Circuit Operation

A circuit with a single SCR and an RL load is shown above. The source v_s is an alternating sinusoidal source. If v_s = E * sin (wt), v_s is positive when 0 < wt < p, and v_s 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. The voltage across an inductor is positive when the current through it is increasing and it becomes negative when the current through the inductor tends to fall. When the voltage across the inductor is negative, it is in such a direction as to forward-bias the SCR.

There is current through the load at the instant wt = p radians and the SCR continues to conduct till the energy stored in the inductor becomes zero. After that the current tends to flow in the reverse direction and the SCR blocks conduction. The entire applied voltage now appears across the diode.

 
TO THE TOP