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Marx Gen proposal

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teravolt

Sat Feb 18 2006, 03:12AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

Where I work we build marxes the size of bread boxes to the size of small houses and we generaly use tygon tubing like you can buy at the hardware store filled with copper sulfate with copper ends.you can also use table salt if your ends are stainless. Genral tap water also conducts high voltage. I used 2 watt 1meg corbon comp with 30 kv 500pf. If you want to use 100 or 60 Kv arangement you can use +and- 50 or 30 delta. Insted of charging resistors you can use charging indutors. I built one of these door knob marxes ,400kv,and it works fine.

Dr. Shark

Sat Feb 18 2006, 01:12PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

Firkragg, surely the rated voltage is for continous use, like charging and dischaging the capacitor 20000 times per second in a TV multiplier and doing that nonstop for a couple of years. As these caps probably wont see more than a 100 dischages in their entire lifetime, I think I can safely overvolt them quite a bit. I don't have any derating curves for the caps, but I am sure that 50kV would be within specs for that kind of duty.

HV Enthusiast

Sat Feb 18 2006, 03:04PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

joe doh wrote ...

Firkragg, surely the rated voltage is for continous use, like charging and dischaging the capacitor 20000 times per second in a TV multiplier and doing that nonstop for a couple of years. As these caps probably wont see more than a 100 dischages in their entire lifetime, I think I can safely overvolt them quite a bit. I don't have any derating curves for the caps, but I am sure that 50kV would be within specs for that kind of duty.

The rated voltage for any use, whether its continuous or pulsed. Designing and using capacitors even to their rated voltage is generally not a good practice. I always use plenty of margin when using capacitors in my circuits, ESPECIALLY with high voltage capacitors.

If you have 40kV capacitors, I am quite sure that you will kill them if you ran them at 50kV, even for a few discharges, especially in a marx generator where voltage erection may not be uniform and some capacitors may well see excursions of the charged voltage considerably.

EEYORE

Sat Feb 18 2006, 07:44PM

Registered Member #99 Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637

I dont think its wise to risk death of the capacitors for just another 10kv more per stage. At 40kv per stage, its going to be quite impressive.Id just stick to the rated input, and if all goes well, buy some 50kv caps later on....You could probably sell this one to pay for a new one later too....

Matt

Dr. Shark

Sun Feb 19 2006, 01:28PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

I still think that the caps should not die immediately when overvolting them a bit, look e.g. at Steve Ward who is using MO capacitors at more than double the rated voltage.
However Dans suggestions about stray inductances and nonuniform errection causing some major overvoltages is much more worrying. I think I will setlle with 25kV for the 30kV capacitors, so they dont die right away

I'll go with 50kV stages of two series / two parallel capacitors.

DAMN, my halfbrige for driving the thing is so not working! I killed almost all my IGBTs over the weekend, so I have to wait until I get new ones unitl I can continue testing.

Marko

Sun Feb 19 2006, 01:33PM

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

Omg, poor igbts, what did you do to them...??

You wound your own transformer, or used flyback(s)?
What supply are you building for that generator??

Dr. Shark

Sun Feb 19 2006, 06:11PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

This is the death toll so far:

I'll be opening a new thread shortly to discuss the supply, basically I am striving for something like Steve Wards CCPS. But it's OT here...

Marko

Sun Feb 19 2006, 06:17PM

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

My table was looking like that when I experimented with high voltage oscillator for SMPS, killed lots of transistors but all from junk (old atx smps, ES bulbs, other smps etc..)

I blew 2 mosfets and 2 2N3055 last few days, dont remember anithing oher...

Dr. Shark

Sat May 13 2006, 01:04PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

This was the first thread when the new 4HV opened, and work is still in progress. I decided to limit myself to a small prototype now, since nothing was working with the big initial design. The water resistors - a nice idea in theory - were leaking and falling apart all the time, so I got some old style 10Meg power resistors instead. I think that with the tiny capacitances involved, this should be a reasonable resistance.
The sparg gaps cannot be seen below the caps, they are still the brass spheres. Darn high ignition voltage with those, really more like 3kV/mm air, rather than the 1kV/mm you see with pointy gaps.

Now it is mounted in a nice perspex bracket, and powered by a simple Marzzilli Flyback driver. Notice I managed to put it on a 3cm x 6cm piece of perfboard

It is running with IGBTs, so I can have the full 24V input voltage at the gates, and do not need to burn it in Zeners. I'll add some spark pictures once I get the tripod for my camera (couple of days), and can do some crazy multiple exposure stuff.

Dr. Drone

Mon May 22 2006, 10:31PM

Registered Member #290 Joined: Mon Mar 06 2006, 08:24PM
Location:
Posts: 1673

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