Thyratrons come in several varieties. All work similarly to the semiconductor Thyristor, one difference being that in many designs (Hydrogen Thyratrons are a common exception) the gate must be biased highly negative in the off state and then biased positive to achieve switching. Like Thyristors, Thyratrons operate like a latching switch, ie. once you have turned them on you can only turn off by cutting the supply to the main circuit. Mercury filled Thyratrons are the slowest, least useful type and are much more restricted environmentally than other types due chiefly to problems with the mercury condensing . They are rarely used as they have few advantages of the thyristor. Hydrogen Thyratrons are *much* faster switching than Thyristors. Some can achieve commutation in under 20ns. Inert gas fillings tend to offer superior performance compared to mercury filled devices, without matching the speed of the Hydrogen filled devices.
Note that Hydrogen Filled Devices employ a hot cathode.
The actual Physical construction/ operation of the thyratron is quite complicated compared to the other devices we have looked at and no attempt will be made to explain it's operation. The reader is advised to consult a wide range of books as devices employing different fillings, or electrode heating methods operate differently. It is not considered to be especially important to consider all these variations here as this is merely an overview of these devices and is not intended to be the final word on the subject. However, in order to differentiate the thyratron form other similar devices and to define it in at least some physical manner here follows Frungel's (Ref.4) definition of the device:
'By the term 'thyratron' there is meant a discharge chamber in which are arranged a cathode, one or several grids, and an anode, and which is filled with an inert gas or metal vapor.'
Some Thyratrons can handle up to 50kV(double gap types) switch thousands of Amperes and handle very high power outputs( e.g. CX 1154 can handle peak powers of 40MW). Typical applications are Radar pulse modulators, Particle accelerators, Lasers and high voltage medical equipment. Another variety of thyratron is filled with Deuterium. These Deuterium filled devices are similar to their Hydrogen filled counterparts but the sparking potential for Deuterium is higher thus allowing even higher voltages to be handled. E.g. E3213 can switch 70kV (double gap type). Specialist Thyratrons with ceramic and metal bodies are encountered. These are designed to be used in extreme environmental conditions. There is a wide variety of grid configurations seen in Thyratrons, it would be impractical to consider them all here. Manufacturers of Thyratrons Include EG&G, GEC, English Electric Valve Co.Ltd, M-O Valve co.Ltd. Big Thyratrons often require you to get a big box full of driver/control circuitry. Prices vary from a couple of dollars to thousands. Hot and cold cathode type devices are encountered.
Note these ratings are the exception rather than the rule in Thyratron devices, devices designed for sub kilovolt voltages and only capable of handling a few tens of amps pulsed are common enough.
Thyratrons typically come in either small multi pin base type packages such as are common in other vacuum tubes or in the case of the higher current devices large tubular packages with hefty end connectors.