# Re: Voltage/Length (fwd)

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---------- Forwarded message ----------
Date: Sat, 17 Jan 1998 13:02:43 -0800
From: Bert Hickman <bert.hickman-at-aquila-dot-com>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Voltage/Length (fwd)

Tesla List wrote:
>
> ---------- Forwarded message ----------
> Date: Sat, 17 Jan 1998 11:05:59 -0600
> From: Jeff Larson <jflarson-at-starnetinc-dot-com>
> To: Tesla List <tesla-at-pupman-dot-com>
> Subject: Voltage/Length
>
> Does any one have the voltage per length approximation formula?  This
> would be used to approximate the voltage of a coil output based on the
> measured spark length.  I imagine the spark length would be dependent on
> the humidity regardless of the voltage?
>
> --
>         Jeff Larson             jflarson-at-starnetinc-dot-com
>         Design Engineer         Home phone 847-934-1877
>         Propheteer INT          Work phone 847-359-8988
>         Palatine IL

Jeff,

Unfortunately, there's no general rule for sparklength versus voltage,
especially for Tesla Coils, since it depends on lots of factors in
addition to humidity. If you can set up the coil to do a "single shot",
i.e., fire only once every second or so, and measure the distance that
the spark jumps to a grounded ball, you can use a very rough
approximation of about 20kV/inch. However, trying to estimate output
voltage on a running coil at fast breakrates by measuring sparklength is
an excersize in futility. A coil that produced "single shot" sparks to a
grounded ball of 20" can easily hit 60-70" at a 400 BPS breakrate.

Another way to estimate maximum voltage is by measuring the primary gap
breakdown voltage (scope and HV probe or AC voltage divider
[carefully!]), and calculating the "effective" secondary + toroid
capacitance by measuring the resonator operating frequency and
inductance and calculating backwards. Assuming that the system is in
tune, the maximum output voltage your coil will deliver is a linear
function of your gap voltage and the varies as the square root of the
ratio of primary and secondary capacitance:

Vout (max) = Vgap* sqrt(Cp/Cs)
where Cp = primary tank capacitance
Cs = the combined secondary and toroid capacitance

A similar calculation can be made using inductances:
Vout (max) = Vgap*(Ls/Lp)
where Ls = resonator (secondary) inductance
Lp = Primary inductance

In reality, various losses conspire to limit you to 70-85% of this value
in a disruptive Tesla Coil.

Safe coilin' to you, Jeff!

-- Bert --

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