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

["Tesla list" <tesla@xxxxxxxxxx>]

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