Forget Tesla coils and check out Marx generators

Article By : Bill Schweber

Building a Marx generator is easier than doing the popular the Tesla coil, since it doesn't require windings or as many high-voltage components.

Low-voltage designs with rails in the single digits get a lot of attention these days, for reasons I don’t need to detail to this audience. Still, there are many situations where rails at hundreds of volts are needed, such as EVs. There are also many important uses for even higher-voltage systems ranging into the thousands of volts, such as physics experiments, safety tests, and even some mass-market consumer products.

I’m always fascinated by the clever ways engineers and scientists have devised to increase a supply voltage by orders of magnitude. You’re undoubtedly familiar with and may have even built a Tesla coil, used for dramatic science demonstrations as well as serious research. (There are many websites showing how to build your own—with suitable safety-related caveats, of course.) As its name implies, the core design uses step-up transformer coils. That’s simple enough in principle, but the “devil” and danger is in the details of the implementation, of course. Another high-voltage scheme is the flyback converter, widely used in CRT-based TVs until they became obsolete, but still used in other applications.

There’s yest another high-voltage topology which is much less known but often used when high-voltage pulses are needed: the Marx generator. It’s not new, as it was first described by Erwin Otto Marx in 1924. Marx generators generate a high-voltage pulse from a low-voltage DC supply, and they are used in high-energy physics experiments, as well as to simulate the effects of lightning on products such as power-line switchgear and aviation equipment.

As with the Tesla coil, the concept is simple, as is the schematic diagram, Figure 1. It operates by charging a number of capacitors in parallel, then quickly connecting them in series. Initially, the capacitors are charged in parallel to voltage VC by a DC power supply through resistors RC. The individual spark gaps are “open” as the voltage VC across them is below their breakdown voltage, thus allowing to capacitors to continue charging. The last spark gap isolates the output of the generator from the load.

Figure 1 The Marx generator schematic diagram shows its simplicity, as a cascaded series of repeated spark-gap, resistor, and capacitor stages. Source: Wikipedia

Once the charged voltage is high enough for the first spark gap to trigger (breakdown), the critical sequential action begins. The short-circuit, which now occurs across the gap, puts the first two capacitors in series, so there is a voltage of about 2VC across the second spark gap. Now the second gap breaks down and adds to the third capacitor, with a cascade of  breakdowns across all of the gaps.

To generate the final spark, the last gap connects the output of the capacitors to the load. In principle, this output voltage is the sum of the voltages across all the capacitors; in practice, it is somewhat less. One of the interesting features about this design is that the voltage across each of the charging resistors is equal to the charging voltage and not the final voltage even as the array charges up; this greatly simplifies component procurement and layout, and also reduces costs.

How much voltage can you generate with this topology? The answer is simple: as much as you want and can afford. It’s used for megavolt-level research-laboratory systems, Figure 2, but you can also generate a few thousand volts from a 1.5 V AA battery.

Figure 2 This megavolt Marx generator used for testing high-voltage power-transmission components at TU Dresden, Germany. Source: Wikiwand

Looking at the schematic diagram, bill of materials, and construction details, it seems that building a Marx generator is easier than doing the popular the Tesla coil, since it doesn’t require windings or as many high-voltage components, Figure 3. There are many web sites showing how to build your own (of course, the usual high-voltage warnings apply).

Figure 3 Compared to the Tesla coil, the Marx generator has a simpler physical construction – but there are still high and dangerous voltages, of course. Source: Electric Stuff – UK

You can also buy one in kit form (the main PC board only, or board plus all components), Figure 4, from Eastern Voltage Research. This unit produces 3 to 4 inches of output arc and 90-kV maximum theoretical output voltage depending on input voltage source, spark gap tuning, and atmospheric conditions. (Note that the company is high-voltage-device agnostic, as they also offer Tesla-coil kits.)

Figure 4 You can also buy a tabletop Marx generator as a ready-to-assemble kit, yielding an output arc up to 4 inches and voltage as high as 90 kV. Source: Eastern Voltage Research

Of course, there are other ways to boost low voltages to much-higher ones. Voltage-multiplier circuits (doublers, triplers, and cascades of these) can also reach thousands of volts. Like the Marx generator, these “multiply” the source voltage by charging capacitors in parallel and discharging them in series. One important difference is that voltage multipliers are powered with alternating current and produce a steady DC output voltage, whereas the Marx generator produces a pulse. Also, there is no open ‘”spark” with the multiplier circuit, which makes it more suitable to use in consumer or mass-market products where higher voltages are needed.

Have you ever built any of these higher-voltage projects, either for personal use or commercial production? If you came from a lower-voltage background, what were the most important lessons you learned about component selection, procurement, or use? What about physical layout?

 

This article was originally published on EDN.

Bill Schweber is an EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features.

 

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