Fully controlled bridge rectifier for a resistive load. A fully-controlled circuit contains only thyristers (semiconductor or silicon controlled rectifiers. SCR1 and SCR2 must be fired simultaneously during the positive half-wave of the source voltage VRs Rto allow conduction of current. Alternatively, SCR3 and SCR4 must be fired simultaneously during the negative half wave of the source voltage.
To ensure simultaneous firing, SCR1 and SCR2 use the same firing signal presents the behavior of the fully controlled rectifier with resistive inductive load (with L→∞). The high-load inductance generates a perfectly filtered current and the rectifier behaves like a current source. With continuous load current, SCR1 and SCR2 remain in the on-state beyond the positive half-wave of the source voltage.
For this reason, the load voltage can have a negative instantaneous value. The firing of SCR3 and SCR4 has two effects, they turn off SCR1 and SCR2, After the commutation, they conduct the load current. This is the main reason why this type of converter is called a ‘‘naturally commutated’’ or ‘‘line commutated’’ rectifier. In this circuit SCR ‘s T1,T2,T3 and T4 are used. The conduction of all these SCR’s can be controlled so that this is called fully convertor.