primary cap

From:  John H. Couture [SMTP:couturejh-at-worldnet.att-dot-net]
Sent:  Thursday, April 02, 1998 4:57 PM
To:  Tesla List
Subject:  Re: primary cap

  Mike -

  You did not give enough information regarding your coil so your question
cannot be answered in full. However, the change in toroids will amount to
only about 4 or 5 picofarads in the secondary circuit. It should be possible
to balance this extra secondary capacitance by adjusting the screws on the
new primary capacitor.

  In order to keep the TC system in tune the equation   LpCp = LsCs   must
be balanced. The Cs = Cl + Ct   where Cl is the sec coil self capacitance
and the Ct is the toroid capacitance. All you need to do is to ensure that
Cp will be the approximate size nescessary for the primary coil tap changes
(Lp). The 15% adjustment of the new capacitor should be capable of doing this.

  It should be noted that certain impedance matching combinations for neon
transformers may give you longer sparks but can overload the neon and damage
it. The step by step design procedure shown in the TCC Guide gives a safe
wattage for the neon transformer.

  If you want to see what happens when you make several changes at one time
you may want to try the JHCTES computer program. It is the only program that
keeps all 46 major TC parameters in tune when changes are made. I am sending
you a flyer via separate email.

  As for the .03 uf capacitor, I would recommend a maximum of .02 uf for the
900 watts of power you are using. If the primary capacitor is too large the
neon transformer will not be able to charge it to full voltage.

  If you have further questions, please let me know. Thank you for your
interest in my books.

  John Couture


At 12:23 AM 4/2/98 -0600, you wrote:
>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
>    Thanks in advance for any suggestions,
>Mike Hollingsworth