Re: 304tl experiments...

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: FutureT@xxxxxxx 

In a message dated 3/24/07 11:38:37 PM Eastern Standard Time, 
tesla@xxxxxxxxxx writes:



> I realize you designed your staccato (the one which
> Steve also uses) circuit to run efficiently due to its
> zero crossing detection capabilities.  Because of this
> it will not trigger the vttc system at arbitrary
> points in the mains cycle.  My main question was,
> wouldn't this zero crossing circuit be uneeded if I
> were to staccato pulse a vttc which was running off of
> rectified and filtered MOT output since the filter cap
> would allow the circuit to deliver a constant energy
> regardless of when the staccato pulse occured?


Brett,

Yes, the zero-crossing feature would not be needed if
you run on filtered DC.  Also you'd be able to obtain
interesting effects not possible with the traditional
staccato design.  I used staccato with filtered DC in
some early tests using mechanical staccato.  The system
was not synchronized to the 60Hz AC waveform.  The problem
with using the "cathode disconnect" circuit with solid state
devices is that they might be looking at the full vttc plate voltage
at times which will tend to blow out the solid state device.

Using the staccato as I use it, the switching occurs
during negative portions of the 60Hz AC waveform, so the
solid state device is not subjected to the full vttc plate
voltage.  Once the solid state device is turned "on", there
is no high voltage across it even when the 60Hz AC
plate voltage waveform goes positive.  This is because
the device functions as a short when it's on.  But if the
device turns off, while the full plate voltage is still on the
tube (due to the positive position along the 60Hz AC
waveform), then the device will see the full plate voltage.
If the plate voltage is 5kV, and the device is rated at
1kV, then the device will be toast.

This is why I used grid control when I used
duty cycle control.  Using grid control, the solid state
switch will never see full vttc plate voltage.  However
an exceptionally high grid voltage may be needed to stop
vttc oscillation during full power.  I actually used a
vacuum tube as a switch in that experiment.  Using
duty cycle control, I was shutting off the vttc basically
at full power operation (full positive plate voltage), so it
was similar to controlling the vttc using filtered DC.

So my point was that the circuit has to be modified when
using filtered DC so the solid state device is not destroyed.
For example a series arrangement of solid state devices
which achieves a rating somewhat above the 5kV plate
voltage might work, but is more complex.  Also the
plate voltage can go higher than that due to resonance.

I think the idea of using a solid state switch for creating
staccato operation of a filtered DC vttc is well worth pursuing.
I just wanted to point out some issues that have to be
considered.

Dave Sharpe has a new vttc typology that overcomes
these difficulties.  It permits staccato operation with
filtered DC and also offers other advantages compared
to a traditional vttc.

John




> Then I would simply need to find an SCR (or IGBT?)
> which would switch fast enough, and would run with
> simple enough driver circuitry to prevent the idea
> from being an excercise in redundancy.
>
> I guess I just thought it would be neat if I could use
> the same remote interrupter box for my SSTCs and my
> VTTCs.  Hmmm, maybe Steve Conner's Brick driver
> circuit might work for this.  Again, this might be a
> waste of time (and an IGBT) unless it can allow me to
> do things with a vttc that I couldn't do with your
> circuit.
>
>
>
> -Brett





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