Actually, the truth probably lies somewhere in between what you figured and
what your father-in-law said. When capacitors (of equal, or near equal
capacitance) are connected in series, their voltage ratings are additive. If
you string capacitors of unequal capacitance in series, then the individual
units will not share an equal voltage drop and the voltage drop will be
inversely proportionate to their capacitance rating (the smaller the
capacitance, the greater the voltage drop). Since you are using identical
capacitor units, you can figure on each unit sharing a near equal voltage
burden.
So, at the end of the day, your string of (25) 600 volt rated caps would be
good for UP TO 15 kV(DC) voltage potential. However, you must consider that
the output of a 15 kV NST is AC and like nearly all standard AC potential
measurements, is measured as the root means squared (RMS). This is to keep
the voltage measurement in compliance with Ohm's law in relation to the
current flow and the resistance and/or impedance of the circuit. This means
that the peak of each positive and negative swing of the sine wave is
actually 1.41x the RMS measurement. This would translate to 15,000(x1.41),
which would be over 21,000 volts! You should either upgrade your capacitors
for a higher voltage rating or opt for a lower voltage NST, like 9 kV. Of
course, a given increase in a capacitor bank's voltage rating, while
maintaining the same cpacitance, requires a squaring of the total capacitive
energy (E = 1/2CV*2). If you want to double the voltage rating of an MMC
bank, while retaining the same capacitance rating, you would need 4x as many
individual capacitor units!
Finally, I'm not sure whether the particular capacitors that you mention are
actually suitable for Tesla coil usage? If they are not designed for the
rapid cycling, significant RMS and peak currents and intense voltage
reversals that are the rule of spark gap Tesla coil firing, they will likely
not survive the rigors of a Tesla coil circuit, even if they have the proper
capacitance and adequate voltage rating. Others more knowledgable with the
part # nomenclautre than myself can probably offer more insight.
Hope this helps,
David
----- Original Message -----
From: "James Janota via Tesla" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Sunday, July 15, 2018 7:42 AM
Subject: [TCML] MMC reference check.
Ok, the old adage about measuring thrice make cutting nice comes to mind
here. I have 25 942c6p15k-f in series. These are rated 600v each. Now my
father in law who worked as an electrician for NASA and Inland Steel, said
that my configuration won’t be enough for a 15kv NST. With my calculator
it said 600x25=15000. But he said each one has to be rated for 15kv each.
I don’t want a $50. Firecracker. I also have 25 1M resistors, one for each
cap.
Please advise.
Regards
James Janota
(727)512-7112 Phone
(727)595-6625 Fax
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