cap specifications

To: tesla list <tesla-at-pupman-dot-com>
Subject: cap specifications
From: bigr-at-teleport-dot-com
Date: Tue, 18 Jun 1996 00:34:02 -0700
I am curious about the capacitor specifications that people are using 
for resonating Tesla coil primaries.  I have seen several posts where 
it appears that a cap working voltage equal to the transformer 
secondary voltage is used.  Normally, caps (except those to be used 
across the AC mains, i.e. UL and VDE listed) are rated for max DC 
voltage. Thus if you had a 15kvac transformer- the equivalent DC rated 
cap would be 14kvac x 2 x 1.414 volts peak-to-peak. 

However, these caps are not just connected to a transformer, they are 
connected to a source of extreme stored energy- namely, the stored 
energy in the magnetic field of the secondary reflected back thru to 
the primary and then subtracting lost energy in the gap.  Who knows 
what the real p-p voltage might be on these caps?  Moreover, if the 
gap is not working perfectly, whatever that quality may be, this 
voltage is probably not anywhere near being constant.

Thus the cap voltage may very well be many times the transformer rms 
voltage depending upon conditions.  If you are skeptical, try this 
experiment:  take a 12v relay and hold the bare wires in your hand as 
you contact a 12vdc source.  You may get quite a jolt.  That because 
when the DC is suddenly removed, the magnetic field in the coil 
collapses producing hundreds of volts (and damped oscillations due to 
the self capacitance of the solenoid).  Thus you can easily get a 10x 
or more increase in voltage by switching a tuned circuit across a 
voltage source.  Something similar may happen when the gap fires in 
the Tesla coil.

I think that the idea that cap working voltage is somehow rigidly tied 
to supply voltage has somehow migrated from the field of power 
amplifiers where using tubes (grid current flowing) or transistors (CB 
junction reverse biased) the output voltage is clamped to near ground 
on the negative going peaks, thus limiting output voltage to some 
multiple of the supply voltage.  There are no clamps in a tesla coil- 
only the maximum transfer of energy to the discharge results in a 
minimum of wasted energy dissipated in caps, etc.

What do others think?  Best Regards, Rob.  

ps: Was my Tesla coil bibliography received in the list?  I either 
screwed up in sending it or accidentally erased my own copy when it 
arrived.  there were some interesting technical articles from the 
professional scientific journal on Tesla coils, along with the 
original article by Duane Bylund in Radio Electronics that inspired my 
mosfet design.  

ps2:  I also see that I reinvented the wheel when it came to idea of 
using an E| core choke with variable gap as ballast.  Well, at least I 
am probably the first to come up with the idea of using one as a bulk 
magnetic tape eraser.  :) :)