Re: SSTC not working as expected

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

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

>From: "Tesla list" <tesla-at-pupman-dot-com>
>To: tesla-at-pupman-dot-com
>Subject: SSTC not working as expected
>Date: Wed, 05 Jun 2002 12:39:25 -0600
>
>Original poster: "rob by way of Terry Fritz <twftesla-at-qwest-dot-net>" 
><rob-at-pythonemproject-dot-com>
>
>Here is my setup:
>
>1. 18" of #26 on 4.5" form with 8" semi-sphere on top
>
>2.  160V applied to latest generation IR IRFP50LC 600V reduced gate
>charge MOSFET, in a flyback configuration. A 100pF-500ohm snubber is
>used.
>
>3. TLC555 oscillator with UC2710 6A driver, coupled to MOSFET with an
>isolation tranformer.  I get very square 8V p-p on the FET gate.
>
>4.  Output tranformer is 10 turns primary, 30 turns secondary, which
>then bottom feeds the coil.  This tranformer core only gets hot after
>very long use. It is 1.5 sq cm core area.
>
>I get lots of RF voltage out of the tranformer.  Enough to create a very
>small arc with a screwdriver.  Yet hardly any output from the coil.
>I've checked it with a scope and I  appear to be applying voltage at its
>resonant freq.  It lights up a neon bulb quite a distance away, but no
>streamers at all.
>
>Perhaps it is a matching problem.  I will see if I can come up with the
>S-Params with the Ansoft Serenade eval version.  The FET only gets
>slightly warm, so it appears that little energy is being coupled into
>the SSTC.  I may have to just go with a primary SSTC winding instead of
>the bottom feed.
>
>Rob.
>
>
>
>--
>-----------------------------
>The Numeric Python EM Project
>
>www.pythonemproject-dot-com
>
>
>


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