thyratron

An electronic valve filled with a gas (usually mercury vapour or an inert gas) at low pressure. It is used for switching, and as a controlled rectifier in applications such as welding. Such valves are now being replaced by semiconductor devices.

A thyratron is a type of gas filled tube used as a high energy electrical switch. Triode, Tetrode and Pentode variations of the thyratron have been manufactured in the past, though most are of the triode design. Gases used include mercury vapor, xenon, neon, and (in special high-voltage applications or applications requiring very short switching times) hydrogen. Unlike a vacuum tube, a thyratron cannot be used to amplify signals linearly.

Thyratrons evolved in the 1920s 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; A thyratron is basically a "controlled gas rectifier".

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. The gas in a thyratron is typically at a fraction of the pressure of air at sea level; Once turned on, the thyratron will remain on (conducting) as long as there is a significant current flowing through it.

Small thyratrons were manufactured in the past for controlling electromechanical relays and for industrial applications such as motor and arc-welding controllers.

Modern applications include pulse drivers for pulsed radar equipment, high-energy gas lasers, radiotherapy devices, 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.

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.