Marx generators, was RE: door knob caps

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

Original poster: "Jim Lux by way of Terry Fritz <teslalist-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

At 08:16 AM 8/20/2003 -0600, you wrote:
>Original poster: "Dave Halliday by way of Terry Fritz 
><teslalist-at-qwest-dot-net>" <dh-at-synthstuff-dot-com>
>
>Besides, looking at their website, they only sell Ceramic Disc
>capacitors with a 2KV range.  These simply will not work.  If you buy
>these for this application, you are taking dollar bills and burning
>them.  No, not even that because you would get some satisfaction from
>the flame.  You are wasting your money.
>
>The reason is that the cap may be rated at 2KV but this is only one part
>of the problem.  The capacitance-at-that-voltage is minimal.  It is the
>capacitance that stores your energy and if you don't have any energy
>storage, you are going to get a less-than-zero performance.
>
>Check this site for a discussion on MMC - they are using them for Tesla
>Coils but they are very applicable to Marx generators too:
>http://www.thegeekgroup-dot-org/projects/MMC/
>
>You _will_ be spending about $100 and this _is_ a good chunk of change
>but, any money you spend at radio shack will be dollars pissed down the
>drain so don't do that, start saving.  What you will be buying will work
>perfectly and deliver good solid fat performance.

Well, I don't know that I'd be so hard on Radio Shack.  Their component 
quality isn't marked worse than any other vendor selling in qty 1 in nice 
blister packed packages. Sure, if you buy 100 pcs, you can easily do better 
other places (Digikey, Mouser, Newark, etc., we all have our 
favorites).  Time is money too... you could spend 10 hours tracking down 
the very best deal on a $100 purchase for a 10% savings.  Hmmm.. $10 saving 
for 10 hours work.

Ceramic caps, while not ideal, will work quite nicely in a Marx (although I 
wouldn't advocate stacking up zillions of little disk bypass types, for 
structural and labor reasons).  Doorknob caps ARE ceramic, and I daresay 
many perfectly functional Marx generators have been made with them, by such 
illustrious workers as Craggs & Meek, among others.  Not to say that they 
are optimum, though.

Here's some practical stuff on building a Marx:

The circuitry is trivial. The mechanical and assembly aspects are not. Pay 
careful attention when designing to such things as:
a) how long will it take to make thousands of solder joints (if you're 
going that route)
b) what about corona from sharp edges/small wires (ceramic disk caps are 
horrid for this, by the way)
c) How will you insulate the thing, what will support it.

Look at some tradeoffs.. Ceramic doorknobs are about $25-30 each (list 
price from C&H or RF parts), much cheaper other sources of varying 
provenance. Say you find some 2200 pF 25 kV units at $30. That's 2.2 nF 
(about 0.687 Joule)

For the same cash, one could gang up 10 geek caps (150 nF -at- 2kV each) to 
make a 15nF, 20kV equivalent.  (that's 3 Joules...)  One could be a bit 
bold, and run the cap at 2.5kV each to give you 4.6875 Joule. (hmmm voltage 
squared...)

In terms of "bang for your buck), the geek MMC is a MUCH better deal. But 
wait, there's more to consider..

You've got to solder all those geek caps together (as opposed to screwing 
in the connection on one doorknob), and the inductance of the MMC might be 
a bit more, slowing down the rise time of your Marx.  And, of course, the 
physical size of the  MMC is bigger, making a bit of a packaging problem.


So, you've got a bit of analysis ahead of you.  I will say, though, that 
for a Marx bank, stored energy is where it's at, from a visual, audio, and 
visceral display standpoint.  Nobody watching is going to care whether your 
rise time is 0.5 microseconds or 50 microseconds.  Unless you need a 
particular waveform for a particular reason, optimize for stored energy.

Say you want 500kV..You're going to need 200-250 geek caps, and it's going 
to store a kilojoule. (and it's going to cost $1000 to build the thing, by 
the time you're done)

For lowest buck designs, the 0.001 uF 10kV ceramic caps from All 
Electronics might work. I think some folks on the list have tried them.


Your next big decision is going to be what the stage voltage should 
be.  Fewer stages is better from a number of sparkgaps and complexity 
standpoint.  However, high stage voltages are a pain to charge and have 
huge corona problems.  If you're going with MMC technology, you've got 
pretty fine grain adjustability (i.e. you can go in 2-3 kV steps for your 
design). If you're using doorknobs, then that will pretty much determine 
your stage voltage (1x or 2x doorknob voltage) I find that stage voltages 
above about 25-30 kV start to get real troublesome from a corona 
standpoint, and 50-60 kV is just plain evil.  If you're contemplating 
40kV+, I suggest you do a bit of fooling with a single stage and charging 
it.  You're going to spend some serious rebuilding time learning how to 
fabricate things that have large radii of curvature, no sharp points, 
etc.  Either that, or immerse the whole thing in oil or pressurized gas.

You need to think about your charging supply.  If your bank stores a 
kilojoule, and you've only got a 50 watt supply, then you're going to spend 
20 seconds charging it (minimum).. actually, you'll probably never charge 
it with 50 watts, because corona losses might be in that ballpark.   A NST 
makes a fine charging supply for lower stage voltages.  The inductive 
current limiting makes it kind of a constant current source, which reduces 
losses in the charging resistors. (What you really don't want is a constant 
voltage supply).  An NST feeding a voltage multiplier might make a good 
charging supply for a bit higher stage voltages, but you're going to have 
to invest in some more MMCS for the multiplier caps.  Again, the poor 
regulation on a CW multiplier doesn't hurt you here.






> > -----Original Message-----
> > From: Tesla list [mailto:tesla-at-pupman-dot-com]
> > Sent: Monday, August 18, 2003 6:56 AM
> > To: tesla-at-pupman-dot-com
> > Subject: Re: door knob caps
> >
> >
> > Original poster: "John by way of Terry Fritz
> > <teslalist-at-qwest-dot-net>" <fireba8104-at-yahoo-dot-com>
> >
> > Thanks Dr. Resonance,
> > I was considering using MMC(tons of radio shack 2kv caps) for
> > caps to if I
> > couldn't find doorknobs. Unfortunately I live in New Jersey(no jokes
> > please) and Chessehold is to far away. Also I hope you win
> > that 250 kv
> > x-ray transformer. I say at least a 4 stage Marx.
> > Thanks,
> > John
> >
> >
> > Tesla list <tesla-at-pupman-dot-com> wrote:
> > Original poster: "Dr. Resonance by way of Terry Fritz "
> >
> >
> > I have a very nice design for a Marx using MMC's and they
> > only cost $3 per cap. If you make it to our Cheesehead
> > Teslathon I can share the data with you.
> >
> > We charge at 60 kV/stage using either a flyback tripler or an
> > x-ray xmfr (faster charging) and run 60 MMC caps on each
> > stage. This gives up .01 MFD total cap per stage at 60 kV and
> > makes a very bright output spark at 600 kV with 10 stages.
> >
> > Less expensive than doorknobs.
> >
> > Dr. Resonance
> >