Firing angle Control of SCR By the UJT Full Wave Circuit

  • 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.



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