The UJT firing circuit uses a unijunction transistor (UJT) as a relaxation oscillator to produce sharp repetitive gate pulses for thyristors, reducing power dissipation in the gate circuit compared to R-firing and RC-firing circuits. The UJT starts conducting when the capacitor voltage reaches its peak voltage VP, causing the capacitor to discharge. It turns off once the capacitor voltage decreases to the UJT's valley voltage VV, after which the capacitor recharges and the process repeats to produce pulses that fire the thyristor. Adjusting the variable resistor RV varies the capacitor charging time and thus controls the firing angle.

1. UJT Firing Circuit :
The above discussed R-firing and RC-firing circuits produce continuous gate pulses
due to which there will be quite high power dissipation at the gate circuit of the
thyristor.
This power dissipation in the gate circuit can be reduced by using a UJT in the firing
circuit.
The UJT will work as a relaxation oscillator that produces sharp repetitive pulses with
good rise time, and it also has good frequency stability under voltage fluctuations
and temperature variations.

3. In the above UJT firing circuit, the UJT starts conducting once the value of the
voltage across the capacitor is equal to or more than the peak voltage VP value
of the UJT.
Now the capacitor starts discharging, and once its voltage decreases to the
valley voltage VV of UJT, the UJT will be turned OFF.
The capacitor again gets charged to supply voltage and the above process
repeats.
Thus whenever UJT conducts it produces pulses at the gate terminal of the
thyristor as shown in the waveform.
The capacitor charging time depends upon the value of variable resistance RV.
Therefore, by varying the value of RV the firing angle can be varied.