Re: Cap Location (fwd)

---------- Forwarded message ----------
Date: Mon, 12 Jan 1998 19:19:22 -0800
From: "Antonio Carlos M. de Queiroz" <acmq-at-compuland-dot-com.br>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Cap Location (fwd)

John H. Couture wrote:

>   Please refer to my reply to Greg Leyh.

Look at these considerations:
Consider the two alternatives, with parasitic capacitances (Cp?) across
the primary winding (Cp1) and across the power transformer output (Cp2):

oo------------C1------o      oo------------gap-----o 
oo   |    |        |  o      oo   |    |        |  o
oo  gap  Cp2      Cp1 o      oo   C1  Cp2      Cp1 o
oo   |    |        |  o      oo   |    |        |  o
oo--------------------o      oo--------------------o

In the first case (left), if, for some reason, the gap opens with significant
current, there is a resonant circuit formed that includes Cp2. As this
capacitance is much smaller than C1, even a relatively low current left
in the primary can cause the development of considerable voltage over
Cp2, across the power transformer output, and varying rapidly, what may
be a cause of problems. In the second case, as soon as the gap opens, 
with or without current, the power transformer is isolated from the 
primary circuit (unless there is too much capacitance across the gap,
unlikely). But in this case, the full main oscillation appears across
the power transformer when the gap is closed. In the second case, Cp1
can resonate with the primary inductance after the opening of the gap,
but at a very high frequency, where the energy can be rapidly dissipated
by irradiation and skin effect losses. In the first case, as Cp2 is much
larger than Cp1, this parasitic oscillation may last longer. Maybe was
about this that Tesla wrote.
Conclusion: In both cases, a good filter between the power transformer
and the primary circuit, with a protective spark gap, is important.

> The capacitor alone is a good protection against overvoltage. It can't charge
> >to more voltage than it had at the start of the cycle.
>   But this wold not be true when the gap opens when voltage transients are
> formed?

There is no energy in the circuit to charge the capacitor with more than the
original voltage. If the primary capacitor is across the power transformer
output, the worst that the power transformer has to support is the main
normal oscillation of the primary circuit.

Antonio Carlos M. de Queiroz