Re: A Marx-like design

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

Original poster: "K. C. Herrick by way of Terry Fritz <twftesla-at-qwest-dot-net>" <kchdlh-at-juno-dot-com>

Jim (& all)-

Comments interspersed...

On Fri, 20 Sep 2002 16:47:50 -0600 "Tesla list" <tesla-at-pupman-dot-com>
writes:
> Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" 
> <jimlux-at-earthlink-dot-net>
> 
> 
> This is very clever....
> Here are some questions that strike me...
> What's the loss across all those gaps... The IR loss will be low, 
> because 
> the gaps are short and the current high, but there's an irreducible 
> cathode 
> drop.  In arc welding (stick, not TIG or MIG), it's really hard to 
> get a 
> stable arc below 20V or so, and that's with a molten electrode on 
> one end, 
> implying that you're going to have 20-30V drop in each gap.

Huh!...I'll accept 20-30 V!  Even IGBTs, according to how I see them (see
recent posting), will drop 10 V or so.
> 
> Each cap gets charged to around 300V (resonant charging, 120V input, 
> etc.), 
> so you're kind of stuck with a 10% hit there..  240V charging would 
> help a 
> bunch, because then you're just looking at 600V total on each cap, 
> with a 
> 5% hit...  I'll bet there's some nice sweet spot where the hassle of 
> 
> voltage multipliers or transformers to get the per gap voltage up 
> (to 
> reduce gap losses) gets to be more than the improved performance.

I'm looking toward 600 V or so, not 300.  I figure I should be able to
reach 600 peak rather easily, from the ~150 mains-peak, with only a
moderate Q in the 60 Hz resonant elements.
> 
> An idea... if you use some sort of gas gap, or maybe even a suitable 
> 
> semiconductor switch, you might be able to get the "gap loss" down.. 
> I've 
> seen Marx configuration pulsers built with avalanche transistors, 
> but I 
> don't know if they have the power handling you need.  Does an SCR 
> (or 
> really, a 4 layer diode, because you'd leave the gate unconnected) 
> have a 
> fast enough "turnon" to work in this application?

Fast enough turn-on, yes, I should think...but paralleled units would be
needed to handle 1000 A peak or so--bringing the annoyance of unequal
current sharing.  It's well worth thought, though.

KCH
> 
> 
> At 01:27 PM 9/20/2002 -0600, you wrote:
> >Original poster: "K. C. Herrick by way of Terry Fritz 
> ><twftesla-at-qwest-dot-net>" <kchdlh-at-juno-dot-com>
> >
> >For some time I've had an idea for a spark-gap-type t.c. with a 
> Marx-like
> >primary, I've fully laid it out on paper (on my HD, that is), and I 
> await
> >a time when my inclination to loaf may diminish.  My idea is to
> >resonantly charge a set of capacitor-modules, from the mains and in
> >parallel, then discharge them in series through the primary.  I've
> >designed it with 20 sections, each to be charged to ~600 V, then
> >disc
> 
> 
> 
> 
>