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RE: Marx generators, was RE: door knob caps
Original poster: "Timothy Myers by way of Terry Fritz <teslalist-at-qwest-dot-net>" <tjmyers-at-nortelnetworks-dot-com>
I've been searching the Web for details on Marx generator construction with
little luck. You just pretty much summed up the answers to all my questions.
Many times I find the design reasoning lacking when analyzing a project. I
don't know if it's just assumed everyone is on the same page or not enough
reasoning is put into the design.
Your details provide enough information to design a custom Marx generator
of you own choosing. Really good information. Thanks,
Tim
-----Original Message-----
From: Tesla list [<mailto:tesla-at-pupman-dot-com>mailto:tesla-at-pupman-dot-com]
Sent: Wednesday, August 20, 2003 12:54 PM
To: tesla-at-pupman-dot-com
Subject: Marx generators, was RE: door knob caps
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/>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>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
> >