Electrical Revolution

14/03/2017

The firing angle control of SCR by UJT full wave circuit is shown in the Figure A.
The firing angle during each positive half cycle has same value in this circuit.
The pulse transformer has two secondary winding S and S’ for each half cycle.
The diode D1 and D2 converts AC in to DC.
The function of the zener diode is to clip pulsating DC ( as shown in the Figure B ).
The charging of capacitor C1 is done through resistor R1.
When the voltage across capacitor becomes peak point voltage, the UJT turns on.
As soon as UJT turns on, the capacitor is discharged through C1 – E – P – C1.
Now the pulse transformer gets pulse. As the secondary winding of pulse transformer S and S’ receives pulse, both SCR1 and SCR2 turns on via gate signal.
The load circuit receives current through terminal b – SCR2 – S’ – LOAD – C during positive half cycle.
Similarly the load current receives current through a – SCR1 – S – LOAD – c during negative half cycle.
As the load receives load current in both positive and negative half cycle, it is called as firing angle control of SCR by the UJT full wave circuit.
The waveform for the firing angle control of SCR by UJT full wave is shown in the Figure B.