Marx Generator misfiring

[Andy Kay]

Hi guys, I've been busy building my first ever Cockcroft-Walton voltage multiplier and my first ever Marx Generator. I don't know if it's permitted to supply video links here but if so then I can show provide a link to a YouTube video that shows the setup. My question concerns misfiring. I'd like to understand what is happening in the Marx Generator when it occasionally misfires, with the minor spark gaps showing weak sparks (or maybe they are just plasma paths). I can't see any obvious shorting pathways, so I'm wondering where all the energy is going on the misfires. Thanks for reading.

[Mads Barnkob]

Please do post the video, if you put in a youtube link, it is automatically embedded so you can play the video directly here in the post.

Misfiring is often due to imbalance in charging resistors/inductors, but lets get a look at your setup before troubleshooting more

[Andy Kay]

Please do post the video, if you put in a youtube link, it is automatically embedded so you can play the video directly here in the post.

Misfiring is often due to imbalance in charging resistors/inductors, but lets get a look at your setup before troubleshooting more

Thank you Mads.
The setup consists of a car battery at 13V driving a two-port CCFL inverter. One port of the inverter is driving a T5 fluorescent lamp in cold cathode mode, the other is driving a 13-stage CW voltage multiplier. I have tapped off the CW at the fifth stage to supply an 11-stage Marx generator. The gap is something like 90mm, but when I set it at 100mm the misfiring begins in earnest.

Here is the link:

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[Andy Kay]

Misfiring is often due to imbalance in charging resistors/inductors,

The resistors are all 1W 1M 1% metal film. I guess I'm really asking about the mechanism of failure when the main gap gets too large. I would have expected all the capacitors to become fully energized just as before, and so the energy should be finding some easier path than the main gap, but I don't see any evidence of that. Perhaps the capacitors aren't getting to full charge, but I don't see how that would follow from just increasing the main gap. Still perplexed.

[Michael_DA]

Hello Andy,
how does the schematic of your Marx generator look like? To my knowledge, the most common is this one, but in principle there are several variations of it, so thats why I ask.



Normally, the output voltage of a Marx generator is the measurable voltage across the load capacitance (in the circuit diagram named C_b) This has the following shape (without a flashover of your main gap. With a flashover, the voltage collapses very quickly.):



This results from the fact that the surge capacitance (C_s) is discharged into the smaller capacitance C_b via the resistor R_d, resulting in the rapid increase of the voltage. The time of the rise can be increased or decreased by the variation of R_d. If the voltage is not large enough to produce a flashover in your main gap, the load capacitance (C_b) will discharge via the resistors R_d and R_e, causing the slower, falling voltage waveform. The drop in voltage (without flashover) is significantly slower, since the discharge resistance R_e is usually chosen to be greater than the resistance R_d.

For a better understanding, I have painted here the two possible paths. The red path is the case if your main gap does not flashover. And the blue path when your main gap rolls over. (Unfortunately, I am not a very good draftsman, but I hope it is obvious  :D).



I hope I could help you.

Kind regards,
Michael