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Re: [TCML] MMC design for 12kV @ 120mA xfmr



John,

Your proposed arrangement would not last very long. The 2 kV rating on the caps is a DC value. Your transformer puts out 12 kV RMS, with a peak voltage very near 17 kV (12 kV x 1.414). The 6 cap string would see 17 kV and fail in short order, and even the 12 cap string would be significantly stressed. Dr. Resonance recommends selecting cap string voltage values of at least 2.5 x V (RMS). Thus, your strings should have a bare minimum of 15 caps in each string for a 30 kV rating. Using 16, 17, or 18 caps will provide additional insurance and longer operating life at a small increment in cost.

Remember that all kinds of spurious voltages are impressed upon the components in a running TC. The extra margin is necessary because the caps are not seeing just a nice clean pure 12 kV sine wave, but a whole mish-mash of resonant and reflected voltages. You will need to use more that two parallel cap strings anyway, as you are using a NST with a very substantial current output. Each cap string is good for only a certain amount of current. The peak currents at the moment of gap firing are substantial, and must be divided across a number of cap strings to prevent overheating. Sorry to say, this project is gonna get expensive.
Dave

John wrote:
Planing out my MMC before I order some CD942c 0.15µF caps. I'm planing
on building my next coil with a 12kV @ 120mA xfmr.  LTR cap value =
0.0398µF so I'm setting my cap value at 0.04µF.

series string A: 2kV @ 0.15µF / 10 = 20kV @ 0.025µF
series string B: 2kV @ 0.15uF / 6 = 12kV @ 0.015uF

Both strings in parallel giving 0.025µF + 0.015uF = 0.04µF

Does that look good?

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