# Re: RMS AC cap rating

```Tesla List wrote:
>
> >From DELCOKEVIN-at-aol-dot-comWed Jul 17 22:21:44 1996
> Date: Wed, 17 Jul 1996 15:21:15 -0400
> From: DELCOKEVIN-at-aol-dot-com
> To: tesla-at-pupman-dot-com
> Subject: RMS AC cap rating
>
> This may seem elementary to some, but I figure that it does not hurt to ask
> when you don't know.
>
> How does one figure how much RMS RF current and peak current your primary cap
> should be able to handle?
>
> Kevin M. Conkey

Kevin,

It depends on how big a coil you plan to run, obviously.  But be aware
that if you move up in power later on, the cap you ordered may be no good
for the new values of current.  Always over specify!!  The RMS current in
a Tesla coil will always be very low compared to the peak current due to
duty cycle of the gap.

The calculation of these values is difficult due to a number of unknowns
and unknowables on a theoretical, un-built system.  A crude guess can be
made by figuring the peak current using the Surge impedance formula
Zsurge= sqrt L/C then use the simple ohms law Ipeak= Epeak/Zsurge.

Example: .03ufd C and 70uh primary L  Z surge=~50 ohms
For 20KV rms input, 28.3KV peak this give Ipk of about 500
amps or for about 350 amps RMS, each pulse.

This example assumes zero loss in the gap and rest of the circuit (never
happens).  So the current should be a good bit little less.

Based on a 20% duty cycle, (No coiler will ever hit this in a disruptive
system), that makes a continous RMS current of 70 amps for the above
circuit.

All the foregoing worst case ideal theoretical conditions considered, you
could spec that the peak current would be 600 amps and the RMS current 70
amps and be OK.

I usually go way overboard and demand 75 amps RMS and 2000 Amps peak in
my Tesla capacitors.  I really rough 'em up when I use 'em!

Impulse caps for water arc explosion or exploding wire work are another
matter.  The RMS current is always spec'd as (0)-zero and the Ipeak is
usually 50,000 amps with 200,000 amp test!

Richard Hull, TCBOR
```