Circuit Operation

The source v_s is an alternating sinusoidal source. If v_s = E * sin (wt), v_s is positive when 0 < wt < p, and it is negative when p < wt <2p. When v_s starts becoming positive, the SCR is forward-biased but remains in the blocking state till it is triggered. If the SCR is triggered at when wt = a, then a is called the firing angle. When the SCR is triggered in the forward-bias state, it starts conducting and the positive source keeps the SCR in conduction till wt reaches p radians. At that instant, the current through the circuit is not zero and there is some energy stored in the inductor at wt = p radians. In the absence of the free wheeling diode, the inductor would keep the SCR in conduction for part of the negative cycle till the energy stored in it is discharged. But when a free wheeling diode is present as shown in the circuit, the current has a path that offers almost zero resistance. Hence the inductor discharges its energy during p < wt < (2p + a) through the free wheeling diode. When there is a free wheeling diode, the current through the load tends to be continuous, at least under ideal conditions. When the diode conducts, the SCR remains reverse-biased, because the voltage v_s is negative.