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.
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