The Mercury-Vapor Rectifier

Author: E.E. Kimberly

Mercury vapor may be used as the conducting medium in a diode of the gas-filled type, and is widely used in heavy-duty rectifiers. Just as in other types of diodes, the cathode of the mercury-vapor rectifier must be heated before conduction can take place. The use of mercury as the cathode permits the rectifier to withstand heavy momentary overloads without damage. During operation the mercury vapor is formed in the evacuated tube over a pool of mercury, which is the cathode. By tipping the tube momentarily, the mercury is caused to touch the anode and to produce an arc and vapor. The heat of the arc produces the necessary ionization of the mercury vapor, and conduction takes place through the external anode circuit if complete. However, after the first half-cycle of conduction is completed, ionization ceases and the conduction cannot restart until the tube is tipped again. Because of the practical difficulty of maintaining the current over the negative half-cycle of plate voltage, the mercury-vapor rectifying tube must be provided with a holding or "keeper" circuit when used for half-wave rectification.

Fig. 27-38 shows a type of full-wave rectifier, together with its circuit, as used in battery charging. The electrodes A and D of the rectifier tube are anodes. The electrode C is a starting anode only. To start the operation, the tube is tilted until part of the mercury from reservoir B spills into contact with C and starts an arc. The arc produces mercury vapor and ionizes that vapor. If, in the half-cycle in which contact' is made, the anode D is positive to B, electrons will pass from B to D and a charging current will flow through the battery. On the next half-cycle, the anode A will be positive to B and the electron flow will be from B to A, causing a current in the battery in the same direction as before.

Fig. 27-38. Mercury-Vapor Rectifier

If the current of one half-cycle were allowed to fall to zero before the next half-cycle started, ionization would cease and rectification would be discontinued, as in the half-wave rectifier without keeper circuit. An inductance X inserted in the cathode lead causes the current flow to persist for more than 180 degrees of time, and so causes overlapping in time with the following half-cycle, as shown in Fig. 27-39 (a). Inasmuch as the resultant current of Fig. 27-39 (b) never falls to zero, the ionization and hence the tube conductance is never zero, and rectification continues from cycle to cycle.

Fig. 27-39. Mercury-Vapor Rectifier Output Current

Mercury-vapor rectifiers of steel-clad construction, like that shown in Fig. 27-40, are in use in street-railway and general-transportation service. They are usually built for operation on 3-phase supply.

Fig. 27-40. Mercury-Arc Rectifier

By proper transformer connections they may be operated as 6-phase or even 12-phase rectifiers. The ripple in the output of a 6-phase unit is less than that from a 3-phase unit, and the ripple from a 12-phase unit is even less than that from a 6-phase unit. The ripple sometimes causes objectionable noise in telephone systems whose lines parallel those of power circuits supplied by such rectifiers. The noise can be reduced to an acceptable minimum, however, by the use of filter circuits.