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# Re: [TCML] LTR Primary Cap on tesla coil

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Ok so according to the chart and to your calculations below I'd be looking for .01uF or 10nF.  I have a static spark gap.  15 of the 942C20P15k in series would get me exactly .01uF. I'll have to check to make sure the transformer is 60 hz when I get home. Thanks guys.

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-------- Original message --------
From: jimlux <jimlux@xxxxxxxxxxxxx>
Date: 01/06/2016  7:58 AM  (GMT-06:00)
To: tesla@xxxxxxxxxx
Subject: Re: [TCML] LTR Primary Cap on tesla coil

On 1/5/16 9:48 PM, Heather & Paul Novotny via Tesla wrote:
> Hi, I have a 12,000 v 30ma NST for my power supply.
>
> Currently I am using a 5gal bucket cap filled with beer bottles and saltwater.  It measures .01315uF on my multimeter.  I am getting about 12-15" streamers off my toroid with this setup.
>
> I would like to try some Cornell Dubilier caps and am wondering what the LTR cap size would be for my power supply?

60 or 50 Hz?

"Resonant" is where the reactance of the capacitor is the same magnitude
as the reactance of the transformer.

The transformer reactance is simply E/I or 12,000/0.030 = 400k
(technically, it's j400k, because it's an inductor)

The reactance of a capacitor is 1/(j*2*pi*frequency * C), where j is
sqrt(-1).  Typically, you'd see the reactance as -1/(j*omega*C) where
omega =2*pi*frequency.

what you want is to rearrange this a bit..

400k = 1/(omega*C) --> C = 1/(omega*400k)

for 60 Hz omega = 377, so C = 1/(377*400k)  = 6.6 nF

So this is the resonant size (one you actually want to avoid)...

You want Larger Than Resonant (LTR) ... typically 1.5-1.7 x the resonant
size, which works out to 10-12 nF.

If you're using the usual 2kV, 0.15 uF caps, 12 of them in series gets
you there-> 12.5 nF @ 24kV or you could use 1600V caps.. that's 19.2 kV,
or 13 in series gives you 20.8 kV and 11.5 nF

with a 15 kV transformer, the peak voltage is 1.414*15= 21.21 kV.

>
> I saw a formula for it but I don't know what all of the variable names mean in the formula.
>
>
> Thanks - Paul
> _______________________________________________
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> Tesla@xxxxxxxxxx
> http://www.pupman.com/mailman/listinfo/tesla

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