TC as pulse transformer
From: Jim Lux [SMTP:jimlux-at-earthlink-dot-net]
Sent: Friday, June 05, 1998 2:20 PM
To: Tesla List
Subject: Re: TC as pulse transformer
Tesla List wrote:
> From: Greg Leyh [SMTP:lod-at-pacbell-dot-net]
> Sent: Thursday, June 04, 1998 5:58 AM
> To: Tesla List
> Subject: Re: TC as pulse transformer
> Jim Lux wrote:
> > And, finally, the energy in the secondary could be dissipated in
> > creating a spark, which we would consider desirable. The trick, then,
> > is to set up the secondary LCR circuit such that it and the spark
> > together are critically damped, because this will provide the maximum
> > energy transfer into the the loss (i.e. the spark).
> Indeed, although relying on a non-linear resistance to critically
> damp an LC ckt might be tricky.
They do it for high power xenon strobes, though, by carefully adjusting
the series L and R. EG&G's sales literature for their linear flashtubes
that talks about this in some detail working from a relationship
developed by Goncz, which models the tube as a nonlinear resistor where
the voltage is proportional to the square root of the current. And, of
course, Edgerton's book talks about it, as well.
> An active switch will be the ultimate way to get the most out
> of a TC -- I've noticed that on Electrum if I can get the gap
> to quench on the first beat, the arcs are about 25% longer and
> capable of gnd strikes.
> It would be nice to avoid the hazards of high speed armatures
> as well...The Electrum rotors store 211kJ of rotational energy
> at full speed, enough to lift a 150lb adult 1000ft in altitude.
> Thyratrons can handle the kV and the di/dt, but fall far short
> on the Iavg rating (typ a few amps -- a TC carries 15-20A avg).
Unless you go to a higher primary voltage, which is desirable for other
reasons! Clearly, if you are running tens of amps in the primary, you
aren't powering it with a NST. Why not go to 50 kV or so. And, if you
can shut the switch off before the voltage reverses on the capacitor,
the capacitor stress will be greatly reduced.
What about a crowbar across the primary L. One switch dumps the current
into the L, then a second one shorts the coil, providing a path for the
current to flow through, instead of back into the capacitor. This
doesn't buy you much for keeping the energy in the secondary, but at
least it is easier on the caps.
> The plasma quench times are tens of uSecs typ, and even longer
> for the higher current deuterium tubes.
> Optimised GTO's can take the di/dt and Iavg, but only come with
> Vdrm (off-state) ratings up to 4500V. Stacking is therefore
> required, with isolated gate drive circuits. Just the gate drive
> alone requires 200A per device, with 1uS risetimes. Dv/dt ratings
> of 200V/uS also limit the practical quenching times.
Much like the power transmission peole use for HVDC inverters. They use
GTO's with optically isolated triggers, etc.
> Ironically, active switching for disruptive TC's is likely to
> be practical only on larger scale coils, due to the quenching
> limitations of the switch.