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Re: SSTC probs



Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" <cwillis-at-guilford.edu>


Hi Matt,

You're getting RF burns from touching the driver circuit parts, but this is
probably because you are picking up the RF, not the driver parts.  I'd
advise not bringing your RF-hot hands near the 555, RF will cause it
trouble with on-and-off timing.  Of course, at 90 V, the drain of the
transistor will have a moderately high RF voltage as well.  You can put the
circuit in a box if you want, but a good ground plane is really all that I
have needed.  Be sure to ground the cases of any potentiometers, like for
555 on- and off-time controls, or you will feed RF back into the timer and
make it go crazy!

About the half-bridge configuration (I think this was what your "half-wave"
question was getting at):  for starters, you need a power supply that has
equal-voltage positive and negative outputs.  The way this is done most
often it seems is with a capacitor divider and a positive-output-only
supply, where the center of the capacitor voltage divider becomes the
equivalent of the DC neutral.  The half bridge circuit uses two
transistors, driven 180 degrees out of phase, which alternately switch the
"hot" end of the primary to the positive and negative supply lines.  The
"cold" end of the primary is at RF ground (although in the case of the
capacitor-divider supply, it is at 1/2 the positive DC voltage as well).
One requirement of using either a half- or full-bridge is that you
incorporate "dead time" into the drive signal (when neither transistor is
turned on) to minimize the likelihood of "shoot through" (both transistors
are still on at the same instant and short the drain supply.) You really
should see Richie's site to get a good explanation.

Something to note for future experiments.  It is very hard (if not
impossible? I have not seen it done anyway) to get a nice,
frequency-adjustable 50% on, 50% off square wave from a 555 UNLESS it is
the CMOS version.  In this case you can use a circuit like Terry Fritz's
"Tesla coil tuner."  So you might want to think about an upgrade to a
different driver soon.

 -Carl