UJT Triggering of SCR

Introduction:
Unijunction Transistor (UJT) is a three-layer, three-terminal semiconductor device that is commonly used for triggering Silicon Controlled Rectifiers (SCRs) in electronic circuits. UJT triggering provides a simple and reliable method to control the firing of SCRs.

Waveform:
The waveform used for triggering an SCR using a UJT is the sawtooth wave. The sawtooth waveform is characterized by a linear voltage ramp followed by a sharp drop. This waveform is suitable for triggering SCRs because it provides a gradual rise in voltage followed by a sudden drop, which allows for precise control of the SCR firing.

Working Principle:
When a sawtooth voltage waveform is applied to the emitter terminal of a UJT, the UJT undergoes a process called "negative resistance region" operation. During this operation, the emitter current increases until it reaches a peak point, known as the peak-point emitter current (IP). Once IP is reached, the UJT switches from high resistance to low resistance, causing a rapid decrease in voltage across the UJT.

SCR Triggering:
The rapid decrease in voltage across the UJT triggers the gate of the SCR, causing it to turn on and conduct current. The precise timing of the SCR firing can be controlled by adjusting the timing of the sawtooth waveform.

Advantages of UJT Triggering:
1. Simple circuitry: UJT triggering of SCRs requires minimal external components, making it cost-effective and easy to implement.
2. Precise control: The sawtooth waveform allows for precise control of the SCR firing, enabling accurate timing in various applications.
3. Reliable operation: UJT triggering provides consistent and reliable triggering of SCRs, ensuring proper functioning of electronic circuits.

Conclusion:
In summary, when used for triggering an SCR, a UJT requires a sawtooth waveform. This waveform enables the UJT to enter the negative resistance region, causing a rapid decrease in voltage that triggers the SCR. UJT triggering offers simple circuitry, precise control, and reliable operation, making it a popular choice in various electronic applications.