# Re: Secondary capacitance

```Original poster: "Dave Larkin by way of Terry Fritz <twftesla-at-qwest-dot-net>" <teslaman15-at-hotmail-dot-com>

Hi Frederick

It is also possible to roughly measure the Vout of a TC,  without any fancy
test gear etc.  Coilers of yesteryear attempted to use output spark length
as a crude guage of voltage.  As we now know stepped leader effects and
wildly non-uniform fields made these guestimates accurate to within an order
of magnitude (or maybe worse!).

However there is a method of determining output voltage from spark length
which yields a far more accurate results.  The coil must be operated in
'single shot' mode (ie. rather than being repetatively pulse by a fast
firing spark gap, the primary cap is dscharged once.)

A small DC supply is used to charge the primary cap, until the spark gap
breaks down.  The output spark length under these conditions _is_ (almost) a
direct relation of voltage.  So if a ground terminal with a large ROC (to
try and make the field a bit more uniform) is used and the single shot spark
length measured, one can determine the approximate output voltage, using the
fact that air breaks down at ~1MV/meter for large gaps.

-Dave-

>From: "Tesla list" <tesla-at-pupman-dot-com>
>To: tesla-at-pupman-dot-com
>Subject: Re: Secondary capacitance
>Date: Thu, 23 May 2002 00:41:41 -0600
>
>Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>
>
>Hi Frederick,
>
>Vs = 0.7(Vp * SQRT(Cp/Cs))
>
>will be pretty darn close!  About 1/2 the coil's power is lost in the gap
>which this takes into account.  For a simple equation, this is as good as
>it gets.  You may be interested in:
>
>http://hot-streamer-dot-com/TeslaCoils/MyPapers/modact/modact.html
>
>For secondary capacitance, see this program and included notes and text
>files...
>
>http://hot-streamer-dot-com/TeslaCoils/Programs/E-Tesla6.zip
>
>Cheers,
>
>	Terry
>
>
>At 12:16 AM 5/22/2002 +0200, you wrote:
> >
> >Hello all,
> >
> >I'm trying to calculate the peak secondary voltage using this formula:
> >
> >Vs = Vp * SQRT(Cp/Cs)
> >
> >Vs = peak secondary voltage
> >Vp = peak charge voltage of tank cap
> >Cs = secondary capacitance (Farads)
> >Cp = tank capacitance (Farads)
> >
> >How do I find out the secondary capacitance (Ctot = Csec+Cterm)?
> >Or more exact, how do I calculate Csec (sec. coil capacitance)?
> >Using Bert Pool's formula for toroid capacitance, I calculated a Cterm
> >value of 2,503 pF.
> >Is the capacitance of the secondary coil significant?
> >
> >Using the first mentioned formula, my 8kV input and my 0,01 uF tank cap
> >gives about 715 kV peak secondary voltage which (even under ideal
> >conditions) seems a little high for my small coil.
> >
> >I wouldn't know what to "expect" from my coil. Here are the specs:
> >
> >Dual 4kV parallel NSTs phased for 8kV
> >0.01 uF MMC
> >Single static gap
> >Flat primary tapped at about 5½ turns
> >1200 turns secondary, 0.3 m high
> >6.5" by 1.8" toroid
> >
> >Sincerely,
> >Fredrik Holmström
> >
>
>

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