Incredibly high voltage via voltage-multiplication.
27 Aug 2003
A Marx Generator is an extremely dangerous High-Voltage pulsed power voltage multiplier device. It CAN kill! Firstly, make sure that no one can come in contact with this - particularly curious onlookers, and yourself. Separating the potential victims from any possible contact with the high voltage is really the only foolproof way of protecting against fools or the unaware. People not familiar with high voltage phenomena can be incredibly naive. Always keep a safe distance from your Marx Generator!
A Marx generator deals with already high voltages, and Multiplies it to an incredibly high level. 100,000 volts will jump about 11cm and 1 million volts will jump over 1 metre! This distance occasionally varies unpredictably with humidity. 1kV jumps about 1.1mm. Don't forget that the Marx generator is a pulsed power device and stores its high voltage charges in capacitors. This makes it extremely dangerous! Always remember to discharge all capacitors with a proper discharge probe before making adjustments. Electrical discharges in air are a producer of ozone which may be a health hazard. Ozone in large amounts is dangerous. The discharges also produce a lot of harmful UV radiation! Protective eye-wear is recommended for extended periods of time. Ear protection is also recommended. The discharges also produce significant Radio Frequency Interference (RFI)...
Do you like the idea of tesla coils and other high-voltage sparking stuff, but don't have the time, money or patience to build something that elaborate? If cheap loud and long sparks are your aim, a Marx generator delivers them with excellent simplicity. Just a slack handful of bits and pieces will build you a respectable noise and light show that can be powered nicely with a flyback or similar source of kilo-voltage DC. Simply said, a small Marx generator is a fun project that can make big, fat, long, noisy sparks, and can be built very quickly and cheaply.
Basically, a Marx Generator is a simple way to generate lots of voltage. A Marx generator is a type of voltage multiplier. It isn't a new idea - charge up a stack of capacitors in parallel, discharge them in series. When you use high voltage capacitors and spark gap, the resulting sparks are quite impressive. Unlike Cockcroft-Walton multipliers, the Marx Generator needs no diodes, and only uses one cap per stage. This reduces their cost enormously, at the expense of the mechanical complexity of the spark gaps.
Marx generators make excellent lightening simulators. They are also very hostile to surrounding equipment, as my pathetic power supply well knows. (It blew it up after a few minutes...) Be careful they can bite very hard too. Always remember to discharge each capacitor carefully before handling it. These caps can store a serious amount of energy!
How it works
The Marx generator consists of an array of resistors, capacitors and spark gaps arranged as above
The capacitors (in green), are charged up in parallel via the 1M (one megohm) resistors, so they each become charged to the input voltage from the high voltage DC power supply unit (with a 10M resistor for current limiting and protection). When the first (leftmost) spark gap breaks down, the voltage across the next stage increases, causing it to break down, and so on for all the other gaps. When all gaps have broken down, the low impedance of the ionised air in the sparks effectively connects all the charged capacitors in series, multiplying the input voltage by the number of capacitors (or stage). The ionised air path has sufficiently low resistance that the charge resistors don't have any significant effect. Basically, a smart way of charging capacitors in parallel, and discharging them in series, creating extremely high voltages.
The 10MegaOhm resistor also has a ballasting effect. This is needed to avoid a continuous arc forming across the first gap after it fires (this prevents further firing). The value will depend on the type of power supply used. High values will reduce the maximum spark repetition rate, so you may be able to get more 'bangs per second' by reducing the resistor value.
Although it is possible to make a Marx generator with just an array of resistors, capacitors and spark gaps, it can be hard to make it fire reliably, as it will depend on the breakdown voltage of the spark gaps, and there can be a fine line between not firing and firing before all the capacitors are fully charged. One solution is to initiate the breakdown of the first gap mechanically, e.g. by waving a suitably insulated screwdriver between the electrodes, or pushing the first gap contacts together with an insulator.
You only need...
3. Solder, Wires, etc...
4. Power supply unit
The most important thing is to make sure that the components are suitable for the input voltage used. This includes the resistors - normal 1/8 or 1/4 watt carbon or metal film resistors are not suitable for high voltage uses like this. Larger (1 or 2 watt) carbon resistors, or high-voltage metal glaze resistors should be used. Alternatively several standard resistors can be used in series, calculated such that each resistor sees no more than its voltage rating (typically about 500V). For example, ten 100K 1/4 watt resistors could be used in place of each 1M HV resistor. The value of the 1M resistors is not too critical - higher values will increase recharge time, lower values will increase losses as some of the spark discharge leaks back through the resistor chain, especially with lower value capacitors.
Capacitors should ideally be ceramic types, as these are best suited to the fast pulses in a Marx generator - polypropylene pulse-rated caps would probably also work well, but are more expensive, and harder to find with high voltage ratings. Use the highest voltage types you can find. Larger values will give fatter sparks, but take longer to recharge... You can often run these in excess of their rated voltage, but obviously this runs an increased failure risk.
An input voltage of about 4-8KV is recommended. Above this, many problems will detract from the 'quick and dirty' approach, requiring much greater care in construction to reduce corona losses from sharp edges, multiple series resistors would be needed, and the capacitors will be harder to find. The high impedance of the charging network mean that corona losses can significantly degrade performance above about 5-6KV unless care is taken to avoid any sharp edges - e.g. by filing the wire ends and joints smooth.. soldering blobs of solder at sharp parts ... potting the whole setup in insulation...
A simple design requires a low-current source of high voltage DC in the range 4-8KV. Small, low-current HV supplies from things like ionisers, photocopiers and laser printers should be suitable. The number of stages you use will determine the output voltage, and hence spark length.
Marx Generator 1
Monday, 27 Oct 2003
This is my first Marx Generator. Marx Generator 1, model 1.
It's a bunch of wires, resistors, and capacitors soldered up together. Very simple. These are 1nF(label on the cap) or 810pF (labeled at the shop where i got them from) (Quite close values anyway) 15kV ceramic caps. They have a nice insulation and generally should be good caps. It's a pity I only got 7 of them. All resistors are 1W 1MegaOhm resistors. At these lower voltages, arc-over should not be a problem... I hope.
Here are the caps, resistor and power supply. The resistor is a 1W 1M resistor. The cap on the left is the 15kV 810pF cap (the one I am using) and the cap on the right is a 30kV 510pF cap. I am using the 50W Liyoda electronic halogen transformer (read more about flyback driver circuits at my flyback page here) to drive the flyback which drives the Marx generator. I am using the small flyback (the one closer to the resistor). It's from a black and white TV and with my wimpy set up, it outputs around 5kV at around 10mA DC (estimated. I need a HV probe..). Perhaps even less.... i need a better power supply! My caps are definitely overrated (at 15kV!) , but that is not a bad thing :).
Here is a close-up of the construction effort. Notic the excess solder usage and the loop-surface-blob gaps. Although not important for correct operation, they are a nice professional touch, making everything more reliable and repeatable when measured. It looks nicer than bent bits of wire too. This all-smooth-surface construction proves to be very useful :)
Gap spacing and charging current are important. The gaps should all trigger at once. In practice the first gap fires and the over-voltage avalanches up the stack. Therefore, I made all the gaps above the first wider, and controlled the trigger voltage from the first gap. If your charging current is too high the first gap may light continuously. This was quite a problem for me, but for now, I'm using a piece of plastic to trigger the first gap...
My current setup (model 1) has 7 stages with an estimated 4kV+- per stage, equates to about 30kV output. Which each cap storing 810pF, and total capacitor charged at 30kV, the whole discharge is 115.7pF at 30kV... 1/2 x 0.0000000001157 x 30000^2 = 0.05207Joules per bang.. not much actually, but produces large healthy 3cm arcs as can be seen in the photo below! First light! It works :D. It's loud, nasty, and really awesome. The blurry orange object on the right is a piece of plastic i used to trigger the spark gap. Notice how bright the arcs are :D
Unfortunately, my 50W power supply unit died soon after this photo was taken :(. The flyback, however, was unharmed. That's all for now, until I get a new power supply. I am considering adding more stages to it.
Marx Generator 1 Updated
15 Mar 2004
Due to the sudden death of my flyback driver PSU, I had to discontinue my high voltage flyback experiments, and this lead me unable to charge up the marx generators capacitors in a suitable way... until now...
I have recently acquired a SONY brand Switch Mode Power Supply unit. 12V at 34A! This is MORE than enough power for driving my flyback! (For more information on the flyback charging circuit, check out my flyback driver page). I used a small flyback from a black and white TV... it's a 'modern' type flyback and is already rectified (outputting DC). Everything has remained the same as Marx Generator 1, a new higher voltage charging supply is used!
So I plugged it in...
Much better! The voltage is MUCH too high! This is about the maximum I can open the spark gaps.. more than that the resistors start arcing over, the capacitors start arcing over and .... well just about everything arcs over... the spark gaps are around 1.3cm... which suggests around 15kV charge in the capacitors... 15x7stages yields 105kV! That's about 0.56J per bang. Very loud and bright. It fires itself about twice a second. In the photo, the main gap is 7cm, but the arc can jump a 10cm gap or more! A lot of corona is evident (From the hissing sound and the large amounts of ozone produced), and a lot of energy is wasted there. I think I need higher voltage capacitors... Immersing it in oil would reduce much of the insulation problems, but I have yet to tried it.
This is a success so far!
Back to main page
(c) Gao Guangyan 2011
Contact: loneoceans [at] gmail [dot] com