There are many ways to create high-voltages and pulses; the Marx generator is a simple one in concept and practice.
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 which require a relatively high voltage to establish an electrostatic field, such as needed for a dust collector.
Of course, the use of high voltage is not limited to practical appliances but also entertainment which can demonstrate scientific principles. For example, there’s the “plasma ball” (coincidentally invented by Nikola Tesla) which uses a voltage source of three to five kilovolts at around 30 to 40 kilohertz to energize the noble gas in a partially evacuated glass globe (Figure 1). The plasma tentacles or filaments leap from the center electrode to the globe’s surface, and their color depends on the gas or gases in the globe.
It’s always fascinating to see 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 many critical safety-related caveats, of course.
As its name implies, the core design of the Tesla coil uses step-up transformer coils. That’s simple enough in principle, but the “devil” and danger is in the details of the implementation. Another high-voltage scheme is the flyback converter, a transformer-based high-voltage DC source topology. It was used in CRT-based TVs until they became obsolete, but it is still widely used in many other applications due to its efficiency and low parts count.
Beyond Tesla Coils
There’s yet another high-voltage topology that is much less known but often used when high-voltage pulses — not a continuous voltage — 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 you can see from the schematic diagram in Figure 2. 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.
Once the charged voltage is high enough for the first spark gap to trigger (breakdown), the critical sequential action begins. The short circuit that 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 sequentially breaking down 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 of 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 3), but you can also generate a few thousand volts from a 1.5 V AA battery (admittedly at a very low current of just a few microamps, but that’s irrelevant for many electrostatic applications, see “Instructables” in the References).
Looking at the schematic diagram, bill of materials, and construction details, it seems that building a Marx generator is easier than doing the popular Tesla coil, since it doesn’t require windings or as many high-voltage components (Figure 4). There are many websites showing how to build your own (see “Electric Stuff” in the References).
(Of course, the usual high-voltage warnings apply — strange and unexpected things happen to basic materials such as insulators and pointed conductors as the voltage increases, even if the current is low).
You can also buy a Marx generator in kit form (the main PC board only or board plus all components) from Eastern Voltage Research (Figure 5). 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).
Of course, there are other ways to boost low voltages to much-higher ones. Voltage-multiplier circuits (voltage 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 relatively steady DC output voltage, whereas the Marx generator produces a pulsed output, somewhat analogous to a lightning strike. 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.
Related EE World Content
Working with higher voltages, Part 1: Voltage boosters
Working with higher voltages, Part 2: Voltage multipliers
Basics of voltage doubler circuits
Flyback power converters, Part 1: Basic principles
Flyback power converters, Part 2: Enhancements and ICs
Key considerations when integrating high voltage dc-dc converters into critical applications
Switched capacitor power conversion for electronic systems
FAQ: What is a charge pump and why is it useful? (Part 2)
FAQ: What is a charge pump and why is it useful? (Part 1)
- Sciencing, “How Does a Plasma Ball Work?”
- Osmo, “How Does a Plasma Ball Work?”
- Autodesk Instructables, “Make a Plasma Globe Out of a Light Bulb!”
- Wikipedia, “Plasma Globe”
- Instructables, “Build a Simple Marx Generator”
- Wikipedia, “Marx generator”
- Electric Stuff (UK), “ ‘Quick & Dirty’ ” Marx generator”
- Eastern Voltage Research, “Marx Generator 2.0 Kit”
- Rensselaer Polytechnic Institute, “Marx Generator”
- Wikiwant, “Marx Generator”
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