Re: Marx generators, was RE: door knob caps

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

Original poster: Ed Phillips <evp-at-pacbell-dot-net> 

Tesla list wrote:
 >
 > Original poster: "Jim Lux by way of Terry Fritz <teslalist-at-qwest-dot-net>" 
<jimlux-at-earthlink-dot-net>
 >
 > Actually, there is a fair amount of info out there, it's just buried..
 > Lyonel Baum has a good page summarizing most of the salient details. My own
 > Marx page doesn't have a lot of design rationale (out of sheer laziness on
 > my part).
 >
 > The other thing is that once you've researched it and built a couple, you
 > tend to figure things are obvious.  Dr. Resonance runs 60 kV in his stages
 > because he's got plenty of experience at that voltage level, and probably
 > instinctively knows by now how to build stuff that doesn't leak everywhere.
 > The other problem with things like Marx generators (at least on the amateur,
 > one-off level) is that they are big and complex enough (mechanically) that
 > you tend to overdesign, because it's such a pain to rebuild.  They also have
 > very long lifetimes.  There are lots of 50 year old Marx systems around
 > still chugging along happily.  Unlike a TC, where the caps get stressed
 > highly, in a Marx, a commercial unit might do a few shots a day on the
 > average (actually, probably more like a couple dozen shots in one day, then
 > 2 weeks of setup for the next test...).  Even if your caps only had a life
 > of 10,000 shots, that's a lot of years of use.
 >
 > High rep rate Marx's (as in radar modulators, flash X-ray, etc.) are a
 > different animal, and are expensive enough that the mfrs can do some
 > research and cut and try.
 >
 > The hard part of a Marx is the spark gaps, by the way, not the caps.  They
 > really need to fire together to get expected performance. That means
 > consistent spacing, consistent voltage, and good alignment.  In many hobby
 > built Marxes, the charging resistors are quite large (100s of K, to several
 > Meg), resulting in a fairly long time constant, which means that the top
 > stages don't get full voltage.  Commercial Marxes, which can have fairly
 > high power charging supplies, tend to use fairly small charging resistors
 > (5-10K).  That gets the arc current (5 Amps is a good design number) in the
 > gaps up, which helps with consistency.

	Doesn't anyone trim his replies???  About three years ago I did some
testing in a Northrop lab which had quite a large Marx generator which
had been in use for many years in doing lightning testing of aircraft
structures.  I can't remember how big the capacitors were or how many in
series, but the thing stood about 10 feet high and had a power supply
which was of the order of 3' x 3' x 4'.  There was a bunch of stuff
piled in front of it so couldn't get a very good view.  Since the thing
was being junked I picked up a couple of spare capacitors which are
0.015 ufd at 25 kV DC, but probably not good for TC service because they
are a lot smaller than the ones we got here a few years back.  Studs on
the end are about 3/16" but they are out in the shop and don't feel like
looking at them right now.  As Marx generators go I don't think this was
a giant, but just a standard commercial outfit made for testing labs.

Ed