Re: A question about SSTC tuning

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

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

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 high voltage at http://www.hut.fi/~jwagner/tesla
 Jan OH2GHR