A double resonance solid state Tesla coil

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

Original poster: "jimmy hynes by way of Terry Fritz <twftesla-at-qwest-dot-net>" <chunkyboy86-at-yahoo-dot-com>







None of the solid state tesla coils I've seen use a resonant primary.  One 
of the downsides of the SSTC is the large magnetizing current, especially 
when you try to get a fast voltage rise. This leads to high losses in the 
power switches and makes it impractical to generate 'normal' rise times and 
streamers.

My approach is to add a primary capacitor. Pspice simulation shows that the 
energy transfer is much more efficient. A parallel  H bridge of IGBTs drive 
at resonance (about 70 kHz) by monitoring the zero crossings of 
instantaneous current with a microcontroller. The current envelope is also 
tracked to detect the peak of the secondary voltage (minimum of the input 
current). At this point, the switches are held closed emulate a spark gap 
before it quenches. This prevents the diodes from sucking power back from 
the secondary. After one or two beats, the switches are left open as the 
secondary decays.

I think this approach combines the advantages of a conventional spark gap 
coil and a SSTC.

Compared to a 19th century conventional coil
1) We don't need a bulky primary transformer
2) We don't have the maintenance and loss of a spark gap (which can absorb 
more power than the streamers)
3) The primary capacitors only have to store about 1/3 as much energy 
because it is continuously being transfered to the secondary
4) The IGBT's can be driven at very high break rates, you just need a 
healthy circuit breaker in your garage
5) It can't be left on in a quiescent state. If there is high voltage 
present, so will there be sparks
(we left our NST plugged in one day and noticed that the wax it was potted 
in had melted!)


Compared to a SSTC
1) The fast rise allows real streamers instead of 'brush discharge'
2) There is no magnetizing current

Compared to an OLTC
1) The current is always in phase with the voltage. In other coils much of 
the current is at a low power factor.
2) Because we get voltage rise from both resonances, we aren't driven to 
extreme winding ratios.

Downsides of the Double Resonance SSTC (DRSSTC)
1) The primary capacitors may be limited by current, rather than voltage
2) As with a spark gap coil, we have to tune the primary to the secondary

This seems like an obvious solution, so tell me if I'm missing something.
I'm assembling it now, I can supply more details (pictures, schematics, 
simulations) for the curious.


Jimmy
(with a lower case i now:-)



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