Industrial Electronics – I (Thyristor)

 

Triggering Circuits of SCR—Explained in Simple Words:-

Silicon Controlled Rectifier (SCR) is one of the most important devices in power electronics. It is widely used in rectifiers, inverters, motor controls, and many other applications where controlled power flow is needed. The SCR can block high voltages and carry large currents, but to actually switch it ON, a special arrangement called a triggering circuit is required.

In this blog, we will explore what triggering means, the different types of triggering methods, and the most commonly used triggering circuits of SCR.

What is the triggering of SCR?

An SCR has three junctions and four layers of semiconductor material (PNPN). By default, if you just apply a forward voltage across it, the device remains in the OFF state (called forward blocking mode). To make it conduct, a small current must be injected into the gate terminal. This process is known as the triggering or firing of SCR.

Once triggered, the SCR latches into conduction mode and continues to conduct until the current through it falls below a certain minimum value called the holding current.

Different Methods of Triggering

There are several ways an SCR can be triggered:

• Forward Voltage Triggering – By applying a high forward voltage, junctions break down. Not practically used because it stresses the device.
• Thermal Triggering – Heating increases leakage current, eventually firing the device. Not reliable.
• Light Triggering – Used in LASCR (Light Activated SCR). Light falling on the gate region turns it ON.
• dv/dt Triggering—A rapid voltage change across the device causes it to turn ON. Usually avoided with snubber circuits.
• Gate Triggering—The most common and practical method. A small positive pulse at the gate terminal makes the SCR conduct at a desired instant.

Out of all these, gate triggering is the most widely used method. Therefore, various triggering circuits are designed around the gate terminal to ensure safe and reliable operation.

Gate Triggering Circuits of SCR

Let us now understand the popular gate triggering circuits one by one.

1. Resistance Triggering Circuit

This is the simplest circuit. A resistor is connected between the gate and the DC supply through a switch. When the switch is closed, gate current flows, and the SCR is triggered.

• Simple and inexpensive
• Not suitable for precise control
• Power loss occurs due to continuous gate current

2. Resistance–Capacitance (RC) Triggering Circuit

To improve control, a capacitor is added in series with the resistor. The capacitor charges through the resistor, and when the switch closes, a sudden pulse of current flows into the gate, turning the SCR ON.

• Provides a sharp triggering pulse
• Better control of firing angle
• Commonly used in AC circuits like light dimmers and fan regulators

3. UJT Relaxation Oscillator Triggering Circuit

One of the most popular firing circuits uses a Unijunction Transistor (UJT) as a relaxation oscillator. The UJT generates a series of sharp pulses, which are fed to the SCR gate. The timing of the pulse (firing angle) can be controlled by adjusting the RC components.

• Provides strong, sharp, and consistent pulses
• Enables smooth control of output power
• Widely used in phase control rectifiers and AC voltage controllers

UJT triggering  waveform

4. Pulse Transformer Triggering Circuit

For high-power applications, gate pulses need electrical isolation. Here, a pulse transformer is used to couple the triggering pulse from the low-voltage control circuit to the high-voltage SCR gate.

• Provides isolation between the control and power circuit
• Suitable for triggering multiple SCRs in series or parallel
• Ensures safety and noise immunity

5. Digital/Microcontroller-Based Triggering

In modern applications, SCRs are triggered by microcontrollers, DSPs, or microprocessors. These digital controllers generate precise timing pulses, which are fed through drivers and opto-isolators into the gate of the SCR.

• Very accurate firing angle control
• Essential for power electronics applications like inverters, UPS, and motor drives
• Easily programmable and flexible

Why Is Proper Triggering Important?

• Efficiency: Proper triggering ensures that the SCR turns ON at the right instant, improving power utilization.
• Safety: Avoids unwanted turn-on due to noise or dv/dt effects.
• Control: Precise triggering allows smooth control of output voltage, current, or power.
• Reliability: Prevents device damage by providing the required gate current pulse.

Conclusion

Triggering circuits are the heart of SCR applications. Whether it is a simple lamp dimmer at home or a complex industrial motor drive, SCR triggering circuits decide when and how the power device should conduct.

From basic resistance triggering to advanced microcontroller-based systems, the choice of triggering circuit depends on the application, required precision, and power level. Understanding these circuits not only helps in designing better power electronic systems but also provides insights into how modern electrical appliances work efficiently.

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