Cockcroft-Walton generator
Encyclopedia
The Cockcroft–Walton generator, or multiplier, is an electric circuit which 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". Less well known is the fact that the circuit was discovered much earlier, in 1919, by Heinrich Greinacher
, a Swiss physicist
. For this reason, this doubler cascade is sometimes also referred to as the Greinacher multiplier. Today Cockcroft-Walton circuits are still used in particle accelerators, but also in many everyday electronic devices that require high voltages, such as x-ray machine
s, television set
s, and photocopier
s.
The CW 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 capacitor
s and diode
s to generate high voltages. Unlike transformer
s, this method eliminates the requirement for the heavy core and the bulk of insulation/potting required. Using only capacitor
s and diode
s, these voltage multiplier
s can step up relatively low voltages to extremely high values, while at the same time being far lighter and cheaper than transformer
s. 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 a multitapped transformer.
Operation of the CW multiplier, or any voltage doubler, is quite simple. Considering the simple two-stage version diagramed within, which is attached to an AC power source on the left side of the diagram. At the time when the AC input reaches its negative pole the leftmost diode is allowing current to flow from the ground into the first capacitor, filling it up. When the same AC signal reverses polarity, current flows through the second diode filling up the second capacitor with both the positive end from AC source and the first capacitor, charging the second capacitor to twice the charge held in the first. With each change in polarity of the input, the capacitors add to the upstream charge and boost the voltage level of the capacitors downstream, towards the output on the right. The increase in voltage, assuming perfect conditions, is two times the input voltage times the number of stages in the multiplier.
, 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 for high-energy physics experiments or lightning
safety testing. CW multipliers are also found, with a higher number of stages, in laser
systems, high-voltage power supplies, X-ray
systems, LCD backlight
ing, traveling wave tube amplifiers, ion pump
s, electrostatic systems, air ioniser
s, particle accelerator
s, copy machines
, scientific instrumentation, oscilloscope
s, TV sets and CRT
s, bug zappers
and many other applications that use high-voltage DC.
, which has the same "ladder" structure but consists of resistors, capacitors and spark-gaps. The Marx generator produces short pulses, whereas the CW generator produces a constant DC.
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
from a low voltage AC
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
or pulsing DC input. It was named after the British and Irish physicists John Douglas Cockcroft and Ernest Thomas Sinton Walton
Ernest Walton
Ernest Thomas Sinton Walton was an Irish physicist and Nobel laureate for his work with John Cockcroft with "atom-smashing" experiments done at Cambridge University in the early 1930s, and so became the first person in history to artificially split the atom, thus ushering the nuclear age...
, who in 1932 used this circuit design to power their particle accelerator
Particle accelerator
A particle accelerator is a device that uses electromagnetic fields to propel charged particles to high speeds and to contain them in well-defined beams. An ordinary CRT television set is a simple form of accelerator. There are two basic types: electrostatic and oscillating field accelerators.In...
, performing the first artificial nuclear disintegration in history. They used this voltage multiplier
Voltage multiplier
thumb|right|280px|Villard cascade voltage multiplier.A voltage multiplier is an electrical circuit that converts AC electrical power from a lower voltage to a higher DC voltage, typically by means of a network of capacitors and diodes....
cascade for most of their research, which in 1951 won them the Nobel Prize in Physics
Nobel Prize in Physics
The Nobel Prize in Physics is awarded once a year by the Royal Swedish Academy of Sciences. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895 and awarded since 1901; the others are the Nobel Prize in Chemistry, Nobel Prize in Literature, Nobel Peace Prize, and...
for "Transmutation of atomic nuclei by artificially accelerated atomic particles". Less well known is the fact that the circuit was discovered much earlier, in 1919, by Heinrich Greinacher
Heinrich Greinacher
Heinrich Greinacher was a Swiss physicist. He is regarded as an original experimenter and is the developer of the magnetron and the Greinacher multiplier....
, a Swiss physicist
Physicist
A physicist is a scientist who studies or practices physics. Physicists study a wide range of physical phenomena in many branches of physics spanning all length scales: from sub-atomic particles of which all ordinary matter is made to the behavior of the material Universe as a whole...
. For this reason, this doubler cascade is sometimes also referred to as the Greinacher multiplier. Today Cockcroft-Walton circuits are still used in particle accelerators, but also in many everyday electronic devices that require high voltages, such as x-ray machine
X-ray machine
An X-ray generator is a device used to generate X-rays. These devices are commonly used by radiographers to acquire an x-ray image of the inside of an object but they are also used in sterilization or fluorescence....
s, television set
Television set
A television set is a device that combines a tuner, display, and speakers for the purpose of viewing television. Television sets became a popular consumer product after the Second World War, using vacuum tubes and cathode ray tube displays...
s, and photocopier
Photocopier
A photocopier is a machine that makes paper copies of documents and other visual images quickly and cheaply. Most current photocopiers use a technology called xerography, a dry process using heat...
s.
Design
Voltage multiplier
thumb|right|280px|Villard cascade voltage multiplier.A voltage multiplier is an electrical circuit that converts AC electrical power from a lower voltage to a higher DC voltage, typically by means of a network of capacitors and diodes....
that converts AC or pulsing DC electrical power from a low voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
level to a higher DC voltage level. It is made up of a voltage multiplier ladder network of capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
s and diode
Diode
In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...
s to generate high voltages. Unlike transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
s, this method eliminates the requirement for the heavy core and the bulk of insulation/potting required. Using only capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
s and diode
Diode
In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...
s, these voltage multiplier
Voltage multiplier
thumb|right|280px|Villard cascade voltage multiplier.A voltage multiplier is an electrical circuit that converts AC electrical power from a lower voltage to a higher DC voltage, typically by means of a network of capacitors and diodes....
s can step up relatively low voltages to extremely high values, while at the same time being far lighter and cheaper than transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
s. The biggest advantage of such circuits is that the voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
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 a multitapped transformer.
Operation of the CW multiplier, or any voltage doubler, is quite simple. Considering the simple two-stage version diagramed within, which is attached to an AC power source on the left side of the diagram. At the time when the AC input reaches its negative pole the leftmost diode is allowing current to flow from the ground into the first capacitor, filling it up. When the same AC signal reverses polarity, current flows through the second diode filling up the second capacitor with both the positive end from AC source and the first capacitor, charging the second capacitor to twice the charge held in the first. With each change in polarity of the input, the capacitors add to the upstream charge and boost the voltage level of the capacitors downstream, towards the output on the right. The increase in voltage, assuming perfect conditions, is two times the input voltage times the number of stages in the multiplier.
Operational characteristics
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 the AC 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. For these reasons, CW multipliers with large number of stages are used only where relatively low output current is required. These effects can be partially compensated by increasing the capacitance in the lower stages, by increasing the frequency of the input power and by using an AC power source with a square or triangular shaped waveform. By driving the CW from a high frequency source, such as an inverterInverter (electrical)
An inverter is an electrical device that converts direct current to alternating current ; the converted AC can be at any required voltage and frequency with the use of appropriate transformers, switching, and control circuits....
, 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 for high-energy physics experiments or lightning
Lightning
Lightning is an atmospheric electrostatic discharge accompanied by thunder, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms...
safety testing. CW multipliers are also found, with a higher number of stages, in laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
systems, high-voltage power supplies, X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
systems, LCD backlight
Backlight
A backlight is a form of illumination used in liquid crystal displays . As LCDs do not produce light themselves , they need illumination to produce a visible image...
ing, traveling wave tube amplifiers, ion pump
Ion pump (physics)
An ion pump is a type of vacuum pump capable of reaching up to 10−11 mbar under ideal conditions. An ion pump ionizes gases and employs a strong electrical potential, typically 3kV to 7kV, to accelerate them into a solid electrode...
s, electrostatic systems, air ioniser
Air ioniser
An air ioniser is a device that uses high voltage to ionise air molecules. Negative ions, or anions, are particles with one or more extra electrons, conferring a net negative charge to the particle. Cations are positive ions missing one or more electrons, resulting in a net positive charge...
s, particle accelerator
Particle accelerator
A particle accelerator is a device that uses electromagnetic fields to propel charged particles to high speeds and to contain them in well-defined beams. An ordinary CRT television set is a simple form of accelerator. There are two basic types: electrostatic and oscillating field accelerators.In...
s, copy machines
Photocopier
A photocopier is a machine that makes paper copies of documents and other visual images quickly and cheaply. Most current photocopiers use a technology called xerography, a dry process using heat...
, scientific instrumentation, oscilloscope
Oscilloscope
An oscilloscope is a type of electronic test instrument that allows observation of constantly varying signal voltages, usually as a two-dimensional graph of one or more electrical potential differences using the vertical or 'Y' axis, plotted as a function of time,...
s, TV sets and CRT
Cathode ray tube
The cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...
s, bug zappers
Bug zapper
A bug zapper, or more formally an electrical discharge insect control system is a device that attracts and kills flying insects that are attracted by light. A light source attracts insects to an electrical grid, where they are electrocuted by touching two wires with a high voltage between them...
and many other applications that use high-voltage DC.
See also
A similar circuit is the Marx generatorMarx generator
A Marx generator is an electrical circuit first described by Erwin Otto Marx in 1924. Its purpose is to generate a high-voltage pulse. Marx generators are often used to simulate the effects of lightning on power line gear and aviation equipment....
, which has the same "ladder" structure but consists of resistors, capacitors and spark-gaps. The Marx generator produces short pulses, whereas the CW generator produces a constant DC.