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Re: Thyratron pair (was DC Drive)



---------- Forwarded message ----------
Date: Mon, 13 Oct 1997 09:43:12 +0300 (EET DST)
From: Harri Suomalainen <haba-at-cc.hut.fi>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Thyratron pair (was DC Drive)  

On Sun, 12 Oct 1997, Tesla List wrote:
> I should point out that this is essentially CW drive and that this 
> has all been done before. Duane Bylund published a CW fed mag 
> some time ago using a 1/2 bridge driver. The real trick would be to 
> get power disruptively into a single resonator. I don't think it can 
> be done without using at least one other coil but I could be wrong.
> I would regard a tap on the resonator as equivalent to using a second 
> coil.

You guessed right. There are at least 3 possibilities for cw-driving:
1) driving with a primary winding
2) so called end-feeding (ie. drive a transformer between gnd and coil
   bottom). My favourite.
3) direct end-feeding. Say you take square wave from a half-bridge fet
   system and take it directly to the bottom of the coil.

Yes, I have tried them all as most of you probably guessed already.
Some comments on ups and downs.
1) primary-secundary breakover is easy when you go for a very high
   coupling constant. You usually need to. Real problem.
2) step-up transformer is great. It provides proper impedance matching
   (ie. voltage step-up). However, to keep it reasonable in size for
   higher power levels you probably need to go for pretty high frequency
   range. Ferrites can still get fairly large. Transformer saturation
   problems will occur if the driver is not using a proper current
   limiting. This occurs especially during/after the spark. (No storage
   scope, no further details, sorry)
3) Impedance. The coil impedance tends to be fairly high. You usually
   need fairly high voltage to drive it. You'd have to design a specially
   low-impedance coil with enormous Q to be able to drive it with low
   voltage (say 400V half bridge). A suitable design for higher voltages
   would be reasonable.

Then there are other impedance matching possibilities. Haven't tried them
out yet. You could probably do something like in the radio transmitters
but you'd need high-voltage coils and high-voltage rf caps. Seems too
expencive.

Transformer line transformers could probably be used too. They just tend
to get huge in size for any coil reasonable for solid-state driving
(the low frequency range).

If you wondered about different wave-forms they can do a lot of good.
With my 1st solid state driver I had not would a suitable coild yet.
My driver was good for around 150-200kHz only and the coil was 450kHz.
Square wave happens to be rich in 3rd harmonic so I tuned the driver
at 150kHz and got output from the 450kHz coil (3rd harmonic).
Unfortunately the harmonic content for square wave is still too low
for any nice power levels. This is still one thing I definately will
look further into.

Probably a lot more choices are still out there to be tried out some day.
For real hot sparks solid state seems no good. A very high-power level
driver would cost quite a lot to build.
--
"99% of the pain in the world is due to misunderstanding" -M. Jarvelainen

Harri.Suomalainen-at-hut.fi - PGP key available by fingering haba-at-alpha.hut.fi