Re: SSTC idea

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux by way of Terry Fritz <teslalist-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

 >
 > >However, the more exciting direction to go is to replace the traditional
 > >spark gap with a solid state version.  This would be a definite challenge
 >
 > I always wonder about this. Would a solid-state gap replacement be any
more
 > efficient than an old-fashioned spark gap? I guess they use them in radar
 > transmitters for repeatability/jitter reasons but what's the point in a
TC?


Hard to beat a spark gap for high peak power switching...

The peak power you need to switch in a disruptive coil is very, very high
(far more than any reasonable tube size)..

Take some example numbers...  100 kHz resonant frequency, 20 kV voltage, 120
bps, 1200 Watts.
First, what's the joules/bang.. 1200Watts/120 bangs/sec = 10 Joule/bang, so
now we know that the capacitor is big enough to hold 10 Joules at 20 kV..
Using E = 1/2 * C * V^2 >>> 20 = C * 400, for V in kV and C in uF.. C =
20/400 = 0.05 uF.. (50 nF)
Now, figure out L to get the fres.. 628E3 = 1E6*sqrt(1/(L*0.05))  (for L&C
in uH and uF) 1/(.628^2)/.05 = L >> L=51 uH
Now figure out peak primary current (assuming energy in L = energy in C)..
10 = 1/2*L*I^2 >> 20/sqrt(51E-6) = I... 2.8 kA

so, you need a switch that can switch 3 kA -at- 20 kV.. pretty darn impressive
switch (it's not quite 60 MW peak power, for a variety of reasons, but
still.. pretty good)

(actual power >> 10 Joules/ 2.5 microseconds (quarter cycle)  around 4 MW)

Not a heck of a lot of tube or solid state switches that can handle that
kind of peak power. (yep, those mondo thyratrons they use at SLAC might be
up in that range..).. and especially not for the  $10-20 that a rudimentary
sparkgap (that CAN switch that kind of power) costs.

Herein lies the fundamental difference between CW (tube and SSTC) coils and
disruptive coils.  A huge tube coil (even a 100 kW one) doesn't have the
peak powers that a disruptive coil does. And that's why the spark
characteristics are so different.  The power levels when the spark is
growing are radically different.


By lowering the fres to something lower, you can get the peak currents down
(L gets big) for the same bang energy, but that effectively increases the
time over which the energy is transferred, reducing peak power to something
reasonable.. Go far enough, and the peak to average ratio starts to get
close to 1 and you have a CW coil.

Staccato operation of a tube coil is sort of in between.. you're going for a
high peak/average ratio.. given that the tube has a long thermal time
constant, and can probably support a much higher peak power than average.
The same idea lets you build 1 MW peak power radar magnetrons that aren't
much bigger than the 600W CW unit in your microwave oven.
 >
 > >I believe though, that a lot of the OLTC (Off-line Tesla Coils) use a
 > >similar solid state switching arrangement however it is at low voltage
 > >(<1kV).
 >
 > That's how mine works at any rate. It's just a DC resonant coil with a
pair
 > of IGBTs dropped in pace of the spark gap.
 >
 > Steve C.
 >