Jochen's High Voltage Page

Marx generator

The Marx generator is a voltage multiplier working on a completely different principle than all other multipliers presented in here. Roughly, the principle is: charge a set of capacitors in parallel, discharge in series. One way to achieve this would be straight-forward via lots of switches. The Marx generator, however, is a clever way of doing this switching automatically, via spark gaps.

Schematic 4-stage Marx generator and actual realization, photographed in the moment of discharge.

The above figure shows the circuit diagram of a Marx generator. The values of the components are not critical, and therefore not given. Obviously, as long as the spark gaps are non-conducting, all the caps are charged in parallel from the 30kV source. Across each gap is the voltage to which the caps have been charged so far. Now they are adjusted in such way, that they fire and become conducting at just below 30kV. When this happens, the gaps act as a very low resistance compared to the charging resistors, so that the caps are suddenly all in series, and their added voltages appear at the output. Of course, even without load on the output, the caps will discharge more or less quickly through the resistors. That´s why the Marx generator is a pulse generator, producing short, but high voltage pulses.

If you look closely, the process of all the gaps becoming conducting at the same time is much more complicated. As the pulse is so short, all the gaps must fire at exactly the same moment - but this is automatically assured: as soon as G1 fires, G2 sees twice the charging voltage, making it fire quickly, and so on with the rest of the gaps. To ensure that this process starts from the very bottom, G1 is usually set slightly closer than the rest of the gaps.

Marx generators are mainly used to simulate the effect of lightning on technical high voltage components. Voltages above 1MV are easily reached. An advantage over cascades is that there is only one cap per stage (and no diodes at all, making it a rather robust device). However, there is one drawback: the maximum spark length is reduced with respect to DC, because a spark needs time to develop.

The simple 4-stage model shown in the photo was built with 2nF caps (homebrew, estimated to 30kV) and 2x 10MOhm resistors between stages. Charging is done with 30kV through a 200MOhm resistor to protect the DC source from any overvoltage kickbacks. The resulting 120kV pulse is enough to jump across a 10cm gap. Note that the same DC voltage (see Super-cascade) is able to jump a gap twice as large.

Capacitors for a Marx generator must ne suitable for pulsed applications. Many cheap high voltage caps (MKT-types) are not suitable. Caps with PP or PE dielectric are good. The resistors can be a lot smaller than in my example, actually they might be substituted by chokes. A large charging resistor is recommended, however, as it protects the DC source and makes the generator controllable by slowing down the repetition rate.

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