Building a high power voltage multiplier

 

A simple high-voltage circuitry project is the Cockroft-Walton voltage multiplier. I first created this as a demonstration for a class in high school, but I’ve altered it over the years in order to improve its performance. The nice thing about this project is that it can be created entirely using cheap, store-bought components—diodes and capacitors—and so it is thus relatively easy to ensure that it will work and perform at the voltage estimated. I got my components from West Florida Components, and the total cost of everything was under $10. A good tutorial that gives possible specifications for the components can be found on Instructables.

The total voltage drop across many capacitors in series is equal to the sum of the voltage drop across each component—this is a consequence of Kirchoff’s circuit laws, and can be mentally visualized as a charge on one end of a capacitor displacing and equal but opposite charge on the opposite plate, which in turn displaces an opposite charge on any other capacitor plates to which it connects, and so on. The Cockroft-Walton multiplier can be visualized as a fancy way of putting a bunch of capacitors in series and charging them so that they each have a voltage drop of 120V, resulting in a total discharge voltage of 120 V times the number of capacitors. This output is roughly DC, and it has a much lower current than the device draws from the mains, hence preserving energy conservation since power = (voltage)*(current). A simple diagram of the half-wave CW multiplier looks like this:

A circuit schematic for a half-wave Cockroft-Walton voltage multiplier.

A circuit schematic for a half-wave Cockroft-Walton voltage multiplier.

The manner by which the CW multiplier can charge each capacitor separately to 120 V is essentially by charging them in parallel and discharging them in series. The concept borrows from the design of a basic half-wave rectifier, which uses a diode and smoothing capacitor to convert the positive portions of AC sine waves to a smooth-ish DC current. The idea is that the first stage in the circuit (capacitor 1 and diode 1) converts the AC to an approximately constant DC signal, which then gets fed forward through diode 2 to the right plate of capacitor 2. During the first positive cycle, that capacitor charges to +120V. During the “off” cycle (the negative portion of the AC sine wave gets blocked by the first diode), the second capacitor discharges through diode 3 into capacitor 3 because, during the off cycle, there’s now -120V on the bottom plate of that capacitor, leading to a potential difference that allows charging. During the next “on” cycle, the current ignores capacitor 3 because it is fully charged (and so it essentially acts like a break in the circuit there), and so now capacitor 4 gets charged instead. During the next off cycle, capacitor 4 discharges through diode 4 to charge capacitor 5, and the cycle repeats itself until, after (number of capacitors)x(charging time) all the capacitors are charged.

There are several equivalent ways of visualizing what is going on in the CW circuit, but the key things to remember are that the capacitors store the charge (differential) and the diodes force the AC to feed forward and charge each capacitor in sequence. The charging time can be adjusted by adjusting the time constants for each capacitor in the circuit relative to the AC cycle frequency (60 hz in the US).

The device would build up charge twice as quickly if one instead uses a full-wave design (which is analogous to a full-wave bridge rectifier), because it would then take advantage of the negative swing of the AC sine wave, which gets lopped off by the first diode in the half-wave version.

I the video above, I have added a switch and fuse for safety reasons (visible in the upper-left hand portion of the screen; I used a plastic lid as a base for the two components). In the first cut, the ~1 mm spark regularly produced by the device is visible. This spark can be used to drive continuously an 8 inch fluorescent tube (shown in the second section), but, curiously, the frequency of the pulses through the fluorescence in the tube depends on the proximity of other conducting objects—in the fourth clip, it is apparent that touching pliers to the glass reduces the frequency of the pulses, rather than increasing them as I would have expected. My best guess for the cause of this effect is charge build-up on the glass interior beneath the metal, leading to low-frequency discharge for the same reason that high-voltage capacitors decrease the sparking frequency in the primary circuit of a spark-gap Tesla coil. The last clip shows the device discharging through a 1 inch xenon flash tube salvaged from a disposable camera. The firing frequency is low due to the relatively large distance that the spark has to cover, despite the low dielectric constant of xenon gas. In other tests, I’ve noticed that large spark gaps that require charge build-up over periods longer than the ~3-5 s for the flash tube will generally result in short circuits occurring upstream in the capacitors in the CW, which probably cause damage to the solder joints and possible the capacitor ceramic due to dielectric breakdown.

Cockroft-Walton generators have a special significance in the history of physics because they were used to generate steering currents in one of the earliest particle accelerators, enabling their creators to win the first Nobel Prize in Physics ever given to a collider project. For this reason, one of the first large-scale CW multipliers (manufactured by Philips Co.) is prominently displayed in the National Science Museum in London:

 

A Cockroft-Walton generator built in 1937 by Philips of Eindhoven. National Science Museum, London, England.

A Cockroft-Walton generator built in 1937 by Philips of Eindhoven. National Science Museum, London, England. Image from Wikimedia Commons.

 

 

Ignition Coil Spark Generator

This is probably one of the easiest high voltage projects I have ever stumbled across. For the cost of an ignition coil and a trip to RadioShack, I was able to intermittently interfere with antenna reception in my household for years.

The cool thing about car ignition coils is that they are essentially miniature solid-state Tesla coils—just like their older cousin, they essentially convert a low voltage, high current power source (such as a 12V, 10 amp car battery) to a high voltage, low-amperage current (such as that irritating 1000V, .003 amp shock you get from a doorknob). Purists will stop me here: technically speaking, Tesla’s original patent called for an open-air circuit, with energy transfer through a “resonant coupling” between two coils with no electrical connection (like a radio antenna and receiver). But the legacy of the Tesla coil lies in the general idea of transformers (the electrical component, not the Hasbro automatons), which are vital to almost every modern electronic device.

Transformers (ie, increasing or decreasing voltage while doing the opposite to current) rely on one of Nikola Tesla’s most important inventions: alternating current. We’ve all heard the description of the electrons wiggling back and forth in the wire instead of flowing though, but that description obscures the true beauty of AC. A better way to visualize AC  is as waves in the lattice of metal atoms. When I took high school chemistry, transition metals were typically described as a “sea of delocalized electrons.” This epithet explains many meaningful properties of metals: they’re literally just a fixed atomic lattice containing a bunch of electrons that constantly float around, uncommitted to a single parent atom . This means that something like an incident electric field can very easily cause the electrons to move: in DC current, a constant electric field makes all the electrons move in one direction. While this carries kinetic energy very well, a common misconception is how it does so: electrons move incredibly slowly, and so the current is not due to a torrent of electrons suddenly cascading through the circuit. Rather, the observed current comes from a large number of electrons getting a tiny boost in velocity– their power is in their sheer numbers, rather than by their individual speeds. In fact, it actually takes about 4 hours for a given electron to make the round trip from your light switch to your ceiling fan– most of the power that travels in the circuit is due to large numbers of slow electrons arriving at the light bulb at the same time.

In alternating current, it’s not the particles themselves carrying the energy—it’s a disturbance in them. In fact, when you flip a light switch, an energetic electromagnetic field travels through the wires in your wall at the nearly the speed of light (because, after all, light is an electromagnetic field). It might slow down some due to other interactions with the metal in the wire (light also slows down when it travels through non-conducting materials, like distilled water), but for all practical purposes the change in current travels super fast. Tesla’s AC was brilliant because it exploited the fact that the particles don’t even have to move at all to carry a lot of energy. Going back to the sea metaphor, imagine how long it takes a single water molecule to travel from Russia to the US, even if currents are steadily eastward. Nonetheless, things like tsunamis can still do a lot of damage; not because of individual particles carrying energy over the entire ocean, but rather because every particle between Russia and the US jumps up and down a little bit to carry the wave. Likewise, when you hit an AC switch, you basically trigger a series of well-timed kicks of energy to the metal lattice (the wire), each of which sends a nice wave traveling through the sea of electrons. Because the light bulb converts the energy of the sloshing electrons into heat and light, you need to keep making waves (using power) in order to keep it going.

The cool thing about kicking the electrons is that the resulting current changes in time– and so it can cause other things to change in time as well. Within an ignition coil, a magnetic field is set up in a wide set of wire loops every time a kick passes through that part of the circuit. When the current stops (the wave passes), the magnetic field collapses. Tesla thought to put a second coil inside the first, so that this collapsing magnetic field has somewhere to go. Just as running current through a coil created a magnetic field, running a dying magnetic field through a different coil will create a current. But say that I’ve made my second coil out of 100 turns of wire, while my original coil only had 10. The voltage induced on my second coil will be 10x my original voltage, and my current will be 1/10 it’s original value. In ignition coils, this ratio is more like 1000:1, allowing me to take a modest input voltage and make a nice, long spark.

It’s important to note that the two coils have no electrical connection between them– the magnetic field literally reaches one coil from the other and transfers the energy (a concept that is the basis of radio communication). Thus ignition coils have four terminals: a primary coil where you can connect a battery, and a set of secondary terminals that are the high voltage outputs.

There’s just one caveat: the magnetic field has to begin to die in order to be resurrected on the second coil. This means that the current that created the field has to come on and off as well– a requirement perfectly suited for AC current. The trouble is that a car battery is DC, and so all it does is pour electrons, rather than make waves.

Your car solves this problem by turning the DC on and off really, really fast using a device known as a distributor, which causes the current to stop and start several times each time a wheel rotates. Each cycle makes a new kick of energy pass through the circuit and thus creates a new ignition spark in the engine each time (and thus creating a steady, continuous set of rapid combustions that allows the car to run smoothly). This used to be done mechanically, but recently integrated circuits have largely replaced physical distributors.

For my project, I faced a similar issue of creating adequate waves in my ignition coil. If I attached the coil to a 12V lantern battery and then rapidly connected and disconnected my leads, I would get sparks, but only as often as I was able to manually connect and disconnect the switch. I considered using modern integrated circuits to do the switching for me, as occurred with distributors in cars, but I realized at the time that I barely knew how to send an email, let alone program an integrated circuit.

A trick I found online is to use what’s known as an electromagnetic buzzer, or a circuit that keeps trying to disconnect itself. Common devices known as N.C. relays have four terminals: two for control, two for output. The control terminals are electrically connected, as are the output (although the latter have a special magnetic switch connecting them). If I connect DC power through the two control terminals, it activates an electromagnet that then pulls open the output switch. If I disconnect power, the field goes away, and so the output terminals close up again. Thus in an ambient state, the N.C. relay is Normally Closed—anything connected to the output terminals will have a continuous circuit through the terminals.

A good way to consider this is that the N.C. relay is a logical NOT gate: if I have power through the inputs, then power is not allowed to run through the outputs. If the inputs lack power, then it is possible for power (in a separate circuit) to run through the output terminals.

N.C. Relay

An N.C. relay blocks current through another circuit when power is run through one of its switches, but allows the other circuit to flow when no power is being run through the input terminals. This illustrates how one circuit can control another, unconnected circuit.

The electromagnetic buzzer uses an NC relay to set up a paradox: what if a circuit were to run into one input terminal, out the other, back in one output terminal, and then out the other and into a circuit? Running power through this configuration would definitely provoke the electromagnet, which would then open the switch- and thus break the circuit. Since the magnet then turns off, the switch closes again—causing the magnet to turn back on, the the cycle to repeat. This means that, if I wire a common relay up correctly, I can end up with an electromagnetic paradox, a circuit that constantly strives to turn itself off.

The two states of an EM buzzer. The circuit constantly oscillates between these positions.

This is the basis of the electromagnetic buzzer, which gets its name from “buzzing” between on and off. If I were to plot the shape of the current in the circuit over time, I would get square waves, where the current runs for a short period at a constant DC value (making a straight line on the plot at this value), and then stops for another regular short period (making another straight line at zero).

Returning to the ignition coil, this means that all I need to put AC through my coil is my battery, the N.C. relay (correctly wired as a buzzer), and my coil. You can see in the video that this process creates a constant spark at a fairly low frequency (you can see it jumping, for example). In true Tesla coils, the sparks jump so fast that they give the illusion of being animated or continuous (at which point they are called “streamers” because they appear to stream out of the terminal).

The full circuit for the continuous ignition coil driver. This should produce 2-3 inch sparks at a steady rate.

For an even higher output voltage (3-5 inch sparks), another coil can be connected in reverse parallel with the original coil primary terminals (ie, in parallel with wires crossed). Sparks will jump between the output terminals of the two coils because they will have opposite polarities due to the reversal.

 

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