Introduction:
When a UJT (Unijunction Transistor) is used for triggering an SCR (Silicon Controlled Rectifier), the waveform obtained from the UJT circuit is a saw-tooth wave.

Explanation:
To understand why the waveform obtained is a saw-tooth wave, let's first understand the basic operation of a UJT and an SCR.

1. UJT (Unijunction Transistor):
- A UJT is a three-layered semiconductor device with one pn-junction and three terminals - emitter (E), base 1 (B1), and base 2 (B2).
- It operates as a switch and is commonly used for triggering other devices like SCRs and triacs.
- The UJT has a unique characteristic called the negative resistance region, where the voltage decreases as the current through it increases.

2. SCR (Silicon Controlled Rectifier):
- An SCR is a four-layered semiconductor device used as a controlled rectifier.
- It can be triggered into conduction by applying a suitable gate pulse.
- Once triggered, it remains in the conducting state until the current through it falls below a certain value called the holding current.

Working:
When a UJT is used for triggering an SCR, the UJT is connected in a relaxation oscillator circuit. This circuit generates a saw-tooth waveform at the emitter terminal of the UJT.

The relaxation oscillator circuit consists of a resistor (R1), a capacitor (C1), and a UJT. The capacitor charges through R1 and discharges through the UJT. This charging and discharging cycle repeats, generating a saw-tooth waveform.

The saw-tooth waveform at the emitter terminal of the UJT is then used as the trigger pulse for the gate terminal of the SCR. The gate pulse triggers the SCR into conduction, allowing current to flow through it.

Advantages of using a saw-tooth waveform:
The saw-tooth waveform has a gradual rise and a sharp fall, making it suitable for triggering SCRs. Here are a few advantages of using a saw-tooth waveform:
- The gradual rise of the waveform ensures a smooth turn-on of the SCR, preventing high current surges.
- The sharp fall of the waveform allows for a quick turn-off of the SCR, minimizing the power loss and reducing the possibility of false triggering.

Therefore, when a UJT is used for triggering an SCR, the waveform obtained from the UJT circuit is a saw-tooth wave.