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).
In electrohydraulic forming, an electric arc discharge is used to convert electrical energy to mechanical energy. A capacitor bank delivers a pulse of high current across two electrodes, which are positioned a short distance apart while submerged in a fluid (water or oil). The electric arc discharge rapidly vaporizes the surrounding fluid creating a shock wave. The workpiece, which is kept in contact with the fluid, is deformed into an evacuated die. A schematic illustration of the electrohydraulic forming process is shown in Fig. 1.
The potential forming capabilities of submerged arc discharge processes were recognized as early as the mid 1940s. During the 1950s and early 1960s, the basic process was developed into production systems. This work principally was by and for the aerospace industries. By 1970, forming machines based on submerged arc discharge, were available from machine tool builders. A few of the larger aerospace fabricators built machines of their own design to meet specific part fabrication requirements.
Electrohydraulic forming is a variation of the older, more general, explosive forming method. The only fundamental difference between these two techniques is the energy source, and subsequently, the practical size of the forming event.
Very large capacitor banks are needed to produce the same amount of energy as a modest mass of high explosives. This makes electrohydraulic forming very capital intensive for large parts. On the other hand, the electrohydraulic method was seen as better suited to automation because of the fine control of multiple, sequential energy discharges and the relative compactness of the electrode-media containment system. (metalformingmagazine.com)
