VTTC controller - First Light

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

Original poster: "sundog by way of Terry Fritz <twftesla-at-qwest-dot-net>" <sundog-at-timeship-dot-net>

Hi All,

    Aside from some glitches to iron out of the design and implementation, 
it works, and surprisingly well.

     After testing the controller extensively, I decided to fire up the 
tube coil and see what happened.  The worst was I could smoke $24 in FETs, 
and some cheaper chips in the controller.

   Quite to the contrary, I got a nice purple fuzz on the breakout point.

   The setup -
     3" PVC secondary, wound with 26ga wire, Fres is ~390khz (best output)
     Primary is 2 layer, helical, with 14 turns per layer and centertapped 
for push-pull.
     Power supply is 2 MOTs in series feeding raw AC to the plate.
     The controller is a TL494 based PWM, with a pair of TC4420 FET driver 
powering 2 -500v -7A FETS
     The grid supply is a 120/480v CT transformer with diodes on the output 
to give me -340v (with a filter cap) for each FET.  With the dropping 
resistor in plate (to limit current to the grid), I get about -300v on the 
grids.

     I'll post waveforms and whatnot on my site as I get time.

    The skinny on this system...  The P channel FETs act as part of a 
resistive divider, feeding the reference voltage to the grids.  When the 
FETs turn on, the voltage on the reference (and grid) drops to zero volts, 
turning the tube on.  The fet turns off, and the voltage comes back to 
-300v.  At -300v, the tube is *hard* off, and I have to crank the variac to 
140v to even begin to get any power out of the tube.  At 4kV on the plate, 
there is no current flow to the tube.

    Switching -  The voltage drops from -300v to 0v in about 1.6uS (not a 
typo), then climbs back up to -300v in about 150-200nS (sometimes as long 
as 300nS for low frequencies).  I attribute this to poor "off" gate 
switching of the FET, as the drive signal is very sharp.  Still, the 
opposite tube turns on while the first has around -170 to -200v on the 
grid, which has the tube "mostly shut off". Still, it's burning up some 
power there, and the matter needs addressed.
   Surprisingly, the FETs don't see that much kickback with a standard RL 
choke at the plate of the tube.  The kickback never surpassed the grid bias 
voltage, so the FETs were in no danger.  Even running the system *way* out 
of tune, there was no damage to the fets or tubes in any way.
    An aluminum enclosure to shield against RF for the drive electronics 
will be forthcoming, as will better adjustment of the TL494's operating 
frequency.


    Now, the issues I ran into -
      Poor FET turn off.  I think a lowish-value resistor to ground from 
the gate should help dump the capacitance once the FET driver turns off, 
giving me a much sharper return to -300v on the grid, and reduced FET 
heating (which isn't even an issue at the moment).
     The tubes refused to pull more than 500w, even with 0v on the grid.  I 
will substitute a pair of 833A's for the RS1026's to compare, as the 
RS1026's should be pulling ~4kW on their own. For now, I think the tubes 
are fine, and my filament tranny's ground and MOT's ground connections are 
suspect.
     Power resistor heating - The dropping resistors in the divider get 
*hot*.  Especially under the CW duty they were under last night.  Resistors 
are current 3kohm 25w aluminum sinked, mounted to a 4x6" heatsink.
     RF kickback into the FET causing spurious triggering -  There was a 
*nasty* ~8khz screeching from the spark at times, which I narrowed down to 
spurious triggering of the FETs.  RF hardening should help eliminate that 
nicely, as will a better FET output waveform.


    Once I get the tube power situation under control, I'll be ready to run 
real full-power (multi kW) tests on the coil.   It's *incredibly* nice to 
be able to run a coil out-of-tune, and not fry anything.  Being out-of-tune 
will matter worse at higher current levels, as the tubes will have to 
dissipate the energy, but they're tough enough to take the overload (within 
reason) without cooking, as FETs do.

ToDo
    Try 833A tubes in place of the RS1026's.
    Get sharper turn-off on the FETs
    Add a 555 timer to enable staccato/pulsed mode to the TL494
    Re-configure the tank circuit to utilize a tank capacitor and run the 
controller as a grid-leak replacement (single-ended)
    Finalize the electrical design and get PCB's etched for easy 
construction/population.

All in all, I'm not surprised that it works, but I am surprised that it 
works as well as it does, with so few problems.  I'm in the process of 
drawing up schematics for the whole thing (TL494, grid-switching circuit, 
etc), which I'll post as I complete them.

   Smile everybody, there was a new flavor of TC born in my garage last 
night. :)

Shad



-----------------------------------------------------------------------
Shad (Sundog)
G-5 #1373
"Ever stop to think, and forget to start again?"
-----------------------------------------------------------------------