Re: 304tl experiments...

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Brett Miller <brmtesla2@xxxxxxxxx>

Shad,

Fascinating work!  Thank you so much for sharing this.
 Actually, as far as I am concerned, you will be free
to drop this on us yourself.  I have a nice single
833A vttc modeled much after Steve Ward's, and with a
few ideas borrowed from Cameron Prince.  I have done
some fun work with an 813 in that same coil (20" 60pps
sparks) and a 304tl.  However, most of my excitement
recently has been directed toward learning the solid
state topologies.  I'll be consumed with my 2nd and
3rd SSTC projects for the forseeable future as well as
an eventual DRSSTC.  I can't wait to see your
grid-driven vttc in action!

-Brett


--- Tesla list <tesla@xxxxxxxxxx> wrote:

> Original poster: Shad Henderson
> <shenderson@xxxxxxxxxxxxxxxx>
>
> Hi All,
>
> Before I got pulled in another direction, I was
> running a grid-driven
> VTTC on a very small scale, using a TV sweep tube as
> the main power
> tube.
>
> I had problems with the circuit, but have since
> realized what the
> trouble was, and how to fix it.
>
> In a nutshell, here's what I did, and how.
>
>
> I used a control transformer (120:480) power supply
> into a full bridge
> rectifier and smoothed it with a 2uF 4kV cap.  That
> was the plate
> supply.
>
> A feedback coil (about 2-3 turns of 20ga wound below
> the primary)ran
> into a comparator, which drove a transistor
> (inverter) and a pair of
> TC4420's (needed an inverted input to one of them).
> A 556 provided BPS
> (~200-1000pps) and "firing time" per pulse, from
> 70-300uS.
>
> The TC4420's drove a small half bridge of IRF740's,
> which in turn drove
> a ferrite toroid [MISTAKE HERE!] that in turn drove
> the grid of the
> tube.  I had a 1:3 step up ratio on the grid-drive
> toroidal ferrite, and
> a single secondary on it.  I connected one end of
> the secondary to a
> -50v supply, and the other to the grid.  Careful
> manipulation of the
> input voltage to the half bridge allowed me to swing
> the grid voltage
> very well.  Plenty of drive for grid current too.
> Beware you don't cook
> the grid, though.
>
> The mistake was using a toroidal ferrite core to
> drive the grid. The
> heavy grid current would walk it into saturation
> very quickly at high
> BPS and long "on" times, leading to the waveform
> going to crap and the
> IRF740's popping.
>
> The "fix" is to use a gapped EE ferrite core in
> place of the toroidal
> core that the grid drive transformer is wound on.
> That'll prevent
> saturation, and keep the IRF740's happy.
>
> It sure beat the heck out of using a piece of
> sillycon to control the
> cathode, because by the time I got all that done, I
> realized I was using
> the tube as a big high voltage resistor in series
> between the B+ supply
> and the mosfet.  Disconnecting the cathode from
> ground just whizzed it
> up to not much below the B+ voltage, necessitating a
> high voltage
> mosfet.  The thought of using a tube as a high
> powered resistor that
> consumed lots of power just struck me as, well,
> silly.  But that's what
> it was in that setup.  I was taking *no* advantage
> of the tube by
> switching it's cathode at Fres, and still beating
> the mosfet silly by
> asking it to interrupt current flow (switching
> losses...).  Much easier
> to switch the tube's grid, and eliminate switching
> losses on the
> high-powered end all together.  After all, a tube
> doesn't care if you
> switch it in the middle of a high current cycle.  It
> just does it.
>
> Once I get my current project off my plate, I'll be
> fixing the hybrid
> VTTC setup and seeing how much I can abuse the
> little sweep tube.  The
> setup is very close to a traditional VTTC, only a
> slightly bigger tank
> cap (I used ~5nF), less primary turns, and the
> feedback winding has
> *far* less turns on it, and less coupling to the
> primary.  You only need
> a few volts feedback.  Be sure to clamp the feedback
> coil with zeners,
> though.  1W or better.
>
> I wanted to spring this on the community as a
> surprise, with pictures
> and a webpage, but since people are contemplating
> similar ideas, I
> figured I'd speak up to share-and-share-alike.
> Maybe save others the
> hassle of repeating my mistake.
>
> Thanks!
>
> Shad H.
>
>
>
>
> On Tue, 2007-03-27 at 00:09 -0600, Tesla list wrote:
>  > Original poster: "Steve Ward"
> <steve.ward@xxxxxxxxx>
>  >
>  > Hi Brett,
>  >
>  >
>  > >  It certainly won't be a
>  > >matter of just plugging in a standary SCR with a
> 555
>  > >oscillator on the gate.  I think one of the
> 1000v 600a
>  > >IGBT bricks would be sufficient, probably
> overkill for
>  > >that duty.  Plus, the driver circuitry wouldn't
> be any
>  > >more sophisticated than the usual staccato.
>  >
>  > Such an IGBT is completely wrong for this use.
> It only needs to run a
>  > few amps, but possibly very high voltages.  The
> proper way of
>  > controlling this would be control of the grid.
> Let me give it some
>  > more thought later...
>  >
>  > Steve
>  >
>  >
>  >
>  >
>
>
>




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