Rectifiers

There are many applications of electricity where, for technical reasons, DC is preferable to AC. It is necessary to have some means of obtaining DC from an AC source and this requirement is met by one or another of the numerous forms of rectifiers.

A metal rectifier, a widely used form of rectifier, is the metallic type consisting essentially of a number of units formed by depositing a layer of semiconducting material, such as copperoxide on a metal conducting base. These units, generally in the form of washers, are assembled on rods under pressure, the number of units employed being determined by the voltage. Each unit has the peculiar property of passing current freely when the semiconducting layer is positive and allowing practically no current to flow in the opposite direction. This form of rectifier has the great advantage of strength, absence of moving parts and relative simplicity.

A valve rectifier. Another type of rectifier is represented by various kinds of two-electrode valves, or diodes. These consist of a glass tube or bulb containing an electron-emitting member (the cathode) and an electrode (called anode) which collects the emitted electrons and causes a flow of current so long as it is positive with respect to the cathode. The cathode heater is supplied with a suitable voltage from the transformer secondary to provide the heating necessary for the emission of electrons. The action of the tube differs somewhat according to whether it is of the high vacuum or gas-filled type. The rectifying property of the diode valve arises from the fact that electrons can flow from cathode to anode only when the latter is positive with respect to the cathode. The electrons, having a negative charge, are attracted by the positive plate, thus causing a flow of current through the tube and the external circuit.

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