Ignitron
Encyclopedia
An ignitron is a type of controlled rectifier
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".
It 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, 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 electron
s 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.
Ignitrons were long used as high-current rectifiers in major industrial and utility installations where thousands of amperes of AC
current must be converted to DC
, such as aluminum smelters. Ignitrons were used to control the current in electric welding
machines. Large electric motor
s were also controlled by ignitrons used in gated fashion, in a manner similar to modern semiconductor devices such as silicon controlled rectifiers and triac
s. Many electric locomotive
s used them in conjunction with transformer
s to convert high voltage AC from the overhead lines
to relatively low voltage DC for the traction motors. For most of these applications, ignitrons have been replaced by solid state alternatives.
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 cycle
s. 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).
Rectifier
A rectifier is an electrical device that converts alternating current , which periodically reverses direction, to direct current , which flows in only one direction. The process is known as rectification...
dating from the 1930s. Invented by Joseph Slepian
Joseph Slepian
Joseph Slepianwas an American electrical engineer known for his contributions to the developments of electrical apparatus and theory....
while employed by Westinghouse
Westinghouse Electric (1886)
Westinghouse Electric was an American manufacturing company. It was founded in 1886 as Westinghouse Electric Company and later renamed Westinghouse Electric Corporation by George Westinghouse. The company purchased CBS in 1995 and became CBS Corporation in 1997...
, Westinghouse was the original manufacturer and owned trademark rights to the name "Ignitron".
It is usually a large steel container with a pool of mercury
Mercury (element)
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...
in the bottom that acts as a cathode
Cathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...
during operation. A large graphite
Graphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...
or refractory metal cylinder, held above the pool by an insulated electrical connection, serves as the anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
. An igniting electrode
Electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...
(called the "ignitor"), made of a refractory semiconductor material such as silicon carbide, is briefly pulsed with a high current to create a puff of electrically conductive mercury plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
. 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 electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s which help to maintain the mercury arc. The mercury surface thus serves as the cathode, and current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
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.
Ignitrons were long used as high-current rectifiers in major industrial and utility installations where thousands of amperes of 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....
current must be converted to 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...
, such as aluminum smelters. Ignitrons were used to control the current in electric welding
Welding
Welding is a fabrication or sculptural process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done by melting the workpieces and adding a filler material to form a pool of molten material that cools to become a strong joint, with pressure sometimes...
machines. Large electric motor
Electric motor
An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...
s were also controlled by ignitrons used in gated fashion, in a manner similar to modern semiconductor devices such as silicon controlled rectifiers and triac
Triac
Triac may refer to:* TRIAC , an electronics component* Triac , a green vehicle* Tiratricol, a common thyroid hormone analogue used for treating thyroid hormone resistance syndrome...
s. Many electric locomotive
Locomotive
A locomotive is a railway vehicle that provides the motive power for a train. The word originates from the Latin loco – "from a place", ablative of locus, "place" + Medieval Latin motivus, "causing motion", and is a shortened form of the term locomotive engine, first used in the early 19th...
s used them in conjunction with 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 to convert high voltage AC from the overhead lines
Overhead lines
Overhead lines or overhead wires are used to transmit electrical energy to trams, trolleybuses or trains at a distance from the energy supply point...
to relatively low voltage DC for the traction motors. For most of these applications, ignitrons have been replaced by solid state alternatives.
Because they are far more resistant to damage due to overcurrent
Overcurrent
In electricity supply, 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,...
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
Pulsed power
Pulsed power is the term used to describe the science and technology of accumulating energy over a relatively long period of time and releasing it very quickly thus increasing the instantaneous power.-Overview:...
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
Refractory
A refractory material is one that retains its strength at high temperatures. ASTM C71 defines refractories as "non-metallic materials having those chemical and physical properties that make them applicable for structures, or as components of systems, that are exposed to environments above...
metal, usually molybdenum
Molybdenum
Molybdenum , is a Group 6 chemical element with the symbol Mo and atomic number 42. The name is from Neo-Latin Molybdaenum, from Ancient Greek , meaning lead, itself proposed as a loanword from Anatolian Luvian and Lydian languages, since its ores were confused with lead ores...
, to handle reverse current during ringing
Ringing (signal)
In electronics, signal processing, and video, ringing is unwanted oscillation of a signal, particularly in the step response...
(or oscillatory) discharges without damage. Pulse rated ignitrons usually operate at very low duty cycle
Duty cycle
In engineering, the duty cycle of a machine or system is the time that it spends in an active state as a fraction of the total time under consideration....
s. They are often used to switch high energy 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...
banks during electromagnetic forming
Electromagnetic forming
Electromagnetic forming is a type of high velocity, cold forming process for electrically conductive metals, most commonly copper and aluminium. The workpiece is reshaped by high intensity pulsed magnetic fields that induce a current in the workpiece and a corresponding repulsive magnetic field,...
, electrohydraulic forming
Electrohydraulic forming
Electrohydraulic forming is a type of metal forming in which an electric arc discharge in liquid is used to convert electrical energy to mechanical energy and change the shape of the workpiece. A capacitor bank delivers a pulse of high current across two electrodes, which are positioned a short...
, or for emergency short-circuiting of high voltage power sources ("crowbar"
Crowbar (circuit)
A crowbar circuit is an electrical circuit used to prevent an overvoltage condition of a power supply unit from damaging the circuits attached to the power supply. It operates by putting a short circuit or low resistance path across the voltage source, much as if one dropped a tool of the same name...
switching).
See also
- ThyratronThyratronA thyratron is a type of gas filled tube used as a high energy electrical switch and controlled rectifier. Triode, tetrode and pentode variations of the thyratron have been manufactured in the past, though most are of the triode design...
- ThyristorThyristorA thyristor is a solid-state semiconductor device with four layers of alternating N and P-type material. They act as bistable switches, conducting when their gate receives a current trigger, and continue to conduct while they are forward biased .Some sources define silicon controlled rectifiers and...
- KrytronKrytronThe krytron is a cold-cathode gas filled tube intended for use as a very high-speed switch, and was one of the earliest developments of the EG&G Corporation. It is somewhat similar to thyratron...
- Triggered spark gap
- Pulsed powerPulsed powerPulsed power is the term used to describe the science and technology of accumulating energy over a relatively long period of time and releasing it very quickly thus increasing the instantaneous power.-Overview:...
- Mercury arc valveMercury arc valveA mercury-arc valve is a type of electrical rectifier used for converting high-voltage or high-current alternating current into direct current . Rectifiers of this type were used to provide power for industrial motors, electric railways, streetcars, and electric locomotives, as well as for...
External links
- Comprehensive Study of High Power Ignitrons Diana Lynn Loree