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S.s.t.c. feedback reconsidered?

Original poster: "K. C. Herrick" <kchdlh@xxxxxxx>

I return to coiling after 6 months or so of inattention. For those who recall my internal bucket-wound primary, I note that some of you were right, I think, in doubting it was a good idea. I've corrected a couple of design errors & mis-wirings in the electronics & that all seems to work OK now. But in firing the system up, gradually and with the secondary in place, I heard the beginnings of a hollow-sounding "zap" and immediately concluded that an arc (an incipient arc, hopefully!) to the secondary was taking place. So...so-much for the internal primary, I think. It's back to square 1 on the physical design.

But I've never really left square 1 on the whole subject of a) tuned vs. untuned primary?; b) feedback from primary or secondary?; c) If a tuned primary and feedback from the primary, then to what frequency should it be tuned with respect to the secondary? (not to mention >by what means< should it be tuned: another subject); d) if a tuned primary, then excite at that frequency or another one?; and finally e) low k or high k?

I remain uninformed even after some study of numerous past List and other postings, viz. "DRSSTC design procedure - draft", "DRSSTC Tuning and Split Frequencies of Dual coupling system", richieburnett.co.uk/operatn2.htm#splitting, and 4HV.org's "Tuning and Frequency Splitting within a DRSSTC (or two coil) system".

A.  Tuned vs untuned primary?

In my one & only prior sstc, the primary was untuned (was it, then, a SRSSTC?), with feedback taken from the secondary's return-current. That worked well though sparks were rather short; and it quit working permanently after I "improved" it once too often.

Is a good reason for using a tuned primary that its current can thereby become increased due to the series resonance? When untuned, its inductance goes up as the square of the turns and so the magnetic flux that it generates rapidly diminishes, as the square of the turns. Resonance cancels out that inductance, so to speak. If that is so, then untuned is not the way to go (and E. below may be ignored).

B.  Whence feedback?

If feedback is from the primary, then the operating frequency, ipso facto, must be that at which the primary circuit resonates. If it is from the secondary, then, clearly I think, primary tuning may be independently varied, with operation at the (or a) resonant frequency of the whole system.

C.  Where to tune the primary?

Low or high with respect to the secondary, and by how much? And should the excitation be at a different frequency altogether, i.e., primary by itself at f1, secondary by itself at f2 and excitation at an independent f3?

Or this, perhaps?: With secondary removed and feedback from the primary, tune the primary to the secondary's measured Fr. Then mount the secondary. Excitation will then be at a primary Fr that is a bit different, due to the proximity of the secondary.

Or this?: Tune the primary to the secondary's measured Fr. Take feedback from the secondary. Then, excitation will be at one of the frequency-split Fr's of the doubly-tuned system--hopefully at the lower one--and not at the Fr of the secondary alone. Alternatively, tune the primary slightly differently (higher or lower?), initially, so that the lower frequency-split amplitude-peak is higher than the upper one, thus ensuring excitation at the lower peak.

D. A quote, "...hard switching that is an inevitable consequence if you don't drive the system at one of the resonances."

That's from Steve Connor in the "DRSSTC design procedure - draft" posting. But wherever feedback is taken, from primary or secondary, in a doubly-resonant coupled-system within a feedback loop is not oscillation necessarily to take place at "one of the resonances" of the system? And so, will not switching occur automatically as desired, i.e. rather near the primary's current-zero-crossing (implementing "soft switching")?

E.  Tuned vs untuned primary?--again

If untuned and feedback is taken from the secondary, what is the phase relation between the primary current and the secondary current or voltage? Is it necessarily so that "hard switching" will occur...or not?

These musings come from someone who is upwards of 60 years beyond his B.S.E.E., so may perhaps be excused. But any illuminations from List attendees will be welcomed by...

Ken Herrick