Electrical Revolution

13/03/2017

Firing of SCR By the UJT half Wave Circuit

Firing of SCR by UJT half wave circuit

The firing of SCR is done by various method. Firing of SCR is done by UJT half wave and UJT full wave method. The firing of SCR is done by UJT half wave circuit is one of the methods.

Explanation of Firing of SCR by 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.