Re: How could a pulse cap operate in TC?

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Steve Conner <steve@xxxxxxxxxxxx>



> it`s useless to write rms rating without its frequency!

Not really. 10A RMS will heat a capacitor just the same whether it's at 1 
kHz or 500 kHz. I ignore skin effect (because it's not significant in thin 
capacitor foils at Tesla coil frequencies) and dielectric losses (because 
they are a function of voltage squared, not current squared)

Of course the _voltage_ needed to force 10A RMS through the capacitor (and 
the resulting dielectric losses) will change radically depending on the 
frequency. But as the designer, that is your problem, not the capacitor 
maker's.

Often the RMS current rating of a plastic film capacitor is not specified. 
Instead the maker gives a "voltage derating vs. frequency" curve and you 
have to read between the lines a little.

Typically the curve is flat up to a certain frequency and then falls off at 
6dB/oct (for every doubling in frequency the maximum voltage halves) This 
implies that dielectric losses are insignificant and the limiting factor is 
the RMS current, and you can calculate it:

Irms=V/Xc = V*2*pi*f*C

where f and V are co-ordinates of any point on the sloping part of the 
curve. You may need to insert a sqrt(2) depending on whether the maker gave 
RMS or peak voltages.


The peak current in a Tesla coil tank circuit is Ipk=Vpk/(sqrt(Lp/Cp))

and the RMS current is roughly Irms=0.5*Ipk*sqrt(ringdown time/time between 
bangs)

Steve Conner