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RE: Terry's DRSSTC actually hooked to a coil now >:-))

Original poster: Terry Fritz <teslalist@xxxxxxxxxxxxxxxxxxxxxxx>

Hi Steve,

At 03:52 AM 2/2/2005, you wrote:
>I guess I am not understanding what the "two paths" are here.

OK. The dual resonator is a fourth order system so it has two totally
separate "modes". You said your coils are both tuned to 85.1kHz. So when
they are coupled, the two modes might sit at say 75 and 95kHz. (the
equations are f1=f0/(1-k^2) and f2=f0/(1+k^2) iirc) They are also called
poles or split frequencies.

These are the two possible resonances the coupled system can have. They are
"orthogonal" which means they work separately. One way to visualise it is
something like a guitar with two strings. You can pluck either string on its
own or both at once.

A discharge in a disruptive coil "plucks" both "strings" with about the same
force and the result is the beat waveform we all know.

However a burst from a DRSSTC driver interacts with the two modes in a more
complex way, depending on the length of the burst. With a self resonant
driver it gets even more complex. Keeping with the guitar analogy, it is
something like plugging your two stringed guitar into an amp and cranking it
up till it bursts into feedback.

Cool!! I ran some models and the upper pole seems dominate right now.

You need some experience of stuff like Fourier transforms and impulse
responses to be able to think about this intuitively. One very relevant
result from Fourier analysis is that a short burst of a sine wave contains a
whole band of frequencies, the shorter the burst, the wider the band.

But generally- If the primary and secondary are tuned exactly to each other
then the self resonant driver will output both frequencies at once and
excite both modes equally. If there is any difference in the tuning, one
mode will be excited more than the other and will grow while the other dies

So one "path" ends up with the lower mode fully excited and the upper one
zero. The other path has the upper mode excited and the lower one zero.

>Yes. Is that possibly bad?

Don't know. With your circuit you will have near zero current switching no
matter what happens so it's probably not a major problem.
>My driver has no predetermined
>frequencies so the drive frequency is totally up to what every the coil
>system wants whenever and however it wants it.

The only problem is if it asks for something stupidly high. With primary
feedback there's nothing stopping it jumping to a harmonic of the resonator
when ground arcs happen, so it could go to 3x or 5x the frequency you
started with. The result is racing sparks and maybe burnt IGBTs. That is why
I chose to use the PLL, so I could put a limit on the frequency range. But
it seems to bring a lot of drawbacks too :(

I should put the buss back on batteries and do a big frequency sweep to see if there are any hidden feedback paths or resonances. I think the logic will be pretty good at rejecting say a 3MHz feedback "squeal", but if there are any bad spots best to find them in a controlled way ;-))

>I "think" it is riding both poles and the resulting primary current
>regardless of frequency or poles and zeros.

I agree. But that is only because you tuned the two circuits carefully to
the same frequency. Once streamer loading comes in I expect it will tip onto
one or other of the poles, probably the upper one. This may or may not
affect the operation much. This is why I recommend mistuning so you can
force it to the pole you want.

It will probably be tuned with the primary a little lower but streamer loads will probably reverse that. I wonder if streamer loading is the real reason the lower pole seems popular? We always tune regular coils lower too for streamer loading reasons. My drive already can do some fast switches if I hit the start up just right with say the current too low to trigger the thing fully. But it does not seem to hurt anything at all.

>If you have like a MicroSim model that would help me to understand I would
>love to study it.

I have done a few Microsim models of DRSSTCs, they are all here.

I also did 10 transient analyses of a self-resonant coil with primary
feedback and a Terry-like streamer load model. I FFT'd the output, to show
how the frequency components of the primary current change as the primary
tuning is adjusted:

Apologies for not having a proper screen dump.

Cool!! I will check these out. I am seeing it in my models too now that I know what to look for! I am also trying to get the FFT figured out again to see the resonances Antonio mentioned. This sort of "chaotic" mode switching should occur in regular coils too but things might be too messy to have ever seen it. In that case, we have to wonder if it helps or hurts the streamers ;-)

The DRRSTC seems to match the models very well!! I think they will help us refine the streamer load models greatly!! My simple 220k+1pF/foot thing has been around way too long ;-))

Thanks a bunch!!



Steve C.