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# Re: Mica Cap Question

```Original poster: "Dr. Duncan Cadd by way of Terry Fritz <twftesla-at-uswest-dot-net>" <dunckx-at-freeuk-dot-com>

Hi Ray!

>I have obtained a .01uF 8KV mica transmitting cap that is rated for
26 Amps
>at 400KHz. Would make a good cap for a smaller MOT power coil?

It certainly would, at least for short runs.  The dc rating of your
cap implies an rf voltage rating of just a shade over 1kV at the test
frequency:

At 400kc/s, a 0,01uF cap (= 1.10^-8 F) has reactance
Xc = 1 / 2.pi.400000.1.10^-8
Xc = 40 ohms approx.
40 ohms x 26 amps ~ = 1kV rms for continuous duty.

Alternatively, using the late Philip Kemp's way of reckoning for a
spark transmitter circuit:

An MOT is likely to be say 2kV -at- 1kVA, let's say 4kV with voltage
doubler and with a 0,01uF cap this will give a bang size of a mere
80mJ (0,5 CV^2) - hence the break rate will be near 12500bps ( = 1000
/ 0,08 )

If the break rate is 12500bps, the circuit resistance is 4 ohms (Terry
has guessed this is close) and we assume the voltage is 4kV then
I = E SQRT (n C / 2R) = 4000 SQRT (12500.1.10^-8 / 2.4)
I = 4000 SQRT (1,56.10^-5) = 15,8A which is within ratings.

[ in the above, I is current, E applied voltage, n the breaks per
second, C capacitance, R resistance ]

Note that two MOTs would put that current well over the rated value.

I think your cap will do fine for short runs, I'd be a little worried
if you wanted to run two MOTs for extended periods.

Dunckx
Geek#1113 (G-1)

```