Athyratron is a type ofgas-filled tube used as a high-power electricalswitch and controlledrectifier. Thyratrons can handle much greater currents than similar hard-vacuum tubes. Electron multiplication occurs when the gas becomes ionized, producing a phenomenon known as aTownsend discharge. Gases used includemercury vapor,xenon,neon, and (in special high-voltage applications or applications requiring very short switching times)hydrogen.[1] Unlike avacuum tube (valve), a thyratron cannot be used toamplify 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 itssensitivity as aradio signal detector, and the German LRS relay tube, which also contained argon gas. Gasrectifiers, which predated vacuum tubes, such as the argon-filled General Electric "Tungar bulb" and theCooper-Hewittmercury-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".[2] Since the 1960s thyristors have replaced thyratrons in most low- and medium-power applications.
Thyratrons resemblevacuum tubes both in appearance and construction but differ in behavior and operating principle. In a vacuum tube, conduction is dominated byfree electrons because the distance betweenanode andcathode is small compared to themean 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 byplasma conductivity. Due to the high conductivity of plasma, a thyratron is capable of switching higher currents than vacuum tubes which are limited byspace 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 avoltage exists between the anode and cathode. Apseudospark switch operates in a similar regime of thePaschen curve as a thyratron and is sometimes called acold cathode thyratron.
A thyratron consists of ahot cathode, an anode, and one or morecontrol grids between the anode and cathode in an airtight glass or ceramic envelope that is filled with gas. The gas is typicallyhydrogen ordeuterium at a pressure of 300 to 500 mTorr (40 to 70 Pa). Commercial thyratrons also contain atitanium 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 initiatesTownsend 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 beionized. This recovery process takes 25 to 75 μs and limits thyratron repetition rates to a few kHz.[3]
Low-power thyratrons (relay tubes andtrigger tubes) were manufactured for controlling incandescent lamps, electromechanical relays or solenoids, for bidirectional counters, to perform various functions inDekatron calculators, for voltage threshold detectors inRC timers, etc.Glow thyratrons were optimized for high gas-discharge light output or evenphosphorized and used as self-displayingshift registers in large-format, crawling-textdot-matrix displays.
Another use of the thyratron was inrelaxation oscillators.[4] Since the plate turn-on voltage is much higher than the turn-off voltage, the tube exhibitshysteresis 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 inpower inverters andoscilloscope sweep circuits.
One miniature thyratron, the triode 6D4, found an additional use as a potentnoise source, when operated as a diode (grid tied to cathode) in a transverse magnetic field.[5] 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 arandom value source.
The miniature RK61/2 thyratron marketed in 1938 was designed specifically to operate like avacuum triode below its ignition voltage, allowing it to amplify analog signals as aself-quenching superregenerative detector inradio control receivers,[6] and was the major technical development which led to the wartime development of radio-controlled weapons and the parallel development ofradio controlled modelling as a hobby.[7]
Some early television sets, particularly British models, used thyratrons for vertical (frame) and horizontal (line) oscillators.[8][self-published source]
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 includeMonarch Machine Tool 10EE lathe, which used thyratrons from 1949 until solid-state devices replaced them in 1984.[9][self-published source]
High-power thyratrons are still manufactured, and are capable of operation up to tens ofkiloamperes (kA) and tens ofkilovolts (kV). Modern applications include pulse drivers for pulsedradar equipment, high-energygas lasers,radiotherapy devices,particle accelerators and inTesla coils and similar devices. Thyratrons are also used in high-powerUHFtelevisiontransmitters, to protectinductive output tubes from internalshorts, by grounding the incoming high-voltage supply during the time it takes for acircuit breaker to open and reactive components to drain their stored charges. This is commonly called acrowbar circuit.
Thyratrons have been replaced in most low and medium-power applications by corresponding semiconductor devices known asthyristors (sometimes calledsilicon-controlled rectifiers, or SCRs) andtriacs. 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 thekrytron, thesprytron, theignitron, and the triggeredspark gap, all still used today in special applications, such as nuclear weapons (krytron) and AC/DC-AC power transmission (ignitron).
The885 is a small thyratron tube, usingargon gas. This device was used extensively in the timebase circuits of earlyoscilloscopes in the 1930s. It was employed in a circuit called arelaxation oscillator. DuringWorld War II, small thyratrons similar to the 885 were utilized in pairs to constructbistables, the "memory" cells used by earlycomputers andcode breaking machines. Thyratrons were also used forphase angle control ofalternating current (AC) power sources inbattery chargers andlight dimmers, 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.
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