The single phase half or semi-controlled rectifier, control is affected only for positive output voltage, and no control is possible when its output voltage, so that a semi-controlled rectifier circuit is made up of both SCR and diodes, tends to become negative since it is clamped at zero volts.
SCR1 can be fired into the ON state at any time provided that voltage VSCR>0. The firing pulses are delayed by an angle a with respect to the instant where diodes would conduct.
SCR1 remains in the ON state until the load current tries to go to a negative value. SCR2 R is fired into the ON state when V SCR > 0, which corresponds to the condition at which SCR2> 0. The mean value of the load voltage with resistive load is given.