Editing Ignitron

[[Image:Ignitron.svg|right|thumb|250px|(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 Electric Corporation|Westinghouse]], Westinghouse was the original manufacturer and owned trademark rights to the name "Ignitron". Ignitrons are closely related to [[mercury-arc valve]]s but differ in the way the arc is ignited.  They function similarly to [[thyratron]]s; 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.

==Construction and operation==
[[Image:Ignitrons.jpg|thumb|Ignitron rectifiers powering industrial process, 1945]]
An ignitron is usually a large steel container with a pool of [[mercury (element)|mercury]] in the bottom that acts as a [[cathode]] during operation. A large [[graphite]] or [[refractory metals|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,<ref> Turner pg. 7-182</ref> is briefly pulsed with a high current to create a puff of electrically conductive mercury [[plasma (physics)|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 [[electrical arc|arc]]. The mercury surface thus serves as the cathode, and [[Electric current|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.<ref> L.W. Turner,(ed), ''Electronics Engineer's Reference Book'', 4th ed. Newnes-Butterworth, London 1976 {{ISBN|0408001682}} pages 7-181 through 7-189</ref>

==Applications==
Ignitrons were long used as high-current rectifiers in major industrial and utility installations where thousands of amperes of [[alternating current|AC]] must be converted to [[direct current|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{{clarify|date=June 2013}} fashion, in a manner similar to modern semiconductor devices such as [[silicon controlled rectifier]]s and [[TRIAC|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]]. The Pennsylvania Railroad's [[PRR_E44|E44]] freight locomotives carried on-board ignitrons, as did the Russian [[:ru:ВЛ60|ВЛ-60]] freight locomotive. For many modern 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 50&nbsp;kV. The anodes in these devices are often fabricated from a refractory metal, usually [[molybdenum]], to handle reverse current during [[ringing (signal)|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 (circuit)|"crowbar"]] switching).
[[File:Philips ignitron pl5551a.jpg |thumb|right| 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==
* [[Excitron]]
* [[Trigatron]]
* [[Thyratron]]
* [[Thyristor]]
* [[Krytron]]
* [[Triggered spark gap]]
* [[Pulsed power]]
* [[Mercury-arc valve]]

==References==
{{reflist}}

==External links==
* [https://web.archive.org/web/20110815202923/http://etd.lib.ttu.edu/theses/available/etd-02262009-31295006979529/unrestricted/31295006979529.pdf Comprehensive Study of High Power Ignitrons] Diana Lynn Loree
* [http://mdk2001.web.cern.ch/mdk2001/Proceedings/Session13/welleman.PDF Semiconductor switches replace thyratron and ignitrons]
* [http://www.britannica.com/EBchecked/topic/282374/ignitron  Britannica online encyclopedia &ndash; Ignitron]
* [http://www.electropedia.org/iev/iev.nsf/display?openform&ievref=531-35-21 Electropedia &ndash; Ignitron]

{{Electronic components}}

[[Category:Gas-filled tubes]]
[[Category:Switching tubes]]
[[Category:Rectifiers]]