Last updated
(1) Anode, (2) Cathode, (3) Ignitor, (4) Mercury, (5) Ceramic insulators, (6) Cooling fluid Ignitron.svg
(1) Anode, (2) Cathode, (3) Ignitor, (4) Mercury, (5) Ceramic insulators, (6) Cooling fluid

An ignitron is a type of gas-filled tube used as a controlled rectifier and dating from the 1930s. Invented by Joseph Slepian while employed by Westinghouse, Westinghouse was the original manufacturer and owned trademark rights to the name "Ignitron". Ignitrons are closely related to mercury-arc valves but differ in the way the arc is ignited. They function similarly to thyratrons; a triggering pulse to the igniter electrode turns the device "on", allowing a high current to flow between the cathode and anode electrodes. After it is turned on, the current through the anode must be reduced to zero to restore the device to its nonconducting state. They are used to switch high currents in heavy industrial applications.

Gas-filled tube arrangement of electrodes in a gas within an insulating, temperature-resistant envelope

A gas-filled tube, also known as a discharge tube, is an arrangement of electrodes in a gas within an insulating, temperature-resistant envelope. Gas-filled tubes exploit phenomena related to electric discharge in gases, and operate by ionizing the gas with an applied voltage sufficient to cause electrical conduction by the underlying phenomena of the Townsend discharge. A gas-discharge lamp is an electric light using a gas-filled tube; these include fluorescent lamps, metal-halide lamps, sodium-vapor lamps, and neon lights. Specialized gas-filled tubes such as krytrons, thyratrons, and ignitrons are used as switching devices in electric devices.

Rectifier AC-DC conversion device; electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction

A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction.

Joseph Slepian American mathematician

Joseph Slepian (February 11, 1891 – December 19, 1969) was an American electrical engineer known for his contributions to the developments of electrical apparatus and theory.


Construction and operation

Ignitron rectifiers powering industrial process, 1945 Ignitrons.jpg
Ignitron rectifiers powering industrial process, 1945

An ignitron is usually a large steel container with a pool of mercury in the bottom that acts as a cathode during operation. A large graphite or refractory metal cylinder, held above the pool by an insulated electrical connection, serves as the anode. An igniting electrode (called the ignitor), made of a refractory semiconductor material such as silicon carbide, [1] is briefly pulsed with a high current to create a puff of electrically conductive mercury plasma. The plasma rapidly bridges the space between the mercury pool and the anode, permitting heavy conduction between the main electrodes. At the surface of the mercury, heating by the resulting arc liberates large numbers of electrons which help to maintain the mercury arc. The mercury surface thus serves as the cathode, and current is normally only in one direction. Once ignited, an ignitron will continue to pass current until either the current is externally interrupted or the voltage applied between cathode and anode is reversed. [2]

Mercury (element) Chemical element with atomic number 80

Mercury is a chemical element with the symbol Hg and atomic number 80. It is commonly known as quicksilver and was formerly named hydrargyrum. A heavy, silvery d-block element, mercury is the only metallic element that is liquid at standard conditions for temperature and pressure; the only other element that is liquid under these conditions is the halogen bromine, though metals such as caesium, gallium, and rubidium melt just above room temperature.

A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic CCD for Cathode Current Departs. A conventional current describes the direction in which positive charges move. Electrons have a negative electrical charge, so the movement of electrons is opposite to that of the conventional current flow. Consequently, the mnemonic cathode current departs also means that electrons flow into the device's cathode from the external circuit.

Graphite allotrope of carbon, mineral, substance

Graphite, archaically referred to as plumbago, is a crystalline form of the element carbon with its atoms arranged in a hexagonal structure. It occurs naturally in this form and is the most stable form of carbon under standard conditions. Under high pressures and temperatures it converts to diamond. Graphite is used in pencils and lubricants. Its high conductivity makes it useful in electronic products such as electrodes, batteries, and solar panels.


Ignitrons were long used as high-current rectifiers in major industrial and utility installations where thousands of amperes of AC must be converted to DC, such as aluminum smelters. Ignitrons were used to control the current in electric welding machines. Large electric motors were also controlled by ignitrons used in gated[ clarification needed ] fashion, in a manner similar to modern semiconductor devices such as silicon controlled rectifiers and triacs. Many electric locomotives used them in conjunction with transformers to convert high voltage AC from the overhead lines to relatively low voltage DC for the traction motors. The Pennsylvania Railroad's E44 freight locomotives carried on-board ignitrons, as did the Russian ВЛ-60 freight locomotive. For many modern applications, ignitrons have been replaced by solid state alternatives.

Alternating current electric voltage which periodically reverses direction; form in which electric power is delivered to businesses and residences; form of electrical energy that consumers typically use when they plug electric appliances into a wall socket

Alternating current (AC) is an electric current which periodically reverses direction, in contrast to direct current (DC) which flows only in one direction. Alternating current is the form in which electric power is delivered to businesses and residences, and it is the form of electrical energy that consumers typically use when they plug kitchen appliances, televisions, fans and electric lamps into a wall socket. A common source of DC power is a battery cell in a flashlight. The abbreviations AC and DC are often used to mean simply alternating and direct, as when they modify current or voltage.

Direct current Unidirectional flow of electric charge

Direct current (DC) is the unidirectional flow of an electric charge. A battery is a prime example of DC power. Direct current may flow through a conductor such as a wire, but can also flow through semiconductors, insulators, or even through a vacuum as in electron or ion beams. The electric current flows in a constant direction, distinguishing it from alternating current (AC). A term formerly used for this type of current was galvanic current.

Welding fabrication or sculptural process for joining materials

Welding is a fabrication or sculptural process that joins materials, usually metals or thermoplastics, by using high heat to melt the parts together and allowing them to cool causing fusion. Welding is distinct from lower temperature metal-joining techniques such as brazing and soldering, which do not melt the base metal.

Because they are far more resistant to damage due to overcurrent or back-voltage, ignitrons are still manufactured and used in preference to semiconductors in some installations. For example, specially constructed "pulse rated" ignitrons are still used in certain pulsed power applications. These devices can switch hundreds of kiloamperes and hold off as much as 50kV. The anodes in these devices are often fabricated from a refractory metal, usually molybdenum, to handle reverse current during ringing (or oscillatory) discharges without damage. Pulse rated ignitrons usually operate at very low duty cycles. They are often used to switch high energy capacitor banks during electromagnetic forming, electrohydraulic forming, or for emergency short-circuiting of high voltage power sources ("crowbar" switching).

In an electric power system, overcurrent or excess current is a situation where a larger than intended electric current exists through a conductor, leading to excessive generation of heat, and the risk of fire or damage to equipment. Possible causes for overcurrent include short circuits, excessive load, incorrect design, or a ground fault. Fuses, circuit breakers, lifersensors and current limiters are commonly used overcurrent protection (OCP) mechanisms to control the risks. An overcurrent can be caused by overloading the circuit or by a short circuit, a ground fault, or an arc fault. Circuit breakers and fuses protect circuit wiring from damage caused by overcurrent.

Pulsed power is the science and technology of accumulating energy over a relatively long period of time and releasing it very quickly, thus increasing the instantaneous power.

In metallurgy, refraction is a property of metals that indicates their ability to withstand heat. Metals with a high degree of refraction are referred to as refractory. These metals derive their high melting points from their strong intermolecular forces. Large quantities of energy are required to overcome intermolecular forces.

An ignitron rated 56 amperes. Cooling jacket connections visible. In use the device was mounted so that the text would be upright. Philips ignitron pl5551a.jpg
An ignitron rated 56 amperes. Cooling jacket connections visible. In use the device was mounted so that the text would be upright.

Comparison with mercury-arc valve

Although the basic principles of how the arc is formed, along with many aspects of construction, are very similar to other types of mercury-arc valves, ignitrons differ from other mercury-arc valves in that the arc is ignited each time a conduction cycle is started, and then extinguished when the current falls below a critical threshold.

In other types of mercury-arc valve, the arc is ignited just once when the valve is first energised, and thereafter remains permanently established, alternating between the main anode(s) and a low-power auxiliary anode or keep-alive circuit. Moreover, control grids are required in order to adjust the timing of the start of conduction.

The action of igniting the arc at a controlled time, each cycle, allows the ignitron to dispense with the auxiliary anode and control grids required by other mercury-arc valves. However, a disadvantage is that the ignition electrode must be positioned very accurately, just barely touching the surface of the mercury pool, which means that ignitrons must be installed very accurately within a few degrees of an upright position.

See also


A trigatron is a type of triggerable spark gap switch designed for high current and high voltage,. It has very simple construction and in many cases is the lowest cost high energy switching option. It may operate in open air, it may be sealed, or it may be filled with a dielectric gas other than air or a liquid dielectric. The dielectric gas may be pressurized, or a liquid dielectric may be substituted to further extend the operating voltage. Trigatrons may be rated for repeated use, or they may be single-shot, destroyed in a single use.

Thyratron type of gas filled tube

A thyratron is a type of gas-filled tube used as a high-power electrical switch and controlled rectifier. Thyratrons can handle much greater currents than similar hard-vacuum tubes. Electron multiplication occurs when the gas becomes ionized, producing a phenomenon known as Townsend discharge. Gases used include mercury vapor, xenon, neon, and hydrogen. Unlike a vacuum tube (valve), a thyratron cannot be used to amplify signals linearly.

Thyristor semiconductor device with three or more p-n junctions, having two steady states: off (non-conducting) and on (conducting)

A thyristor is a solid-state semiconductor device with four layers of alternating P- and N-type materials. It acts exclusively as a bistable switch, conducting when the gate receives a current trigger, and continuing to conduct until the voltage across the device is reversed biased, or until the voltage is removed. A three-lead thyristor is designed to control the larger current of the Anode to Cathode path by controlling that current with the smaller current of its other lead, known as its Gate. In contrast, a two-lead thyristor is designed to switch on if the potential difference between its leads is sufficiently large.

Related Research Articles

Vacuum tube Device that controls electric current between electrodes in an evacuated container

In electronics, a vacuum tube, an electron tube, or valve or, colloquially, a tube, is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied.

High-voltage direct current

A high-voltage, direct current (HVDC) electric power transmission system uses direct current for the bulk transmission of electrical power, in contrast with the more common alternating current (AC) systems. For long-distance transmission, HVDC systems may be less expensive and suffer lower electrical losses. For underwater power cables, HVDC avoids the heavy currents required to charge and discharge the cable capacitance each cycle. For shorter distances, the higher cost of DC conversion equipment compared to an AC system may still be justified, due to other benefits of direct current links. HVDC currently uses voltages between 100 kV and 800 kV, with an 1,100 kV link in China due to become operational in 2019.

Cold cathode Type of electrode and part of cold cathode fluorescent lamp.

A cold cathode is a cathode that is not electrically heated by a filament. A cathode may be considered "cold" if it emits more electrons than can be supplied by thermionic emission alone. It is used in gas-discharge lamps, such as neon lamps, discharge tubes, and some types of vacuum tube. The other type of cathode is a hot cathode, which is heated by electric current passing through a filament. A cold cathode does not necessarily operate at a low temperature: it is often heated to its operating temperature by other methods, such as the current passing from the cathode into the gas.

Schottky diode semiconductor diode formed by the junction of a semiconductor with a metal, semiconductor diode with a low forward voltage drop

The Schottky diode, also known as Schottky barrier diode or hot-carrier diode, is a semiconductor diode formed by the junction of a semiconductor with a metal. It has a low forward voltage drop and a very fast switching action. The cat's-whisker detectors used in the early days of wireless and metal rectifiers used in early power applications can be considered primitive Schottky diodes.

Spark gap

A spark gap consists of an arrangement of two conducting electrodes separated by a gap usually filled with a gas such as air, designed to allow an electric spark to pass between the conductors. When the potential difference between the conductors exceeds the breakdown voltage of the gas within the gap, a spark forms, ionizing the gas and drastically reducing its electrical resistance. An electric current then flows until the path of ionized gas is broken or the current reduces below a minimum value called the "holding current". This usually happens when the voltage drops, but in some cases occurs when the heated gas rises, stretching out and then breaking the filament of ionized gas. Usually, the action of ionizing the gas is violent and disruptive, often leading to sound, light and heat.

Silicon controlled rectifier semiconductor electronic device with three p-n junctions, mainly used in devices where the control of high power is demanded

A silicon controlled rectifier or semiconductor controlled rectifier is a four-layer solid-state current-controlling device. The principle of four-layer p–n–p–n switching was developed by Moll, Tanenbaum, Goldey and Holonyak of Bell Laboratories in 1956. The practical demonstration of silicon controlled switching and detailed theoretical behavior of a device in agreement with the experimental results was presented by Dr Ian M. Mackintosh of Bell Laboratories in January 1958. The name "silicon controlled rectifier" is General Electric's trade name for a type of thyristor. The SCR was developed by a team of power engineers led by Gordon Hall and commercialized by Frank W. "Bill" Gutzwiller in 1957.


The krytron is a cold-cathode gas-filled tube intended for use as a very high-speed switch, somewhat similar to the thyratron. It consists of a sealed glass tube with four electrodes. A small triggering pulse on the grid electrode switches the tube on, allowing a large current to flow between the cathode and anode electrodes. The vacuum version is called a vacuum krytron, or sprytron. The krytron was one of the earliest developments of the EG&G Corporation.

Power electronics application of solid-state electronics to the control and conversion of electric power

Power electronics is the application of solid-state electronics to the control and conversion of electric power.

Mercury-arc valve electrical equipment for converting high-voltage or -current alternating current into direct current

A mercury-arc valve or mercury-vapor rectifier or (UK) mercury-arc rectifier is a type of electrical rectifier used for converting high-voltage or high-current alternating current (AC) into direct current (DC). It is a type of cold cathode gas-filled tube, but is unusual in that the cathode, instead of being solid, is made from a pool of liquid mercury and is therefore self-restoring. As a result, mercury-arc valves were much more rugged and long-lasting, and could carry much higher currents than most other types of gas discharge tube.

Phase-fired controller

Phase-fired control (PFC), also called phase cutting or "phase angle control", is a method for power limiting, applied to AC voltages. It works by modulating a thyristor, SCR, triac, thyratron, or other such gated diode-like devices into and out of conduction at a predetermined phase of the applied waveform.

In electronics, a crossatron is a high-power pulsed modulator device, a cold cathode gas-filled tube that combines the best features of thyratrons, vacuum tubes, and power semiconductor switches. This switch is capable of operating with voltages in excess of 100 kilovolts by the use of deuterium gas fill to increase the Paschen breakdown voltage, axial molybdenum cathode corrugations to provide a higher current capability, and a Paschen shield that is formed from molybdenum. The terminal curvature of the Paschen shield and of the adjacent portion of the anode are selected to establish a voltage stress at the curved Paschen shield surface within the approximate range of 90-150 kV/cm in response to a 100 kV differential. The cold cathode gives the crossatron an advantage of achievable lifetime and reliability in comparison to a hydrogen-filled thyratron.

Fleming valve a vacuum tube used as a detector for early radio receivers

The Fleming valve, also called the Fleming oscillation valve, was a thermionic valve or vacuum tube invented in 1904 by Englishman John Ambrose Fleming as a detector for early radio receivers used in electromagnetic wireless telegraphy. It was the first practical vacuum tube and the first thermionic diode, a vacuum tube whose purpose is to conduct current in one direction and block current flowing in the opposite direction. The thermionic diode was later widely used as a rectifier — a device which converts alternating current (AC) into direct current (DC) — in the power supplies of a wide range of electronic devices, until beginning to be replaced by the selenium rectifier in the early 1930s and almost completely replaced by the semiconductor diode in the 1960s. The Fleming valve was the forerunner of all vacuum tubes, which dominated electronics for 50 years. The IEEE has described it as "one of the most important developments in the history of electronics", and it is on the List of IEEE Milestones for electrical engineering.

In the years 1942-1944, the Radio Manufacturers Association used a descriptive nomenclature system for industrial, transmitting, and special-purpose vacuum tubes. The numbering scheme was distinct from both the numbering schemes used for standard receiving tubes, and the existing transmitting tube numbering systems used previously, such as the "800 series" numbers originated by RCA and adopted by many others.

The pseudospark switch, also known as a cold-cathode thyratron due to the similarities with regular thyratrons, is a gas-filled tube capable of high speed switching. Advantages of pseudospark switches include the ability to carry reverse currents (up to 100%), low pulse, high lifetime, and a high current rise of about 1012 A/sec. In addition, since the cathode is not heated prior to switching, the standby power is approximately one order of magnitude lower than in thyratrons. However, pseudospark switches have undesired plasma phenomena at low peak currents. Issues such as current quenching, chopping, and impedance fluctuations occur at currents less than 2-3 kA while at very high peak currents (20-30 kA) a transition to a metal vapor arc occurs which leads to erosion of the electrodes. Pseudospark switches are functionally similar to triggered spark gaps.

Electric discharge in gases occurs when electric current flows through a gaseous medium due to ionization of the gas. Depending on several factors, the discharge may radiate visible light. The properties of electric discharges in gases are studied in connection with design of lighting sources and in the design of high voltage electrical equipment.


  1. Turner pg. 7-182
  2. L.W. Turner,(ed), Electronics Engineer's Reference Book, 4th ed. Newnes-Butterworth, London 1976 ISBN   0408001682 pages 7-181 through 7-189