This was my first high voltage circuit that eventually led me into building other high voltage generators, supplies and Tesla coils.
A Marx generator works by the principle of charging up a number of capacitors in parallel and when the voltage is high enough to break down the spark gaps, the capacitors will be discharged in series. When a Marx generator fires, it is said to be erected.
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A Marx generator can work as a electromagnetic pulse generator, at high energy levels, if it is not shielded and grounded correctly. You can risk damaging electronic equipment if this circuit is not constructed or operated in a safe manner with these precautions in mind.
In the construction of the Marx generator it is very important to give great thought to distances that works as insulation between stages and components. An advantage against spark gap jitter can be achieved easily and for free by designing all the spark gap in one line where they can see each other. The UV light from the first spark gap will help break down the following spark gap and so forth.
The ideal Marx generator circuit will deliver n times the input voltage with n stages. So with a 4 kV supply and 6 stages, ideally I should have 24 kV output.
Each stage of 1 nF is charged to 4 kV and gives me 0.008 Joule energy per stage. With all 6 stages in series the capacitance is now 1/6 and the voltage is 6 times higher. So the erected capacitance is now 167 pF with 24 kV across it resulting in 0.048 Joule discharge energy.
|Voltage supply||4 kV at 20 mA / 20 kHz from a solid state neon sign transformer.|
|Stage design||6 stages of 2x 1M5 3500 V charging resistors and 1 nF 7500V capacitors|
|Discharge||24 kV at 0.048 Joule|
|Longest arc||20 mm sparks|
14th April 2008
The 4 kV supply used is a solid state neon sign transformer that delivers 20 mA. The output comes at 20 kHz from the switching and normally is no problem for a neon tube, but I need to rectify it to use it in the Marx generator as DC supply is needed. A string of 10 1N4007 diodes are used for a 10 kV rectifying diode.
3500V 1M5 metal-oxide resistors were used with two in series for each stage and 1 nF 7500V ceramic capacitors, this gave a good head room for a 4 kV stage voltage.
The long leads on the capacitors were used as the spark gap by soldering them to the string of resistors as far up the legs as possible and then bending them into forming spark gaps.
A Marx generator in this size and level of supply voltage is a very forgiving and hard to break circuit. This makes it perfect as a entry level circuit for high voltage experiments.
The few number of components makes it cheap and it is easy to source the high voltage capacitors on Ebay.
The sparks generated from the Marx generator is very loud and it is easy to gain higher voltages and thus longer sparks by just adding more stages. A circuit that is easy to upscale just by adding more stages and where output is quickly calculated with number of stages.
Definitely a must-build circuit for everyone with a interest in high voltage generation and experimentation.
28th April 2008
7 thoughts on “24 kV Marx generator”
Regarding the theoretical maximum output voltage and real world results, can stray inductance in the discharge path impact the final output voltage and max spark length?
I know parasitics limit the peak current but I am wondering is it the same for the peak voltage.
I think it is fair to say that 20 mm spark is about as good as it can get with a theoretical output of 24 kV, the real output voltage depends on the firing voltage of the spark gaps and energy loss in the spark gaps.
To determine air break down voltage there is some sets of formulas for sphere to sphere, rod to rod, these types to a plate etc. The 1 kV to 1 mm estimation is fair enough for irregular discharge points and little to none accuracy on setting all spark gaps the same.
So the parasitic inductance of the discharge path doesn’t really effect the final output voltage, just the peak current?
It’s just that I have a marx generator sitting on my desk now and was wondering in general if parasitic inductance comes into play since it would mean a slower rise time.
can i get your formulas which you use it ?
thanks for your help
Hi Mohammed Zardawi
You calculate the charge time from the RC time constant of each stage and the voltage multiplication is just the number of steps times the input voltage.
Hi, Are Knox Generators powered by DC voltage or AC voltage?
They need DC voltage as you are charging up capacitors to the breakdown voltage of the spark gap. In this project I use 10 diodes to half-wave rectify the 20 kHz AC output of the high voltage power supply.