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1 MV bipolar Marx

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LAN_403

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Tue Aug 28 2007, 07:58PM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

Yeah, long ago i wanted to build a big marx generator exceeding the 1MV range.
Now i started building it.
I was lucky to find these caps on ebay and decided to use them in a marx generator.
for the marx i decided to build 16 stages at 587pF 32kV dc out of 8 wima fkp1 4,7nF 4kV dc caps in series.

I already ordered 8 metres 8mm threaded rod and nuts out of polyamid (nylon). for the sparkgaps i want to use brass rod and castellated nuts (mhh, right word?)

when finished, it should look like this (one Tower) Dont know if a toroid would work, maybe with or without one.

This is just a simple 10minute design to show you how i want to assemble it. 16 stages with 32kV dc for 512kV max for each tower. 1.024MV for both.

Here are the caps

Instead of resistors iÂ´ll use inductors with 527ÂµH and 0,17mm wire. I simulated one tower with swcad (Spice) and it seems to work well.
Now im waiting for my pcbs and threaded rods :)

Marko

Tue Aug 28 2007, 08:08PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

I fear that some people around reported problems with inductors.. they need to be really large, I think your 500uH isn't closely enough.. and even then you may run into voltage standoff problems.

Steve Ward is going to be able to give you much more help on that part.

Other than that, it looks like awesome project. Tough and brutalistic construction, I'm looking forward to it

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Tue Aug 28 2007, 08:44PM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

yeah, im not sure about the inductors. in the simulation it seems to work without problems, but im not sure cause there are so few marxÂ´s with chokes around. but thats no problem.
i wanted to wind the chokes on 6cm 15mm pe tubing between each stage. if this doesnt work, i can use normal resistors instead, or reduce the wire to 0,1mm (thats the thinnest i can wind for myself) for around 1mH

I want to use inductors cause high voltage resistors that can stand up 32kV are hard to find.

Sadly, i have to wait another week for the rods and nuts.. much time to etch the pcbs .i hope for a successfully project cause i spend much money in it and will spend much more time in it to build it,, i looked for some suitable resistors but the only i found was a 15k resistor 5 watts which is long enough to withstand the voltage..

vasil

Wed Aug 29 2007, 03:25PM

Registered Member #229 Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506

I thought once to use flyback secondaries as inductors in a Marx setup. I guess that will upstand at 30 kV, but I am not sure that will have enough inductance.

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Wed Aug 29 2007, 04:37PM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

Today i started building the mmcÂ´s. 8caps in one pcb with fat soldered tracks. of course iÂ´ll varnish the back with a special varÂ´nish that holds up 70kV/mm.

4 of 32 are finished. Still waiting for all the other stuff.

And, i found a shop for high wattage resistors and looked for some 11 watt 82k resistors. dont know if they hold 30kV.
Measured 617 to 619pF per mmc :)

Marko

Wed Aug 29 2007, 09:43PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Simple small 1 or 2W resistors in most cases proved good for >10kV. Few in series should do well..

Cap arrangement looks good, but.. why are your MMC output connections so close? That looks like it is hardly going to withstand 32kV.. With lacquer maybe, but nothing works like good air clearance. And I don't see why can't you make it wider...?

I just don't want your marx to blow up and you to be sad after it.

It may be good idea to hi-pot test the boards before mass-producing them and putting them together.

Hey, you may want to put the resistors on those boards too. (?).

Doh, I really hope somebody with experience will show up...

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Thu Aug 30 2007, 05:18PM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

to calm you down marko, the distance between the connections is wide enough. i testet them with my 7.5kV neon with 4x cw multiplier (30kV) and it looks good with and without varnish.

I needed to make the connector in the middle for the spark gaps. i have 2 different board designs. the first connects the sparkgap to the left, the second to the right side of the caps. the two holes beneath the big hole for the threaded rods is for te resistor or inductor connections. The resistors are not on the pcb but between each mmc (cause the design is optimized for inductors on tubes on the threaded rod between each mmc pcb.

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Sat Sept 01 2007, 03:38PM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

Some spark gaps.

brass is a pain to bend and after a few tries i thought it would be better to solder every 45 degree angle by hand.

i got some nice 10mm brass spheres from my local surplus store and thought that they would make great spark gaps. Really expensive though but i think this project is worth the costs.

my pcbs and diodes and resistors arrived today so i can hook up a cw multiplier to test the resistors . i hope they will hold up 32kV

Marko

Sun Sept 02 2007, 12:10PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

If it was me I would probably find a way to mount the gaps without bending them, but this looks cool, balls give a really beautiful touch!

It's not ashamed of chris H. 's marx

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Sun Sept 02 2007, 03:45PM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

heres what it will look like in the final version except the nylon thingis will replaced by m8 nylon threaded rod.

Heres my 3 stage test setup. still have to build some sort of flyback driver to try it out

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