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

```On 1/5/16 9:48 PM, Heather & Paul Novotny via Tesla wrote:
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```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?
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60 or 50 Hz?

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"Resonant" is where the reactance of the capacitor is the same magnitude as the reactance of the transformer.
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The transformer reactance is simply E/I or 12,000/0.030 = 400k (technically, it's j400k, because it's an inductor)
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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.
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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)...

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You want Larger Than Resonant (LTR) ... typically 1.5-1.7 x the resonant size, which works out to 10-12 nF.
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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
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with a 15 kV transformer, the peak voltage is 1.414*15= 21.21 kV.

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I saw a formula for it but I don't know what all of the variable names mean in the formula.

```