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BIG Capacitors
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From: Bill Lemieux [SMTP:gomez-at-netherworld-dot-com]
Sent: Wednesday, February 18, 1998 4:57 PM
To: Tesla List
Subject: Re: BIG Capacitors
Edward V. Phillips wrote:
> Any idea how big a 100 uF 18 kV capacitor would be?
Good point. I just required a very nicely-designed, massive pulser
which contains 6ea 63uF / 10kV caps. Each unit is about 8x12x24" and
weighs over 100lbs. The whole machine weighs nearly 1000 pounds, and
was an iron-clad galloping bitch to transport.
Which reminds me, does anyone have a good source for surplus ignitrons?
I prefer the National 7703H so as to match the originals. I may have
damaged the tubes by moving the machine- National's literature says
that in crowbar and cap-discharge applications, moving the tubes may
damage the ignitor and shorten the life of the tube. I am hoping that
proper re-conditioning the tubes will alleviate this...
> You can get 4800 ufd 450 volt electrolytic capacitors pretty
> cheaply at swap meets (less than a buck around here).
Ed- eletrolytics are really lousy for fast pulse-discharge work.
1. they have high-self-inductance, which slows down their discharge time
too much for 1uS pulse times. Most pulse-discharge research is looking
for very high peak power, and the narrower you can get your pulse, the
higher the peak power for a given stored energy.
2. they have high effective series resistance, limiting their discharge
current and thus, discharge time.
> 40 of
> those in series would be 120 ufd at 18 kV, if you could
> equalize the voltages with shunt resistors across each
> capacitor. Since typical leakage is of the order of 2 ma,
> that wouldn't take too much power. I have used one of these
> very successfully in a magnet charger, and think the series
> resistance isn't more than a couple of ohms. Have looked at
> the can crusher page, but can't remember how many turns on
> the primary, so hard to judge what peak current is. This
> may or may not be a foolish idea, but I'm sure discharging
> such a bank would be pretty spectacular.
It's less than you would expect. I built a 1.4 kJ bank out of Aerovox
high voltage filter caps, and could not get discharge times faster than
some hundreds of milliseconds. It was loud, but dangerously so.
Later, I found a single pulse-dischage cap beautifully designed for
low-inductance, fast-pulse work, of about 1.5 kJ at roughly the same
voltage. Its discharge times were on the order of less than 1uS with
a rather lousy switch and transmission design, and the noise was
stunning.
-G
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