A thyratron is a type of gas-filled tube used as a high-power electrical switch
and controlled rectifier. Because of the gas fill, thyratrons can handle much
greater currents than similar hard-vacuum tubes, since electron
multiplication occurs in the ionized gas, the phenomenon called Townsend
discharge. Gases used include mercury vapor, xenon, neon, and hydrogen. Unlike
a vacuum tube, a thyratron cannot be used to 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
as a radio signal detector; and the German LRS Relay tube, which also
contained argon gas. Gas rectifiers, which predated vacuum tubes, such as the
argon-filled General Electric "Tungar bulb" and the Cooper-Hewitt 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 didn't
appear until around 1928. A solid-state device with similar
operating characteristics is the thyristor, sometimes also known as the
silicon controlled rectifier. The term "thyristor" was derived from a
combination of "thyratron" and "transistor". Since the 1960s thyristors
have replaced thyratrons in most low- and medium-power applications.
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 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 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 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 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.
Another use of the thyratron was in relaxation oscillators. Since the plate
turn-on voltage is much higher than the turn-off voltage the tube has hysteresis
and with a capacitor across it could function as a sawtooth oscillator. The
voltage on the grid controlled the breakdown voltage and thus the period of
oscillation. Thyratron relaxation oscillators were used in power inverters
and oscilloscope sweep circuits. One miniature thyratron, the triode 6D4,
found an additional use as a potent noise source, when operated as a diode
in a transverse magnetic field. Sufficiently filtered for "flatness" 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.
The miniature RK61/2 thyraton valves marketed in 1938 were designed
specifically for radio control circuits, and were the major technical development
which led to the wartime development of radio-controlled weapons and the
parallel development of radio controlled modelling as a hobby.
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 and in the tool's field
regulator. Examples include Monarch Machine Tool 10EE lathe, which used
thyratrons from 1949 until solid-state devices replaced them in 1984.
High-power thyratrons are still manufactured, and are capable of
operation up to tens of kiloamperes and tens of kilovolts. 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 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 and AC/DC-AC power transmission.
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 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
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
External links tpub.com
GLVAC Hydrogen Thyratron Article about switch tubes by John
Pasley Thyratron collection at the Virtual
Valve Museum Article on gas filled thyratrons