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 – R_L – 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 HALF WAVE 
CIRCUIT DRAWING.PDF

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