Why UJT is Used in Firing SCRs

The unijunction transistor (UJT) is frequently employed in the firing circuit of a Silicon Controlled Rectifier (SCR) due to its unique characteristics that make it highly suitable for triggering applications. This article will explore the key reasons why UJTs are preferred for this purpose and provide a deeper understanding of their role in SCR firing circuits.

Key Reasons for Using UJT in Firing SCRs

1. Triggering Capability

UJTs are renowned for their ability to generate sharp pulses when triggered, thanks to their negative resistance region. This capability is crucial for providing precise triggering signals to SCRs, ensuring reliable and accurate turning on of the SCRs at the desired moments.

2. Simplicity

UJT circuits are relatively simple and require fewer components compared to other triggering methods. This simplicity not only makes the circuit design and implementation easier but also reduces the overall complexity of the system, leading to fewer potential failure points.

3. Low Triggering Current

UJTs can operate with low input currents, which is particularly important for triggering SCRs without requiring significant power. This characteristic is advantageous in scenarios where power efficiency and cost savings are critical.

4. Pulse Generation

These transistors can produce narrow pulses that can be finely tuned to turn on the SCR at the exact moment required within the AC cycle. This feature is especially useful in applications like phase control, where precise control over power is essential.

5. Reliability

UJTs are recognized for their robustness and reliability. This makes them highly suitable for industrial applications where SCRs are widely used for power control. Their durability ensures consistent and dependable performance over time.

6. Adjustable Timing

The timing of the UJT pulse can be adjusted by changing the resistor and capacitor values in its circuit. This flexibility allows for precise control over when the SCR is fired, making UJTs highly versatile in different applications.

Typical Application

In practical circuits, the UJT is often used in conjunction with a resistor-capacitor (RC) network. When the capacitor charges to a certain voltage, the UJT turns on and generates a pulse that triggers the SCR, allowing it to conduct current.

Conclusion

In summary, the UJT is an effective choice for firing SCRs due to its ability to generate precise trigger pulses, the simplicity of its design, low current operation, and reliability. This makes it widely used in various applications involving SCRs such as light dimmers, motor speed controls, and power regulators.

The forward breakover voltage and negative resistance of the UJT ensure reliable triggering of the SCR. In breakdown, the UJT provides a rapidly rising current, making it well-suited for triggering purposes. The programmable unijunction transistor (PUT) is still used for SCR triggering, but the original UJT is no longer utilized in this manner. This is because SCRs, which are typically used in phase-controlled motor control, require consistent trigger points that do not vary rapidly between cycles. This is particularly important for maintaining consistent timing, especially in 50 Hz systems where precise and consistent timing is crucial for optimal motor performance.

For more detailed information on UJTs and their applications, refer to the Unijunction Transistor - Wikipedia page.