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Resistor Choices for Marx Gen

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Robert Francis

Wed Mar 28 2018, 04:49PM

Registered Member #2355 Joined: Thu Sept 10 2009, 09:12PM
Location:
Posts: 71

Hi, I am building a Marx generator, about 760kV, (+380kV, -380kV) but I am having some design issues.

I built the marx gen with inductors instead of resistors for decreased charging times.

But what do I do about the partially charged capacitors in the marx gen when I turn the device off?

I heard that I should put some resistors across the positive and gnd and negative and gnd.

Basically I want to fire up the Marx gen and not get electrocuted when I pack it up and put it away.

Any advice?

Oh, I also used a Cockcroft Walton generator, 1 stage, to increase the initial voltage coming out of the flyback transformers before going into the Marx gen.

Could/should I put my resistors across the CW or Marx or both?

Sulaiman

Wed Mar 28 2018, 06:37PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

For high voltage circuits, quickly draining away charge when power is off, for safety,
is always in conflict with power consumption.

One method is to have one or more relay contact sets held open when power is applied,
when power is removed the contacts close and can be used to discharge capacitors via resistors etc.

johnf

Thu Mar 29 2018, 08:06AM

Registered Member #230 Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284

clever people use a shorting bar
big solid bit of copper to short out each stage
the use of resistors is frowned upon by experts as an open cicuit resistor looks the same as a good one

stay safe

AndreiRS

Thu Mar 29 2018, 04:44PM

Registered Member #62109 Joined: Sun Jan 28 2018, 10:00PM
Location: Porto Alegre
Posts: 56

You could use aluminum to short circuit. It would not be so agressive for the capacitors as it is less conductive than copper... Maybe it is a bad ideia, but sounds good to me. It will need to be cleaned sometimes, because it may form an oxide layer due to oxidation.

Robert Francis

Thu Mar 29 2018, 05:11PM

Registered Member #2355 Joined: Thu Sept 10 2009, 09:12PM
Location:
Posts: 71

I'm going to use epoxy to pot the entire marx gen. Should I still worry about the caps in the marx gen discharging?

Obviously I can't use a shorting bar cause the caps will be buried in epoxy, 600volts/mil

Flachzange

Thu Mar 29 2018, 05:41PM

Registered Member #61569 Joined: Sat Apr 15 2017, 05:12PM
Location: Germany
Posts: 28

Professional made impulse generators are made with at least 3 ways to discharge it.
The first is a discharge resistor that gets connected with switch when the power is turned off.
It discharges the generator within several seconds completely.
The second one is either a string or band that gets pulled through the generator that shorts all the capacitors hard without any resistors. This band or string is seperated in an insulating part and a metal part. So when the generator is operating the insulator is pulled through the generator. When it gets discharged the metal part is pulled through the generator.
And the third safety feature is always a grounding stick.

So for the diy generator i would at least use a discharge resistor that gets connected somehow.

teravolt

Thu Mar 29 2018, 08:08PM

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

I prefer my marxes open for servicing. Shorting each and every cap is necessary in the event a inductor or resistor opens. Also charge can take more time to drain away where large with charge resistances or be pushed to other parts of the marx during shorting. HV requires respect, and wisdom is learned from being shocked

Robert Francis

Fri Mar 30 2018, 05:19PM

Registered Member #2355 Joined: Thu Sept 10 2009, 09:12PM
Location:
Posts: 71

I was thinking, at the first stage of my marx the voltage is as high as 42kV. I could use a diy knife switch (those for purchase are too small to use with high voltages) and wear lineman gloves when manually switching the marx gen off.

With a SPDT knife switch I could put a bunch of resistors on one path and a direct wire for the other.

What resistors would you guys recommend?

teravolt

Mon Apr 02 2018, 04:31PM

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

how many uf in each cap? you probably don't want to be near it. if it is big a gas switch would be fine. if it is a small diy then let it self break by making the gap at the bottom smaller. do you have any pictures?

Robert Francis

Mon Apr 02 2018, 10:46PM

Registered Member #2355 Joined: Thu Sept 10 2009, 09:12PM
Location:
Posts: 71

The caps are rated to 2200uF

The first stage in the marx gen is a homemade trigatron. I have one piece of g10 tube, electrodes on each end and one in the middle that will be used to trigger the first stage.

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