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Re: Capacitor Calculations
On Fri, 17 Jan 1997 23:25:20 -0700, Tesla List
<tesla-at-poodle.pupman-dot-com>, you wrote:
[snip to save chip, et al]
><< Hi All!
>
> I've amazingly been able to get hold of a 15KVA 22KV Single Wire Earth
> Return transformer from the local electricity board.
> Anyway, I'm trying to work out exactly what value caps to use with this
> xformer.
>
> When I plug all of this into the formula, it gives me 0.09uF.
>
> Strange thing is that I've seen other coils that use this sort of power
> level and transformer, and they use 2 0.5uF caps in series to yeild 0.25uF.
>
> Which ones do I go for, and is there a particular formula I should be
> using for these xformers other than the above?
>
Rod,
That is the correct use of the formula to make the capacitor
and mains transformer resonate at your power line frequency. Remember
that in this resonant mode the output of the transfomer/cap
combination will be the 31.1kV of the transformer times the Q of the
secondary circuit. If the Q were as low as 4 (it will probably be
higher) the output would go to 124.4kV! For a Q of 10 -> 311kV!!!!
Of course, at these voltage levels something is going to arc over long
before the maximum voltage is reached. When that happens, the cap will
dump all of it's energy in one BIG BANG. (energy = 1/2 * C * V*V) At
100kV you will have 450 joules of energy just waiting to blow anything
within reach.
I would suggest starting out smaller. Follow Ed's suggestions for
sizing the primary cap. Use a Variac to throttle down the output
voltage of the transformer.
Cheers,
jim