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 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 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 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 (grid tied to cathode) in a transverse magnetic field. 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. The miniature RK61/2 thyratron marketed in 1938 was designed specifically to operate like a
vacuum triode below its ignition voltage, allowing it to amplify analog signals as a
self-quenching superregenerative detector in
radio control receivers, and was 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. 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. 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. 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==