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capacitor discharge
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From: Jim Lux [SMTP:jimlux-at-earthlink-dot-net]
Sent: Friday, March 06, 1998 12:49 AM
To: Tesla List
Subject: Re: capacitor discharge
>
> ----------
> From: C.D. Rakes [SMTP:cdrakes-at-ipa-dot-net]
> Sent: Thursday, March 05, 1998 10:04 AM
> To: Tesla-at-pupman-dot-com
> Subject: capacitor discharge
>
> Hi, This is my first inquiry to the Tesla list and what I would like help
> with is not actually Tesla coiling, but does have to do with high
voltage.
> Many year ago I purchased 10 70-uF, 10,000-volt pulse type capacitors,
> and had hoped to use them in a rail gun experiment. What I've have done
is
> to connect two of these capacitors in series to use as a capacitor
> discharge system to experiment with high energy electromagnetic fields.
> I'm using a heavy duty 5750-volt plate supply transformer to charge each
of
> the capacitors to the transformer's peak voltage, which should be
somewhere
> near 8-KV.
That is, about 2 kJ for each cap.
Since they are in series the total voltage would be around
> 16-KV. What I would like to know is how far apart must the discharge
> electrodes (which are two, 2-inch trailer hitch balls) be positioned to
> avoid self discharge?
Roughly 30 kV/cm, so for your 16 kV, you are looking at around 0.5 cm or a
0.2 inches. With a gap that small, the 2 inch diameter ball creates an
almost uniform field. So, if you put the balls 1/2 inch apart, I wouldn't
expect a self discharge.
I've seen several experimenters using a steel ball
> as the discharging element, but I would like to consider using an
> insulating material, such as Plexiglas or similar material, that would be
> placed between closed spaced balls and quickly removed to allow the gap
to
> fire. Would appreciate any suggestions and ideas.
I would mount one of the balls so that it can move and drive it with a
spring with some sort of release catch. Use a fairly big spring so that it
moves fast. Alternately, you could use a spring (or air cylinder, or
whatever) to force a metal bar across both contacts. Or, a piece of copper
pipe to bridge the gap (sideways, that is).
If you want really fancy, use a thin metal sheet with a hole in the center
between the gap, bias it appropriately with high value resistors (e.g. 100
Meg) and then capacitively couple a trigger pulse (say from a auto ignition
coil) to the midplane electrode. The overvoltage will distort the field
between your main electrodes, which will then break down.
Or you could construct a classic three ball gap along the same lines as
above.
My own preference would be a mechanical switch, though, unless you need
millisecond or better timing accuracy.
For long cap life, you want to make sure the discharge circuit is
critically damped (i.e. not much voltage reversal) because that is the
quickest way to shorten the life of a discharge cap.