical field distribution! Say you put one roll
just on top of another. Ground the lower end. The high point has
air or whatever around. Then the conductor foil is actually capasitively
connected to the air. It may well have too high E-field. Make sure this
never can happen. Pay attention on getting E field right. One easy
method is to have each cap roll in a conducting cylinder etc connected
to one end of the roll. Then the foil in the cap will be only capasitively
connected to the conductur around and E-field will not be huge *inside*
the cylinder. Do take care no cylinder-to-cylinder breakdown occurs.

Do not assume equal leakage currents in the rolls. If leakage currents
cannot be measured parallel each roll with a resistor making the leakage
current high enough. Then calculate the worst case voltage for the
largest resistor according to the tolerances. This is very essential
if there is any dc bias. For general purpose use dc bias should probably
be assumed.

I think that after those matters have been taken care of it should
work pretty fine. However, I hope the ESR/ESL of each roll is low
enough. Do notice that if one roll has ESR of say 0.01 ohms 100 of
them in series will make a cap with 1 ohm ESR.
--
Harri.Suomalainen-at-hut.fi - PGP key available by fingering haba-at-alpha.hut.fi

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