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TC as pulse transformer


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From:  Jim Lux [SMTP:jimlux-at-earthlink-dot-net]
Sent:  Wednesday, June 03, 1998 2:36 PM
To:  tesla-at-pupman-dot-com
Subject:  TC as pulse transformer

here are some ideas that have occurred to me while reading over Terry's
most recent post.

At some point, particularly as the Ctop gets big, can't the TC be
considered as a air core pulse transformer.

You charge up Cpri, then discharge it into the primary of a transformer
(which happens to have inductance Lpri). The flux of the primary is
linked to that of the secondary, so the current in Lpri induces a
current in Lsec. That current charges Csec (=Cself + Ctop), which then
breaks down the air dissipating the energy.

 Assume for a moment that your spark gap can act as an ideal switch. You
close the switch, and at the perfect moment (i.e. when the capacitor
voltage is zero, and the inductor current is maximized) you open the
switch. All that energy has to go somewhere, and the somewhere is the
secondary. Isec = Ipri * sqrt(Lpri/Lsec). (It is, of course, quite a
trick to open a sparkgap switch when the current is at a maximum).

Now that the energy is in the secondary, where does it go? The current
in Lsec starts to charge the Ctop (and Cself), causing its voltage to
rise. If nothing else happens, Lsec and Csec ring, and the power is
dissipated in Rsec (mostly the winding resistance, because the
dielectric loss of air is quite low). Another loss source is the
effective resistance from the top elecrode to ground created by corona.

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).

For a real spark gap switch, it is impossible to turn off the switch
while the primary current is still flowing (although a rotary gap can
approximate this). So, what you do is put some sort of load on the
secondary of the transformer that can take the energy out of the primary
and do useful work before the energy has a chance to flow back through
the switch and into the capacitor.  That load is a spark.

Or, if the resonant frequency of the secondary is different from that of
the primary, you can get a lower frequency "beat" as the energy goes
back and forth between primary and secondary (while the switch is
closed), and hopefully, while the energy is in the secondary, and the
current is low in the primary, you can "quench" the spark, opening the
switch.

Returning to making big sparks. We know from laboratory research that
making a big spark requires a slow rise time voltage pulse (many, many
microseconds, if not milliseconds). We also know that we want to get
energy from our storage reservoir (the primary cap) into the
transformer, and then open the switch. This is starting to look like the
desired switch is something that is unidirectional, reasonably fast and
low loss. Perhaps a thyratron or an SCR?

We want a slow rise time (necessary for developing a big spark), which
means low resonant frequency in the secondary, and high inductances for
both the primary and secondary.

This is what Terry and Dave have arrived at, although by another route.
They advocate high inductances, small C, and high primary voltages (to
get the energy up).