Ignitron
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An ignitron is a type of controlled rectifier dating from the 1930s. Invented by Joseph Slepian while employed by Westinghouse, Westinghouse was the original manufacturer and owned trademark rights to the name "Ignitron".
It is usually a large steel container with a pool of mercury in the bottom, acting as a cathode. A large graphite cylinder, held above the pool by an insulated electrical connection, serves as the anode. An igniting electrode (called the "ignitor") is briefly pulsed to create an electrically conductive mercury plasma, triggering heavy conduction between the cathode and anode.
Ignitrons were long used as high-current rectifiers in major industrial installations where thousands of amperes of AC current must be converted to DC, such as aluminum smelters. Large electric motors were also controlled by ignitrons used in gated fashion, in a manner similar to modern semiconductor devices such as silicon controlled rectifiers and triacs. Many electric locomotives used them in conjunction with transformers to convert high voltage AC from the catenary to relatively low voltage DC for the motors.
Because they are far more resistant to damage due to overcurrent or back-voltage, ignitrons are still manufactured and used in preference to semiconductors in certain installations. For example, specially constructed pulse rated ignitrons are still used in certain pulsed power applications. These devices can switch hundreds of kiloamperes and hold off as much as 50,000 volts. The anodes in these devices are fabricated from a refractory metal, usually molybdenum, to handle reverse current flow during ringing (or oscillatory) discharges without damage. Pulse rated ignitrons usually operate at very low duty cycles. They are often used to switch high energy capacitor banks during electromagnetic forming, electrohydraulic forming, or for emergency short-circuiting of high voltage power sources ("crowbar" switching).
