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4hv.org :: Forums :: High Voltage
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Small Marx Generator (and questions)

Author Post
thedatastream
Mon Jan 29 2007, 02:55PM Print View
Registered Member #505
Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329
I've just built a small Marx generator with the following specifications

  • 2 x 3n9 1600V PE film in series to make 2n 3200V capacitors
  • 8 stages of these caps being charged by 2 x 470k resistors in series
  • All above parts scavenged from junk box
  • Donated flyback transformer, driven from a 555 timer + MOSFET as per Mike's site

Some nice 2" sparks have resulted

Problem is repeatable triggering (suprise). I was thinking of using some gas discharge tubes in series as the spark gaps as I've got access to several different types. I would set the first gap to be a slightly lower voltage than the rest by using different tube voltages to get better triggering.

Has this been tried before? I haven't found any references on t'internet. Failing that, I shall have to bodge a trigger coil together.

Thanks
James
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Dr. Shark
Mon Jan 29 2007, 04:05PM
Registered Member #75
Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711
Hi James, I don't think you would have much sucess with those tubes, since they would prevent the sparks from "talking" to eachother. Ideally the first gap braking down creates UV light that ionises air molecues in the other gaps. This helps to achive synchronous firing of all gaps.
You also only need a trigger coil if you want to control the firing time and not just have the generator fire away steadily. From my experience erratic triggering (I guess that's the problem you are describing?) is due to pointy bits in the wiring spraying off charge where they are not supposed to. Also check your spark gap spacing.
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thedatastream
Mon Jan 29 2007, 04:48PM
Registered Member #505
Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329
joe wrote ...

Hi James, I don't think you would have much sucess with those tubes, since they would prevent the sparks from "talking" to eachother. Ideally the first gap braking down creates UV light that ionises air molecues in the other gaps. This helps to achive synchronous firing of all gaps.


I was looking at this page and it seems to contradict itself in terms of the method by which the other gaps trigger.

wrote ...
This effectively puts the bottom two capacitors in series, overvoltaging the next gap up, which then puts the bottom three capacitors in series, which overvoltages the next gap, and so forth. This process is referred to as "erecting".


...and then...

wrote ...
The traditional Marx generator operating in air has all the gaps in a line with the electrodes operating horizontally opposed. This allows the UV from bottom gap to irradiate the upper gaps, reducing their jitter.


Which statement is correct?

joe wrote ...
From my experience erratic triggering (I guess that's the problem you are describing?) is due to pointy bits in the wiring spraying off charge where they are not supposed to. Also check your spark gap spacing.


Sounds like I need to improve my spark gaps for more reliable firing - some careful adjustment may well be necessary.

Thanks
James
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Steve Conner
Mon Jan 29 2007, 05:22PM
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Stray capacitance to ground is the only thing making the other gaps in a Marx bank fire when it "erects". There's a formula for the minimum stray capacitance for acceptable triggering. If that is marginal (it usually is) then the UV light from the other gaps is needed to help out.
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Steve Ward
Mon Jan 29 2007, 06:14PM
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
Stray capacitance to ground is the only thing making the other gaps in a Marx bank fire when it "erects".


I always thought that too, but wasnt sure because the stray C is often really small it would seem. I wonder if there is any benefit to trying to increase it at all since some of my generators had problems with firing properly from time to time.
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thedatastream
Thu Feb 01 2007, 08:28AM
Registered Member #505
Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329
Well I've tidied up my spark gaps. They used to be bits of bent wire which resulted in some coronal discharge around the points and across the gaps.

I've removed these and replaced them with bits of wire with large blobs of solder on one end as below.



The Marx now fires repeatably once all the gaps have been adjusted to prevent premature firing. However, after a few strikes the bottom cap starts a thin arc across it that I have to blow out. I'm presuming that is because my ballast / charging resistor at the bottom of the Marx chain is too low in value (its 1Meg). I shall try a higher value resistor later.

James
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Dr. Slack
Thu Feb 08 2007, 02:03PM
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1578
You might try fewer stages with more series caps per stage. It makes everything a bit less twitchy, increases the effect of the stray capacitance, increases the tolerance you are allowed on the gaps, all good really. That's if you have the voltage to charge them with.
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