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The Ignitron

Author: E.E. Kimberly

Of all gas-filled tubes the ignitron is capable of passing the highest plate current without injury, its physical dimensions being considered. Its construction is shown in Fig. 27-36. It consists of a double-walled cylindrical steel jacket enclosing a graphite anode, a mercury-pool cathode, and an "igniter" finger of silicon carbide or other substance of high resistivity which dips into the mercury pool. Water is circulated between the two walls of the jacket to keep the mercury vapor in the inside space at the proper pressure for the best results. The mercury, being an almost indestructible source of electrons, is capable of passing unusually high currents of electrons to the graphite anode. The anode is made of graphite to enable it to withstand the intense bombardment of these electrons. Because of these features the ignitron is capable of passing unbelievably large currents for heavy-power duty such as is found in electric spot-welding and the electrolytic reduction of ores.

Fig. 27-36, Cut-away View of an Ignitron and Also its Ciccuit Symbol

The characteristics of operation of the ignitron are similar to those of the thyratron, but the control of ionization is accomplished differently.

When an "igniting" current of a few amperes is passed between the igniter and the mercury cathode, a small arc appears at the area of contact and ionizes a portion of the vapor at that point. If at that time a sufficient voltage is present between the anode and the cathode, complete ionization of the vapor space occurs and conduction begins. Just as in the grid-glow tube and the thyratron, the igniter has no further control over the action of the tube until such conduction has been stopped by opening the plate circuit or by reducing the plate voltage below that required for maintaining the conduction.

Unlike the thyratron, the ignitron does not require a warming-up period, and so it is always ready for instant use. It does, however, require much greater firing power in its igniter circuit than the thyratron does in its grid circuit. The power required for its firing is somewhat variable, and so the power in the igniter circuit must be comfortably higher than that required on the average in order to insure certainty of firing. If an ignitron fails to fire on an impulse from a thyratron, the igniter-to-cathode voltage will be permitted to rise far above normal, the igniter current will be excessive, and the protecting fuses in the igniter circuit will be blown. In the thyratron the grid-to-cathode voltage is held negative to prevent firing before the desired instant; whereas in the ignitron the conduction is started by positive action by the arcing between the igniter and the mercury-pool cathode.

Last Update: 2010-10-06