Re: A question about SSTC tuning

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "K. C. Herrick by way of Terry Fritz <twftesla-at-qwest-dot-net>" <kchdlh-at-juno-dot-com>

I don't see why you other s.s.t.c. folks don't make your coils
self-tuning like I do.  Your comments below and also Daniel McCauley's
posting of today bring this to my mind.  If you will just shift the
secondary return's signal 90 degrees you will find that it becomes in
phase (or 180-out depending on winding-sense) with the signal that you
need drive the primary at Fr.  

I take my 12"-dia x 35"-high coil's return and connect it via a 0.15 uF
capacitor to ground, 0.15 uF having low Xc at my ~140 KHz Fr.  Then
connect the small voltage across that capacitor (90 degrees shifted from
its current) through a 1K resistor to a pair of diodes back-to-back and
again to ground.  The square wave of voltage across those diodes, when
oscillations get going, is just what you want, appropriately amplified,
to drive your primary at resonance.  >Dynamically< at resonance, I might
add: sparks or no sparks, it's always at the exact phase necessary for
excitation at the instantaneous Fr of the secondary.  This is assuming,
of course, that the primary is untuned, as it should be, making the
secondary the only tuned element in the system.

With sufficient gain, circuit noise in the amplifying chain will get the
first half cycle going quite smartly, and then the diode voltage will
take over.

It has worked like a champ for me.

Ken Herrick


On Sat, 03 Aug 2002 12:33:03 -0600 "Tesla list" <tesla-at-pupman-dot-com>
writes:
> Original poster: "Jan Wagner by way of Terry Fritz 
> <twftesla-at-qwest-dot-net>" <jwagner-at-cc.hut.fi>
> 
> Hi!
> 
> > MY SSTC has been up and running for some time now with
> > no blown FETs. I do have a question about tuning the
> > thing though I use a dual channel oscilloscope with
> > one channel on one of my FET gates the other is hooked
> > to a small piece of foil;
> 
> Another, IMHO more revealin measuremetn is to place a small high 
> wattage 1
> Ohm or smaller resistor in series with the TC ground lead, then 
> scope
> accross the resistor (one channel).
> 
> If out of tune, the signal will still contain chopped bits of
> the correct frequency so it is very easy to guess which way you have 
> to
> adjust tuning, up or down.
> 
> When you approach in-tune, the signal will look more an more like a 
> clean
> sine wave. Umm yeah and of course the amplitude will rise as well. 
> Once
> you get a perfectly clean sine, you've tuned your SSTC. :o)
> 
> (tuning with a 12V test supply, not mains 110vac or 230vac)
> 
> > any way I had thought tuned
> > would be when the two waveforms were 90% out of phase.
> > MY gate wave looks the worst at this point however and
> > calms down with the two waves slightly off of 90% the
> > out put of the coil is not noticeably better at either
> > of these points. any thoughts..
> 
> In my opinion it is better to scope on one end of the primary coil, 
> not
> the mosfet gates - again, not mains but 12V test supply. If mains
> then watch out how you connect the scope probe ground clip...! :o)
> 
> The gate drive and actual pri coil voltage waveform are always 
> slightly
> out of phase. This is because of the mosfet turn-on and turn-off
> delays. The less the total series resistance from the driver to the 
> gate
> structure inside the mosfet, the shorter the delays and thus phase 
> shift.
> 
> About bad looking gate drive: if slighlty out of tune then the 
> mosfets
> won't be switched at zero current, and you get a little bit of 
> voltage
> spike mess on the mosfet source pin and the copper tracks there.
> So, if you haven't soldered the two gate drive leads _directly_ the
> where the mosfet pins leave the package, the you may end up with 
> some
> significant garbage in the gate-to-source waveform. "Typically" not 
> more
> than about 2V max spikes/mess - otherwise it starts getting 
> dangerous for
> the fets...
> 
> good luck!
> cheers,
>  - Jan
> 
> --
> *************************************************
>  high voltage at http://www.hut.fi/~jwagner/tesla
>  Jan OH2GHR
> 
> 
> 
> 
>