A Marx generator charges capacitors in parallel then discharges them in series.

I read that the voltage across each capacitor accumulates when they are connected in series, which causes subsequent spark gaps to break down after the initial gap is triggered.

How do I choose the voltage ratings for the capacitors?

  1. Should I use capacitors of higher voltage ratings towards the load in order to factor in voltage accumulation?
  2. Should there be a minimum voltage rating (or charging voltage) for all capacitors? This is considering how the total voltage of the first two capacitors needs to exceed the breakdown voltage of the smallest possible air gap.

1 Answer 1


Voltage rating equal to input. The "accumulation" happens in the instant of everything sparking over, effectively connecting the capacitors in series; no individual capacitor sees more than its initial voltage.

Note that capacitors may see reverse voltage (due to uneven discharging, or ringing), so a symmetrical voltage limit is desirable. Capacitors are sometimes rated DC only (electrolytics, some film types). Easy enough, choose caps with an AC rating.

Probably for values you'll want to use here, it's not even an issue -- ceramic or pulse-rated film will be fine.

  • \$\begingroup\$ It seems the spontaneous breakdown of subsequent spark gaps is a result of higher voltage, otherwise, the triggering mechanism cannot be explained. \$\endgroup\$
    – coulombs
    Jul 3, 2022 at 23:19
  • \$\begingroup\$ Not so. Just before the instant of total breakdown, all voltages are very close to threshold. Then one fires, emitting intense UV radiation. This ionizes nearby air, and may cause photoelectron emission from other electrodes. This lowers the threshold for nearby spark gaps, causing a chain reaction. Marx generators work poorly (if at all) when shielded spark gaps are used (no UV/light emission). \$\endgroup\$ Jul 3, 2022 at 23:30

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