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.