Firing of SCR By the UJT half Wave Circuit

  • The firing angle control of SCR by UJT is shown in the Figure A.
  • When switch SW is on, the charging of capacitor C1 is done through resistor R1. 
  • When the voltage across capacitor becomes equal to peak point voltage, the UJT turns on.  
  • As soon as the UJT turns on, the gate of SCR receives gate pulse through resistance R4 resulting SCR turns on. 
  • The load gets supply through P – RL – A – K – N. 
  • The discharging of capacitor C1 is done through C1 – P – E – B1 – R3 – C1. 
  • The charging and discharging waveform of the capacitor C1 is shown in the figure B. 
  • When the voltage across capacitor becomes equal to 2.0 V, the UJT turns off and capacitor once again start to charge. 
  • The charging rate of the capacitor decreases as the resistance R1 increases. 
  • This will result increase of firing angle and load current decreases. 
  • The firing angle has different value in each positive half cycle because the UJT gets DC supply and SCR gets AC supply so there is no synchronism between them.  
  • In order to keep firing angle equal during each positive half cycle, the UJT gets DC supply from the same source as that is given to the SCR.
  • The AC supply is converter in to DC supply before it is given to UJT.
  • There is no filter used during conversion of ac supply into DC supply
  • The load receives supply only during half cycle therefore it is called as firing of SCR by UJT half wave circuit.

firing of scr by ujt full wave circuit

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