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TheCockcroft–Walton (CW)generator, ormultiplier, is anelectric circuit that generates a highDCvoltage from a low-voltageAC.[1] It was named after the British and Irish physicistsJohn Douglas Cockcroft andErnest Thomas Sinton Walton, who in 1932 used this circuit design to power theirparticle accelerator, performing the first accelerator-induced nuclear disintegration in history.[2] They used thisvoltage multiplier cascade for most of their research, which in 1951 won them theNobel Prize in Physics for "Transmutation of atomic nuclei by artificially accelerated atomic particles".
The circuit was developed in 1919, byHeinrich Greinacher, a Swissphysicist. For this reason, this doubler cascade is sometimes also referred to as theGreinacher multiplier. Cockcroft–Walton circuits are still used in particle accelerators. They also are used in everyday electronic devices that require high voltages, such as dentalX-ray machines andair ionizers.
The CW generator is avoltage multiplier that converts AC electrical power from a lowvoltage level to a higher DC voltage level. It is made up of a voltage multiplier ladder network ofcapacitors anddiodes to generate high voltages. Unliketransformers, 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 thevoltage 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 voltageVi such thatVi =Vp sin(t + π), i.e. with a peak value ofVp, which after power-on is 0 volts and starts with a negative half-cycle. After the input voltage is turned on
At any given moment, either the odd-numbered diodes are conducting, or the even-numbered ones, never both. With each change in the derivative of input voltage (i.e.dVi/dt), current flows up to the next level in the "stack" of capacitors through the diodes. Eventually, after a sufficient number of cycles of the AC input, all capacitors will be charged. (More precisely, we should say theiractual voltages will converge sufficiently close to theideal ones ‒ there will always be some ripple from the AC input). All the capacitors are charged to a voltage of 2Vp, except forC1, which is charged toVp. The key to the voltage multiplication is that while the capacitors are charged in parallel, they are connected to the load in series. SinceC2 andC4 are in series between the output and ground, the total output voltage (under no-load conditions) isVo = 4Vp.
This circuit can be extended to any number of stages. The no-load output voltage is twice the peak input voltage multiplied by the number of stagesN or equivalently thepeak-to-peak input voltage swing (Vpp) times the number of stages
The number of stages is equal to the number of capacitors in series between the output and ground.
One way to look at the circuit is that it functions as a charge "pump", pumping electric charge in one direction, up the stack of capacitors. The CW circuit, along with other similar capacitor circuits, is often called acharge pump. For substantial loads, the charge on the capacitors is partially depleted, and the output voltage drops according to the output current divided by the capacitance.
In practice, the CW has a number of drawbacks. As the number of stages is increased, the voltages of the higher stages begin to "sag", primarily due to theelectrical impedance of the capacitors in the lower stages. And, when supplying an output current, the voltage ripple rapidly increases as the number of stages is increased (this can be corrected with an output filter, but it requires a stack of capacitors in order to withstand the high voltages involved). For these reasons, CW multipliers with large number of stages are used only where relatively low output current is required. The sag can be reduced by increasing the capacitance in the lower stages, and the ripple can be reduced by increasing the frequency of the input and by using a square waveform. By driving the CW from a high-frequency source, such as aninverter, or a combination of an inverter and HV transformer, the overall physical size and weight of the CW power supply can be substantially reduced.
CW multipliers are typically used to develop higher voltages for relatively low-current applications, such as bias voltages ranging from tens or hundreds of volts to millions of volts forhigh-energy physics experiments orlightning safety testing. CW multipliers are also found, with a higher number of stages, inlaser systems, high-voltage power supplies,X-ray systems, CCFL LCDbacklighting,traveling-wave tube amplifiers,ion pumps,electrostatic systems,air ionisers,particle accelerators,copy machines, scientific instrumentation,oscilloscopes, television sets andcathode-ray tubes,electroshock weapons,bug zappers and many other applications that use high-voltage DC.
TheDynamitron is similar to the Cockcroft–Walton generator. However instead of being powered at one end as in the Cockcroft-Walton, the capacitive ladder is charged in parallel electrostatically by a high frequency oscillating voltage applied between two long half-cylindrical electrodes on either side of the ladder column, which induce voltage in semicircular corona rings attached to each end of the diode rectifier tubes.[3]
