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primary cap


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From:  Hollmike [SMTP:Hollmike-at-aol-dot-com]
Sent:  Wednesday, April 01, 1998 3:00 PM
To:  tesla-at-pupman-dot-com
Subject:  primary cap

Hi all,
      I have a question for D.C. Cox, John Couture and any one else who cares
to reply.  When I first started building coils(about four years ago),  I
purchased two books from the ITS museum in Colorado Springs.  The first was
JHC's "Tesla Coil Construction Guide" and the second was DCC's "Modern
Resonance Transformer Design Theory".  Both of these are great reference
sources IMHO.  I am glad to find out that both of you are in this discussion
group.  I now have a question that should be easy for each of you to answer.
     I am planning to make a modification to my primary cap.  Presently, I am
using two 15kV, 30 ma neons to power my coil.  I believe I have constructed my
primary gap to be adjustable to about any voltage rating up to about 30kV.
Presently, I can get about 36 inch streamers from my setup using a 0.0115 uF
primary cap, which is great considering I was only expecting about 18 - 20
inches when I designed it.  I recently constructed a plate cap, that if it
works as expected, will give me a great deal of latitude in modifying my
system (I can adjust the capacitance by about 15% by adjusting the torque on
the screws holding it all together).  I can put this new cap in parallel with
the existing cap or use it in place of that cap, and will decide this based on
feedback from you.  
     I have been studying the various design programs on the web and they all
seem to adhere to the impedance matching of the primary cap.  My present cap
is a fairly close match due to the fact that I purchased a number of HV caps
from a local surplus store and bought all of the ones they had.
     In DCC's book, it states that due to the short duty cycle the primary cap
can be much larger than the impedance match dictates.  In JHC's manual, you
pick the cap size and then size the tranny to be able to power it.  I have
obtained various values for the cap size by working the equation from JHC's
book in reverse and assuming different voltages(RMS, peak, 2X RMS, etc).  So,
my first question is which of these values is the one to use?  My best guess
is the result that is most conservative(or lowest cap value).  
     My real question is what would you use for the primary cap in my system
that would provide the best output and yet be safe for the neons.  I have RF
chokes and safety gaps in place, so there is no need to remind me of using
these. As is, I can run my coil for thirty minutes without problems so my
system must be reasonably well built.  I have never seen the safety gap fire,
so things must be ok as far as I know.  
     I have tentatively concluded that I could use up to 0.03uF for my primary
cap, but wanted to first get advice from those more knowledgable than myself
before jumping in.  My main reason for wanting to increase the primary cap is
that I just replaced the 4 X 16 toroid with a 6 X 20.  I found that to retune,
I will have to splice about 1/4 of a turn of tubing to my primary.  This
wouldn't present a problem, but I would rather increase the capacitance and
reduce the number of turns.  I just want to get some advice before I decide
this.
    Thanks in advance for any suggestions,
Mike Hollingsworth