Single-phase Midpoint controlled rectifier with Resistive load:
1) During positive half cycle of AC supply, “a” is positive with respect to “b”, this makes T1 forward biased and T2 is reverse biased. But since no triggering pulse is applied, both are in off state. When SCR T1 is triggered at firing angle α, current flows through load from “a”, T1 and back to centre-tap “n” of the transformer. This current flow is continuous till angle π when the supply voltage reverses the polarity and T1 is turned off.
2) During negative half cycle of AC supply, “b” is positive with respect to “a”, this makes T2 forward biased and T1 is reverse biased. But since no triggering pulse is applied, both are in off state. When SCR T2 is triggered at firing angle α+π, current flows through load from “b”, T2 and back to centre-tap “n” of the transformer. This current flow is continuous till angle 2π, when the supply voltage reverses the polarity and T2 is turned off. The operation is as shown in waveforms.