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

Original poster: "Daniel McCauley" <dhmccauley@xxxxxxxxxxxxxxxxxxxxxxxxxx>

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