Thyratron
A thyratron is a thermionic valve with one or more grids, into which a small amount of gas has been introduced. In function, it behaves like a switched rectifier. In its ‘off’, non-conducting state, it will hold off high voltages of either polarity; when triggered ‘on’, it will conduct large currents with very precise timing. In its simplest triode form, a thyratron consists of an anode and cathode separated by a control grid.
It differs from a conventional vacuum triode in that it can conduct large currents with low dissipation using a very much smaller cathode. When it is triggered, ionization processes in the gas provide large numbers of extra current carriers; effectively this amplifies the cathode capacity many times.
Low power thyratrons were used initially in rectification and motor control and have now been replaced by solid state devices, principally the thyristor, a solid-state analogue of the thyratron. Hydrogen or deuterium filled thyratrons are used extensively in radar, laser switching, medical linear accelerators and switching applications in high energy physics.
Construction and operation
A typical hot-cathode thyratron uses a heated filament cathode, completely contained within a shield assembly with a control grid on one open side, which faces the plate-shaped anode. In the off situation the voltage on the control grid is negative with respect to the cathode. When positive voltage is applied to the anode, no current flows. When the control electrode is made less negative, electrons from the cathode can travel to the anode because the positive attraction from the anode prevails over the negative repulsion caused by the slightly negative voltage on the control grid. The electrons will ionize the gas by collisions with the gas in the tube and an avalanche effect results, causing an arc discharge between cathode and anode. The shield prevents ionized current paths that might form within other parts of the tube. The gas in a thyratron is typically at a fraction of the pressure of air at sea level; 15 to 30 millibars (1.5 to 3 kPa) is typical. For a cold cathode thyratron the trigger voltage on the control grid will typically be positive, and a flash over from control grid to cathode will initiate the arc discharge in the tube.
Both hot- and cold-cathode versions are encountered. A hot cathode is at an advantage, as ionization of the gas is made easier; thus, the tube's control electrode is more sensitive. Once turned on, the thyratron will remain on (conducting) as long as there is a significant current flowing through it. When the anode voltage or current falls to zero, the device switches off.
Applications
Low-power thyratrons (relay tubes and trigger tubes) were manufactured in the past for controlling incandescent lamps, electromechanical relays or solenoids, for bidirectional counters, to perform various functions in Dekatron calculators, for voltage threshold detectors in RC timers, etc. Glow thyratrons were optimized for high gas-discharge light output or even phosphorized and used as self-displaying shift registers in large-format, crawling-text dot-matrix displays.
One miniature thyratron, the triode 6D4, found an additional use as a potent noise source, when operated as a diode (grid tied to cathode) in a transverse magnetic field.[1] 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 random value 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 include Monarch Machine Tool 10EE lathe, which used thyratrons from 1949 until solid-state devices replaced them in 1984,[2] 13EE and EE1000. For a brief period, an SCR-based "Monarch Sidney" drive was offered, which incorporated so-called SCR "power blocks", but this model was quickly discontinued. The various national laboratories are still utilizing thyratron-based 10EE and 13EE/EE1000 machines and Richardson Electronics, as the successor to Electrons Inc, still makes the thyratrons which these machines utilize.[citation needed][dubious – discuss]
High-power thyratrons are still manufactured, and are capable of operation up to tens of kiloamperes (kA) and tens of kilovolts (kV). Modern applications include pulse drivers for pulsed radar equipment, high-energy gas lasers, radiotherapy devices, particle accelerators and in Tesla coils and similar devices. Thyratrons are also used in high-power UHF television transmitters, to protect inductive output tubes from internal shorts, 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.
Thyratrons have been replaced in most low and medium-power applications by corresponding semiconductor devices known as thyristors (sometimes called silicon-controlled rectifiers, or SCRs) and triacs. 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 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
The 885 is a small thyratron tube, using xenon gas. This device was used extensively in the timebase circuits of early oscilloscopes 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 bistables, the "memory" cells used by early computers and code breaking machines. Thyratrons were also used for phase angle control of alternating current (AC) power sources in battery chargers and light 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.
Notes
- ^ "Sylvania: 6D4 Miniature triode thyratron data sheet" (PDF). Retrieved 25 May 2013.
- ^ http://www.lathes.co.uk/monarch/page2.html Lathes.co.uk, retrieved 2012 July 27
References
- Stokes, John, 70 Years of Radio Tubes and Valves, Vestal Press, NY, 1982, pp. 111–115.
- Thrower, Keith, History of the British Radio Valve to 1940, MMA International, 1982, p. 30, 31, 81.
- Hull, A. W., "Gas-Filled Thermionic Valves", Trans. AIEE, 47, 1928, pp. 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. 875
- Radio and Electronic Laboratory Handbook, M.G. Scroggie 1971, ISBN 0-592-05950-2