The UJT (Unijunction Transistor) firing circuit is used to trigger or control the firing angle of devices such as thyristors or SCRs (Silicon Controlled Rectifiers). In this circuit, the pulses are generated when the capacitor discharges.

Explanation:
1. UJT:
- The UJT is a three-terminal semiconductor device that has a unique characteristic of having a negative resistance region in its characteristics curve.
- It consists of a bar of lightly doped n-type material with a p-type region at one end and a heavily doped n-type region at the other end.
- The two n-type regions are called the emitter and the base, while the p-type region is called the intrinsic standoff layer.

2. UJT Firing Circuit:
- The UJT firing circuit consists of a UJT, a capacitor, and a resistor.
- The capacitor is initially charged through a resistor.
- The resistor limits the charging current to the capacitor.
- The UJT is connected in a relaxation oscillator configuration.
- The voltage across the capacitor determines the firing angle of the thyristor or SCR.

3. Pulses Generation:
- Initially, the capacitor is fully charged to a voltage higher than the peak voltage of the AC input.
- When the AC input voltage reaches its peak and starts to decrease, the voltage across the capacitor exceeds the peak voltage, causing the UJT to trigger.
- The UJT triggers when the voltage across the capacitor reaches a certain threshold known as the peak-point voltage (Vp).
- Once triggered, the UJT goes into the negative resistance region, causing a sudden drop in voltage.
- This sudden voltage drop across the capacitor discharges it rapidly.
- As the capacitor discharges, the voltage across it decreases below the peak voltage, causing the UJT to turn off.
- This completes one pulse generation cycle.
- The cycle repeats as the AC input voltage rises again.

Therefore, in the UJT firing circuit, the pulses are generated while the capacitor discharges. The discharge of the capacitor triggers the UJT, leading to the generation of pulses.