The simplest of all rectifier categories is the diode rectifier. In the simplest form, the diode rectifier does not provide any means to control the output current and voltage values. In order to be suitable for industrial processes, the output value must be controllable within a certain range. This control can be accomplished by applying a mechanical so-called on-load tap changer. As a typical case, the on-load tap changer controls the input AC voltage on the primary side of the rectifier transformer, so it can control the output DC value within a certain range. Usually the on-load tap changer is used in combination with a saturable reactor connected in series in the output circuit of the rectifier. By introducing direct current into the reactor, a variable impedance is generated in the line. Therefore, by controlling the voltage drop across the reactor, the output value can be controlled within a relatively narrow range.
Very close to the diode rectifier in design is the thyristor rectifier. Because the electrical parameters of the thyristor rectifier are controllable, there is no need for an on-load tap changer and a saturated reactor.
Because the thyristor rectifier contains no moving parts, the maintenance of the thyristor rectifier system is reduced. One advantage noted is that the regulation speed of the thyristor rectifier is faster than that of the diode rectifier. During the process characteristic step, the thyristor rectifier is often adjusted so quickly that overcurrent can be avoided. As a result, the overload capacity of the thyristor system can be designed to be smaller than that of the diode system.