The Cockcroft–Walton (CW) generator, or multiplier, is an electric circuit that generates a high DC voltage from a low-voltage AC or pulsing DC input. It was named after the British and Irish physicists John Douglas Cockcroft and Ernest Thomas Sinton Walton, who in 1932 used this circuit design to power their particle accelerator, performing the first artificial nuclear disintegration in history. They used this voltage multiplier cascade for most of their research, which in 1951 won them the Nobel Prize in Physics for "Transmutation of atomic nuclei by artificially accelerated atomic particles".
The circuit was developed in 1919, by Heinrich Greinacher, a Swiss physicist. For this reason, this doubler cascade is sometimes also referred to as the Greinacher multiplier. Cockcroft–Walton circuits are still used in particle accelerators. They also are used in everyday electronic devices that require high voltages, such as X-ray machines, microwave ovens and photocopiers.
Operation
The CW generator is a voltage multiplier that converts AC or pulsing DC electrical power from a low voltage level to a higher DC voltage level. It is made up of a voltage multiplier ladder network of capacitors and diodes to generate high voltages. Unlike transformers, this method eliminates the requirement for the heavy core and the bulk of insulation/potting required. Using only capacitors and diodes, these voltage multipliers can step up relatively low voltages to extremely high values, while at the same time being far lighter and cheaper than transformers. The biggest advantage of such circuits is that the voltage across each stage of the cascade is equal to only twice the peak input voltage in a half-wave rectifier. In a full-wave rectifier it is three times the input voltage. It has the advantage of requiring relatively low-cost components and being easy to insulate. One can also tap the output from any stage, like in a multi-tapped transformer.
To understand the circuit operation, see the diagram of the two-stage version at right. Assume all capacitors are initially uncharged, and the circuit is powered by an alternating voltage Vi such that , i.e. with a peak value of Vp, which after power-on is 0 volts and starts with a negative half-cycle. After the input voltage is turned on When the input voltage Vi is decreasing and approaching its negative peak −Vp, current flows from the bottom terminal of the source, through diode D1 and then through capacitor C1, charging it. Vi eventually reaches the negative peak −Vp, at which point C1 is charged to a voltage of Vp. Vi then starts increasing ‒ its derivative reverses sign from negative to positive. When this happens, the current reverses its direction, since the load placed on the source is almost purely capacitive and thus current leads voltage by almost 90°. When Vi is increasing and approaching its positive peak +Vp, current flows from the top terminal of the source, through C1 (discharging it), through diode D2, and finally through capacitor C2 (charging it). Eventually, Vi reaches +Vp, and when we add to it the voltage of C1 (which is now slightly below +Vp), we get the resulting voltage of almost 2Vp ‒ this is the voltage to which C2 is charged. In this phase, diode D1 is reverse-biased, so no current flows through it. When Vi starts decreasing again ( is negative), current flows from the bottom terminal of the source, through C2 (discharging it), through diode D3, through C3 (charging it to a voltage of almost 2Vp), and finally through C1 (recharging it to Vp, after it was partially discharged in the previous phase). Since some voltage is dropped also on C1 and not just on C3, C3 will not be charged to 2Vp immediately, but only in later iterations. The same applies to C1 and Vp respectively. Also, in this phase, C2 discharges to a voltage below 2Vp, similarly to C1 in the previous phase. It will be recharged in the next phase. When Vi begins to increase again, current flows from the top terminal of the source, through C1 and C3 (discharging them), through diode D4, through C4 (charging it to a voltage of almost 2Vp), and finally through C2 (recharging it). During this phase, C1 and C3 discharge below Vp and 2Vp respectively, and will be recharged in the following phase.