Understanding Thyristor Firing Angle (SCR)

The firing angle of a thyristor, specifically a Silicon Controlled Rectifier (SCR), refers to the delay angle between the instant when the voltage across the SCR becomes positive and the moment when the SCR is triggered into conduction. Think of it as the ‘wait time’ before the SCR is allowed to start conducting.

The firing angle is usually measured in degrees and is a crucial parameter in controlling the power flow through the device. By adjusting this angle, we can precisely regulate how much power is delivered to the load.

Ways to Enable an SCR

There are several methods to enable an SCR, which essentially means triggering it into conduction – allowing it to act as a closed switch. Here are a few common methods:

1. Gate Triggering

This is the most common and widely used method. It involves applying a positive pulse to the gate terminal of the SCR. This positive pulse triggers the device into conduction, allowing the current to flow from the anode to the cathode. It’s like giving the SCR the ‘go-ahead’ signal.

2. Voltage Triggering

Applying a sufficiently high voltage across the anode and cathode terminals can also trigger the SCR. However, this method is less commonly used compared to gate triggering because it’s harder to control precisely.

3. Light Triggering (Optical Triggering)

In this method, light is used to trigger the SCR. A light-sensitive semiconductor device, such as a photodiode or phototransistor, is used to sense the light and generate a triggering signal. Imagine a light switch that turns on the SCR.

4. Temperature Triggering

By exposing the SCR to a sudden temperature change, it is possible to trigger it into conduction. However, this method is less precise and less commonly used compared to gate triggering. It’s generally not a reliable or controllable approach.

5. dv/dt Triggering

Triggering can also occur due to a rapid rate of change of voltage (dv/dt) across the SCR terminals. Imagine the voltage suddenly spiking. Special precautions and circuitry are required to control and utilize this method effectively to avoid unwanted triggering.

6. RC Triggering

The gate of the SCR can be triggered using an RC (resistor-capacitor) network. The capacitor is charged through a resistor, and when the voltage across the capacitor reaches a certain level, it triggers the SCR. It’s like a timed release mechanism.

The choice of triggering method depends on the specific application requirements, precision, and the characteristics of the load being controlled. Gate triggering is the most widely used method due to its precision and ease of control.