Re: SSTC not working as expected

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

Original poster: "rob by way of Terry Fritz <twftesla-at-qwest-dot-net>" <rob-at-pythonemproject-dot-com>

Tesla list wrote:
> 
> Original poster: "John Tomacic by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <tesla_ownz_u-at-hotmail-dot-com>
> 
> With a single MOSFET in flyback configuration, one would not expect to get
> more than a few inches discharge to a grounded object. To get to this level,
> the tuning is very critical and requires a "fine tuning" control. Also, you
> will need a breakout point on your sphere as there is not enough power
> delivered for streamers to break out on their own. The bottom of the
> secondary has to be connected to a very low impedance ground, this is more
> critical in SST work than conventional coils.
> 
> The fine tuning can be accomplished by placing a 20 turn 500 ohm or 1K ohm
> pot in series with the  "normal" frequency control pot. The "normal"
> frequency control pot is usually much larger in resistance, 100K ohm or more
> depending on the circuit used, it should also be a 10 or 20 turn control.
> Initially set the fine tune pot to it's mid position, then use the normal
> control to get close to resonance.  The waveform at the MOSFET drain at
> resonance will look as in the pic attached, with large voltage spikes. These
> spikes should not exceed the rating of your MOSFET or it will fry. Once you
> are close to resonance you can use the fine tune pot to tweak for best
> output.  Note that if you are using a filtered DC 160V supply, and are not
> using an interrupt circuit (ie you are in CW mode) the discharge will be a
> plasma flame, not a streamer or arc.
> Your gate drive voltage should be at least +10V (positive peak), if it is
> only 8 v peak to peak (ie, positive peak +4v and negative peak -4v), then
> your MOSFET is not being turned on fully.
> 
> There is nothing easy about making a SST coil work, you need good test
> equipment, know what you are doing, and be patient and persistent to
> overcome lots of pitfalls along the way.  My SST coil is a MOSFET full
> bridge driving a ferrite transformer bottom connected to the secondary in a
> magnifier configuration. I get 24 inch arcs, at peak input power just over
> 9KW, in interrupt mode.  See my website for pictures and description of coil
> controller:
> 
> http://www.angelfire-dot-com/electronic/teslacoil/
> 
> Hope this helps, good luck.
> 
> John Tomacic
> Ottawa,
> Canada
>  
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I've built big ones before that worked like yours, so I have plenty of
experience with SSTC's.  This is an experimentation coil. I found the
problem was gate drive.  I also found that "Warp Speed" IR IGBT's
survive longer than the latest 3rd generation IR FETs.  So you really
can switch IGBT's at 250khz.  

Rob.  

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