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Re: Solid State Spark Gaps

Tesla List wrote:
> 
> >From leyh-at-ix-dot-netcom-dot-comThu Sep 19 22:28:41 1996
> Date: Tue, 19 Sep 1995 00:12:24 +0000
> From: "G.E. Leyh" <leyh-at-ix-dot-netcom-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Solid State Spark Gaps
> 
> On Fri, 13 Sep 1996, Tesla List wrote:
> 
> > > I gotta' chime here, too.  The mosfet would be the ideal switch if it
> > > could just handle 22,000 volts and 600 amps of current at the same time.
> <snip>
> > > When all the smoke clears and theoretical stuff is pushed asside,
> > > when the wishes, hopes, and dreams for the future are
> > > relegated to that future,  FETs, IGBTs and all the other stuff of this
> > > century is pretty pathetic when it comes to big power at high voltages.
> 
> > Well.. how abuot using many (fast) thyristors or mosfets in series?
> > Series using *is* possible with very carefull design. Anyone given that
> > a greater thought?
> > Thyristors could easily handle high peak currents. For fast switching
> > they are still not as good as mosfets. Then, of couse there probably
> > is quite few high current high voltage (say 1-2kV) capable GTOs
> > around somewhere..
> 
> Actually, in recent years solid state devices have improved dramatically in
> their capacity for switching large peak currents, at high voltages.
> The Pacific Intertie, which IMHO should be considered one of the seven wonders
> of the world, delivers over 1,500,000,000 continuous watts from the Washington
> State Hydroelectric Power Authority to the LA basin nonstop via a 1 megavolt DC
> transmission line.  How is it that the receiving station located just north of
> LA can convert this immense amount of HVDC power back into 60Hz 3 phase and
> apply it to the local grid?  Solid State Switches!!
> 
> The primary reason that high peak capacity devices such as SCRs and GTOs
> have not been applied to fast pulsed power applications (such as Tesla Coils)
> is due to their di/dt limitation, which for a good inverter grade GTO is
> about 500 amps/uS.  A typical TC has primary current risetimes on the order
> of 1000 to 2000 amps/uS.
> 
> The 500 A/uS limitation is due to the finite rate of growth of the conduction
> area away from the gate structure, when the GTO is fired. However, A new
> gate design called the 'interdigitated gate structure' promises to raise this
> di/dt limit significantly.  At work we are testing such a device, which is
> called the Modified GTO.  At present, the device will hold off 4200V, and
> switch pulsed currents of over 50,000 amps peak, at risetimes of 15,000 A/uS!!
> 
> People at work imagine replacing the 6000 amp thyratrons along the accelerator
> with MGTOs, but I already have dreams of sticking one into my coil.  My present
> rotary gap drops about 1200V at 2000A, and takes over 200uS to quench.  The
> MGTO at work (stacked appropriately) would drop about 10V, and 'quench' in 20uS.
> 
> Perhaps, without all the rotating inertia of a rotary gap, I'll finally be able
> to play 'Claire de Lune' on my coil.
> 
> -GL


Greg,

Its good to know that technology will catch up.  For the average coiler 
with the average pocketbook, series systems of thyristors, FETS, IGBTs 
are out of sight in terms of dollars and when they go, they all go.  
Our own Dave Sharpe works with these systems a lot in power controls (I 
got out of industrial electronics in the 70's).  He constatnly talks of 
this and that new silcon device handling kiloamps, etc., etc.  I always 
burst his bubble which it comes to the di/dt question and voltage.  The 
IGS gating sounds nice.  How is this device on flyback voltages from a 
ringing primary in the average coilers poorly designed system?  (Get out 
the checkbook guys!)

The new gating technology will ultimately allow some pair or more of 
these plus-ultra devices to switch with the proper characteristics,  but 
again, the single device cost will be incredible!  Certainly, for the 
next 10-20 years, little is on the horizon which will fall in the 
"Digi-key" phone order classification, where a fist full of these devices 
can be ordered by those who wish to dink around with them for coil use.

The hydrogen thyratron seems ideal if it weren't for the need for having 
a mechanical termination of the conduction current when used in a simple 
Tesla coil system. (no reversals allowed)

Thanks again for the update!

Richard Hull, TCBOR