60 vs. 30 ma
From: Edward V. Phillips[SMTP:ed-at-alumni.caltech.edu]
Sent: Wednesday, June 18, 1997 1:10 PM
Subject: Re: 60 vs. 30 ma
It would seem to me that if one switches to a transformer with
twice the current rating, using the same capacitor, that the
capacitor would simply charge up to the spark gap voltage twice
Depends on how close you are to series resonance with the n ew
transformer. Should charge up at least twice as fast.
"Modeling the transformer as a voltage source Vt and series R.
In a series V-R-C circuit, solving for the time to charge the cap
to a certain voltage Vc, t = -RC ln((Vt-Vc)/Vt). Doubling the
transformer current rating would halve the R value. "
Not a good model. Use a voltage source in series with an inductor
having a reactance (at the power line frequency) about equal to
the secondary voltage divided by the secondary current. This
gives you a very large resonant voltage rise when the capacitor
is "matched", limited by the resistance of the winding,
saturation of the core, or most likely insulation failure in
the transformer. The transformers I have measured here have
resistance such that the Q would be over 10, giving at least
150 kV at resonance for a 15 kV transformer. Obviously, something
will give before that. This all points out the importance of
using a safety gap across the transformer, set to break down
at close to the normal open-circuit voltage.