Thyratron

Fig 1: R.C.A. brand 885 Triode Thyratron

A thyratron is a type of gas filled tube used as a high energy electrical switch and controlled rectifier. Triode, tetrode and pentode variations of the thyratron have been manufactured in the past, though most are of the triode design. Because of the gas fill, thyratrons can handle much greater currents than similar hard vacuum valves/tubes since the positive ions carry considerable current. 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. 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. A thyratron is basically a "controlled gas rectifier". 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 silicon controlled rectifier (SCR).

Construction : 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. When positive voltage is applied to the anode, if the control electrode is kept at cathode potential, no current flows. When the control electrode is made slightly positive, gas between the anode and cathode ionizes and conducts current. 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. 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.

 Fig 2: Rare Z806W relay tube used in elevators

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

Fig 3: Giant GE hydrogen thyratron, used in pulsed radars, next to miniature 2D21 thyratron used to trigger relays in jukeboxes

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.

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" ("white noise") in a band width of interest, such noise was used for testing radio receivers, servo systems and occasionally in analog computing as a random value source.

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 electrically identical to the 884/6Q5, which uses an octal base.