S.s.t.c. feedback reconsidered?

["Tesla list" <tesla@xxxxxxxxxx>]

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