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RE: SS Variac - Part II < IT WORKS W/ HV :^) >

Original poster: Sparktron01@xxxxxxxxxxx

Steve, Terry All

I just finished running system for better then 15 minutes into a 5kV, 60mA
NST transformer,  I can (and will) provide pictures, voltages and currents.

I am supplying up to 105Vrms, at up to 2.5A measured into NST, sparking into
a horned spark gap (minature Jacob's Ladder). NO problems, no transformer
diode IGBT problems whatsoever. LP inductor running hot (ok put a 4"
muffin fan on the whole thing and say chuck it). My efficiency is not the
greatest, (I am passing through 2 isolation transformers, and are only
putting ~ 30Vrms on IGBT). But end to end is at least 75% and once controls
are fiinalized and powered by 120VAC, conduction lossses will drop on IGBT, due to operation at much higher Vce. Also I had NO EMI FILTERING AT ALL on the control stage, and it worked fine (no failures). NST are definitely strange
beasts looking at primary side, at least 3 "resonant" points were detected
as voltage was adjusted upward. IDK if that was LPF interaction with
NST , or luck of the draw.

So I guess I'm the first to use this on control on a HV transformer, it does
look EXTREMELY promising...   :^D

Dave Sharpe, TCBOR/HEAS
Chesterfield, VA. USA

> Original poster: "Steve Conner" <steve.conner@xxxxxxxxxxx>
> >The basic topology is a full wave DC bridge in
> >series with hot leg, with a 600V, 40A IGBT across +/- of bridge as a
> >commutating
> >switch.
> Before you do any more work on this, please give some thought to the $64,000
> question- When you're driving an inductive load (like a NST etc) through
> this device, the current will try to keep flowing when the IGBT turns off.
> Where will it flow?
> I tried to figure it out but couldn't think of any path for it. I concluded
> that the diode bridge or IGBT would be destroyed by inductive kickback when
> running with an inductive load, so I gave up on this topology.
> If anyone can prove me wrong I'd be happy as it's a very simple and
> attractive circuit. I know a guy who successfully used it to do PWM dimming
> of filament lamps but of course they are pure resistive loads.
> Steve Conner