Editing Thyratron

{{Short description|Gas-filled tube, electrical switch, rectifier}}
{{Use mdy dates|date=May 2025}}
[[Image:thyratronsmall.jpg|thumb|upright|Giant [[General Electric|GE]] hydrogen thyratron, used in pulsed [[radar]]s, next to miniature 2D21 thyratron used to trigger [[relay]]s in [[jukebox]]es. Reference 2D21 tube is {{cvt|2+1/8|in}} tall.]]
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 a [[Townsend discharge]]. Gases used include [[Mercury (element)|mercury]] vapor, [[xenon]], [[neon]], and (in special high-voltage applications or applications requiring very short switching times) [[hydrogen]].<ref>{{cite book |editor-first=L. W. |editor-last=Turner |title=Electronics Engineer's Reference Book |edition=4th |publisher=Newnes-Butterworth |location=London |date=1976 |isbn=0-408-00168-2 |at=pp. 7-177 and 7-180}}</ref> Unlike a [[vacuum tube]] (valve), a thyratron cannot be used to [[Amplifier|amplify]] signals linearly.

In the 1920s, thyratrons were derived from early vacuum tubes such as the UV-200, which contained a small amount of argon gas to increase its [[Sensitivity (electronics)|sensitivity]] as a [[radio]] signal detector, and the German LRS relay tube, which also contained argon gas. Gas [[rectifier]]s, which predated vacuum tubes, such as the argon-filled General Electric "[[rectifier#Argon gas electron tube|Tungar bulb]]" and the [[Peter Cooper Hewitt|Cooper-Hewitt]] [[Mercury-arc valve|mercury-pool rectifier]], also provided an influence. [[Irving Langmuir]] and G. S. Meikle of GE are usually cited as the first investigators to study controlled rectification in gas tubes, about 1914. The first commercial thyratrons appeared circa 1928.

The term "thyratron" is derived from Ancient Greek "θύρα" ("thyra"), meaning "door" or "valve". The term "[[thyristor]]" was further derived from a combination of "thyratron" and "[[transistor]]".<ref>{{cite web |url=http://home.roadrunner.com/~mathematikos/Etymology/thyristor.pdf |title=Etymology of thyristor |access-date=2014-01-28 |url-status=dead |archive-url=https://web.archive.org/web/20120905132123/http://home.roadrunner.com/~mathematikos/Etymology/thyristor.pdf |archive-date=2012-09-05 }}</ref> Since the 1960s thyristors have replaced thyratrons in most low- and medium-power applications.

==Description==
[[File:Thyratron Symbols.svg|thumb|alt=Thyratron Symbols|Most commonly used symbols in the US and Europe of a thyratron (variations are usually related to the representation of the filament and the cathode)]]

Thyratrons resemble [[vacuum tube|vacuum tubes]] both in appearance and construction but differ in behavior and operating principle. In a vacuum tube, conduction is dominated by [[Electron|free electrons]] because the distance between [[anode]] and [[cathode]] is small compared to the [[mean free path]] of electrons. A thyratron, on the other hand, is intentionally filled with gas so that the distance between anode and cathode is comparable with the mean free path of electrons. This causes conduction in a thyratron to be dominated by [[Plasma (physics)|plasma]] conductivity. Due to the high conductivity of plasma, a thyratron is capable of switching higher currents than vacuum tubes which are limited by [[space charge]]. A vacuum tube has the advantage that conductivity may be modulated at any time whereas a thyratron becomes filled with plasma and continues to conduct as long as a [[voltage]] exists between the anode and cathode. A [[pseudospark switch]] operates in a similar regime of the [[Paschen's law|Paschen curve]] as a thyratron and is sometimes called a [[cold cathode]] thyratron.

A thyratron consists of a [[hot cathode]], an anode, and one or more [[control grid|control grids]] between the anode and cathode in an airtight glass or ceramic envelope that is filled with gas. The gas is typically [[hydrogen]] or [[deuterium]] at a pressure of 300 to 500&nbsp;m[[Torr]] (40 to 70&nbsp;[[Pascal (unit)|Pa]]). Commercial thyratrons also contain a [[titanium hydride]] reservoir and a reservoir heater that together maintain gas pressure over long periods regardless of gas loss.

Conductivity of a thyratron remains low as long as the control grid is negative relative to the cathode because the grid repels electrons emitted by the cathode. Space charge limited electron current flows from the cathode through the control grid toward the anode if the grid is made positive relative to the cathode. Sufficiently high space charge limited current initiates [[Townsend discharge]] between anode and cathode. The resulting plasma provides high conductivity between anode and cathode and is not limited by space charge. Conductivity remains high until the current between anode and cathode drops to a small value for a sufficiently long time that the gas ceases to be [[ionization|ionized]]. This recovery process takes 25 to 75&nbsp;μ[[second|s]] and limits thyratron repetition rates to a few&nbsp;k[[Hertz|Hz]].
<ref>{{cite book
|title=Gas Discharge Closing Switches
|publisher=Springer Science+Business Media, LLC
|isbn=978-1-4899-2132-1
|date=1990}}</ref>

==Applications==
[[Image:Z806W.JPG|left|thumb|Rare Z806W ''relay tube'' used in elevators]]

Low-power thyratrons (''relay tubes'' and ''trigger tubes'') were manufactured for controlling incandescent lamps, electromechanical relays or solenoids, for bidirectional counters, to perform various functions in [[Dekatron]] calculators, for voltage threshold detectors in [[RC circuit|RC]] timers, etc. ''Glow thyratrons'' were optimized for high gas-discharge light output or even [[phosphor]]ized and used as self-displaying [[shift register]]s in large-format, crawling-text [[dot-matrix display]]s.

Another use of the thyratron was in [[relaxation oscillator]]s.<ref name="Gottlieb">{{cite book
 | last1  = Gottlieb
 | first1 = Irving 
 | title  = Practical Oscillator Handbook
 | publisher = [[Elsevier]]
 | date   = 1997
 | pages  = 69–73
 | url    = https://books.google.com/books?id=e_oZ69GAuxAC&q=thyratron+%22relaxation+oscillator%22&pg=PA70
 | isbn   = 0080539386
 }}</ref> Since the plate turn-on voltage is much higher than the turn-off voltage, the tube exhibits [[hysteresis]] and, with a capacitor across it, it can function as a sawtooth oscillator. The voltage on the grid controls the breakdown voltage and thus the period of oscillation. Thyratron relaxation oscillators were used in [[power inverter]]s and [[oscilloscope]] sweep circuits.

One miniature thyratron, the triode 6D4, found an additional use as a potent [[Noise generator|noise source]], when operated as a diode (grid tied to cathode) in a transverse magnetic field.<ref>{{cite web |url=https://frank.pocnet.net/sheets/137/6/6D4.pdf |title=''6D4 Miniature triode thyratron'' data sheet |publisher=[[Sylvania Electric Products|Sylvania]] |access-date=25 May 2013 |archive-date=18 October 2021 |archive-url=https://web.archive.org/web/20211018232318/http://www.frank.mif.pg.gda.pl/sheets/137/6/6D4.pdf |url-status=live }}</ref> Sufficiently filtered for "flatness" ("[[white noise]]") in a band of interest, such noise was used for testing radio receivers, servo systems and occasionally in analog computing as a [[hardware random number generator|random value source]].

The miniature RK61/2 thyratron marketed in 1938 was designed specifically to operate like a [[Triode|vacuum triode]] below its ignition voltage, allowing it to amplify analog signals as a [[Regenerative circuit|self-quenching superregenerative detector]] in [[radio control]] receivers,<ref>{{cite web |url=https://frank.pocnet.net/sheets/138/r/RK61.pdf |title=''Subminiature gas triode type RK61'' data sheet |publisher=[[Raytheon|Raytheon Company]] |access-date=20 March 2017 |archive-date=18 October 2021 |archive-url=https://web.archive.org/web/20211018232501/http://www.frank.mif.pg.gda.pl/sheets/138/r/RK61.pdf |url-status=live }}</ref> and was the major technical development which led to the wartime development of radio-controlled weapons and the parallel development of [[radio-controlled models|radio controlled modelling]] as a hobby.<ref name=Honnest-Redlich>George Honnest-Redlich ''Radio Control for Models (1950)'' p. 7</ref>

[[File:Wynn-Williams Scale-of -Two Counter from the Cavendish Laboratory, University of Cambridge,UK.jpg|right|thumbnail|[[C. E. Wynn-Williams#Prewar research|Wynn-Williams's scale-of-two counter]] using thyratrons (with permission of the [[Cavendish Laboratory]], [[University of Cambridge]], UK.)]]
Some early television sets, particularly British models, used thyratrons for vertical (frame) and horizontal (line) oscillators.<ref>{{cite web |url=http://www.earlytelevision.org/british_american_comparison_prewar.html |title=Comparison of British and American Pre-1945 Sets |publisher=Early Television Museum of Hilliard OH |access-date=4 February 2018}}</ref>{{self-published inline|certain=yes|date=May 2025}}

Medium-power thyratrons found applications in machine tool motor controllers, where thyratrons, operating as phase-controlled rectifiers, are utilized in the tool's armature regulator (zero to "base speed", "constant torque" mode) and in the tool's field regulator ("base speed" to about twice "base speed", "constant horsepower" mode). Examples include [[Monarch Machine Tool]] 10EE lathe, which used thyratrons from 1949 until solid-state devices replaced them in 1984.<ref>{{cite web |url=http://www.lathes.co.uk/monarch/page2.html |website=Lathes.co.uk |accessdate=2012-07-27 |title=Monarch 10EE Toolroom Lathe}}</ref>{{self-published inline|certain=yes|date=May 2025}}

High-power thyratrons are still manufactured, and are capable of operation up to tens of [[ampere|kiloamperes]] (kA) and tens of [[volt|kilovolts]] (kV). Modern applications include pulse drivers for pulsed [[radar]] equipment, high-energy [[gas laser]]s, [[radiotherapy]] devices, [[particle accelerator]]s and in [[Tesla coil]]s and similar devices. Thyratrons are also used in high-power [[Ultra high frequency|UHF]] [[television]] [[transmitter]]s, to protect [[inductive output tube]]s from internal [[short circuit|short]]s, by grounding the incoming high-voltage supply during the time it takes for a [[circuit breaker]] to open and reactive components to drain their stored charges. This is commonly called a ''[[Crowbar (circuit)|crowbar circuit]]''.

Thyratrons have been replaced in most low and medium-power applications by corresponding semiconductor devices known as [[thyristor]]s (sometimes called [[silicon-controlled rectifier]]s, or SCRs) and [[TRIAC|triac]]s. However, switching service requiring voltages above 20 kV and involving very short risetimes remains within the domain of the thyratron.

Variations of the thyratron idea are the [[krytron]], the [[krytron|sprytron]], the [[ignitron]], and the triggered [[spark gap]], all still used today in special applications, such as nuclear weapons (krytron) and AC/DC-AC power transmission (ignitron).

==Example of a small thyratron==
[[Image:885 Thyratron.jpg|thumb|upright|R.C.A. brand 885 Triode Thyratron]]
The ''885'' is a small thyratron tube, using [[argon]] gas. This device was used extensively in the timebase circuits of early [[oscilloscope]]s in the 1930s. It was employed in a circuit called a [[relaxation oscillator]]. During [[World War II]], small thyratrons similar to the 885 were utilized in pairs to construct [[flip-flop (electronics)|bistable]]s, the "memory" cells used by early [[computer]]s and [[code breaking]] machines. Thyratrons were also used for [[Phasor|phase angle]] control of [[alternating current]] (AC) power sources in [[battery charger]]s and [[light dimmer]]s, but these were usually of a larger current handling capacity than the 885. The 885 is a 2.5 volt, 5-pin based variant of the 884/6Q5.

==See also==
* [[Krytron]]
* [[Spark gap#Power-switching devices|Triggered spark gap]]

==Notes==
{{reflist}}

==References==
*Stokes, John, ''70 Years of Radio Tubes and Valves,'' Vestal Press, NY, 1982, pp.&nbsp;111–115.
*Thrower, Keith, ''History of the British Radio Valve to 1940,'' MMA International, 1982, p.&nbsp;30, 31, 81.
*[[Albert W. Hull|Hull, A. W.]], "Gas-Filled Thermionic Valves", Trans. AIEE, 47, 1928, pp.&nbsp;753–763.
*Data for 6D4 type, "Sylvania Engineering Data Service", 1957
*J.D. Cobine, J.R. Curry, "Electrical Noise Generators", Proceedings of the I.R.E., 1947, p.&nbsp;875
* Radio and Electronic Laboratory Handbook, M.G. Scroggie 1971, {{ISBN|0-592-05950-2}}

==External links==
*[http://www.electricstuff.co.uk/pulse.html Article about switch tubes by John Pasley]
*[http://www.pocketmagic.net/?p=1745 Article on gas-filled thyratrons]

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[[Category:Products introduced in 1928]]
[[Category:Gas-filled tubes]]
[[Category:Switching tubes]]
[[Category:Rectifiers]]