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Re: S.s.t.c. feedback reconsidered?

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

OK--thanks once again. Looks as if you have the correct take on it. From a practical standpoint I'm pleased with that because a) I'm currently configured to take the feedback from the primary circuit, and that works well; and b) primary-feedback handily takes care of the absent-secondary condition.

Now onward toward reconfiguring the primary coil. I'm leaning toward a flat one, hopefully with a scheme for continuously varying its inductance, for tuning. I take the feedback from a single turn adjacent to my single-turn of capacitors (see the photo, posted some time ago), so that will not be affected by reconfiguring the actual coil.


Tesla list wrote:
Original poster: "Mccauley, Daniel H" <mailto:daniel.h.mccauley@xxxxxxxx><daniel.h.mccauley@xxxxxxxx>

>1.  The frequency at which the system resonates is, I think, a
>property of the entire system (primary and secondary) ONLY if the
>feedback is taken from the secondary.  If the feedback is taken from
>the primary, especially with k fairly low, then the system Fr will
>strongly tend to be that of the primary alone.

Frequency at which the system resonates is a function of the entire
system for both primary and secondary feedback based systems.  In each
instance, there is a distribution of harmonics (within the primary
current) between the upper and lower frequency pole, as well as
additional harmonics of these.  The only thing that changes is that
magnitude of these particular

>2.  When feedback is taken from the secondary and the primary is
>initially tuned to essentially the Fr of the secondary, the system Fr
>could be at either of the frequency-split frequencies (the "split"
>depending upon k, of course).  Perhaps it would tend to be at the
>lower one because the circuit would operate more efficiently there.

I have shown through both simulation and experimental results, that with
secondary feedback, regardless of the tuning of the primary, most of the
harmonics occur at the lower frequency pole.  Not sure why this is

>3.  But when feedback is taken from the secondary and the primary is
>tuned lower, the system will always resonate at the lower of the
>split-frequencies.  The primary may then be tuned as desired so as to
>maximize the voltage gain of the primary:secondary tuned-transformer,
>with the system Fr staying at the lower gain-peak--even if the
>primary's stand-alone Fr is not precisely that.  (Were the primary
>tuned higher, the system Fr would be higher, but there'd be no point
>in doing that.)

I think you have your feedbacks mixed up.

>4.  So secondary-feedback is best.  However...  When one wants to
>test the system absent the secondary, there's no feedback to
>implement oscillation.  If one substitutes another signal, then
>oscillation will not be in sync with the only resonant element
>remaining in the system: the primary.  Result: "hard"-switching.

Primary feedback would be best, not secondary feedback.

>5.  So...  What is wanted is a) feedback from the secondary when it
>is present, and b) feedback from the primary when the secondary is
>not present.  Preferably, automatically selected.

Primary feedback is the preferred method.


How about that...anyone?


Tesla list wrote:
>Original poster: "Daniel McCauley"
>>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.
>The frequency at which the system resonates is a property of the
>entire system, not simply
>the primary or secondary resonant frequency.  Remember with a dual
>resonant system, that
>frequency splitting occurs and that the system has two peaks in its
>frequency response at which
>it could operate.
>With primary feedback, you are able to control the ratio at which
>the primary current will be divided among these two peaks (as
>well as harmonics of them).  If primary tuning is less than the
>natural frequency of the secondary, more of the lower peak will be
>excited, and if
>primary tuning is greater than the natural frequency of the
>secondary, more of the upper peak will be excited.  Either way,
>primary current will
>contain harmonics of each frequency pole.  I (as well as others)
>have shown this in both simulation and with using a spectrum
>analyzer to monitor
>primary current in a primary driven DRSSTC.
>With secondary base feedback, no matter how the primary is tuned,
>the system will operate at the LOWER frequency pole of the
>system.  I'm not sure
>why this is exactly, but I have made both simulation and
>experimental measurements (again using spectrum analyzer) and have
>found that regardless of
>primary tuning, the system will always operate in the purely lower
>frequency mode.
>>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?
>As far as tuning the primary, this is a complicated subject.  With
>low power DRSSTCs, the general practice (as of right now anyways) is
>to tune the primary
>coil lower than the natural resonant frequency of the secondary.  If
>tuned low enough, the majority of primary current harmonics will
>occur at the lower
>frequency node in the split frequency response.  This (as you have
>read on Richie Burnett's site on frequency splitting), eliminates
>primary notching and
>allows the primary current to ring up continually over the entire
>pulsewidth delivering maximum energy in the shortest amouint of
>time. How much lower to
>tune depends on the system, coupling, etc... and can be gathered by
>simulation or experimentation.
>I'll leave it to Steve Ward to comment on the high power DRSSTCs . . .
>Daniel McCauley
>DRSSTC : Building the Modern Day Tesla Coil Book
>Check it out at <http://www.drsstcbook.com>http://www.drsstcbook.com