Understanding Firing Angle and Delay Angle in SCR Thyristors

The control of thyristors, particularly Silicon-Controlled Rectifiers (SCRs), is crucial in many electronic applications. Two key parameters in this control are the firing angle and the delay angle. These parameters determine when and how the thyristor is turned on and off, thus impacting the functionality of the circuit it is used in. This article provides a detailed explanation of both the firing and delay angles, and their significance in the operation of thyristors.

What is a Thyristor?

A Silicon-Controlled Rectifier (SCR)is a type of semiconductor device used in electronics, primarily for rectification and control of electric power. Unlike simpler diodes or transistors, an SCR requires triggering to turn it ON and it remains ON until the current through it drops below a certain threshold, typically at zero crossing of the input waveform. This makes SCRs useful in switching and control applications but demands careful control.

Firing Angle in SCR Thyristors

The firing angleis a critical parameter that determines when an SCR begins conducting. Specifically, it is the angle at which the AC supply voltage waveform crosses the zero line but before the triggering signal is applied to the gate of the SCR. This angle is measured in degrees from the start of the positive half of the AC cycle.

Typically, if the firing angle is set to zero, the SCR is triggered at the zero crossing of the AC voltage, which is a common starting point. However, by adjusting the firing angle, one can control the point in the AC cycle at which the SCR conducts. This adjustment is often used to control the amount of power transferred from the AC source to the load, making the SCR useful in applications such as dimming lights, motor control, and voltage regulation.

Effect of Firing Angle

As the firing angle is increased, the time at which the SCR turns on later in the AC waveform, leading to less average power being drawn from the AC source. Conversely, a smaller firing angle means the SCR turns on earlier, leading to more average power. Therefore, the firing angle effectively serves as a means to vary the average output power of the AC to DC conversion process, offering significant flexibility in control of power electronics applications.

Delay Angle in SCR Thyristors

While the firing angle determines when the SCR turns ON, the delay angledetermines when the SCR will turn OFF. The delay angle is defined as the angle at which the current through the SCR reaches zero during the negative half-cycle of the AC waveform. This angle is measured from the start of the negative half-cycle of the AC supply.

In simpler terms, after the SCR is triggered, it conducts until the current through it drops to zero at the negative half-cycle of the AC waveform, which is determined by the delay angle. This delay ensures that the SCR can fully turn off before the next positive half-cycle, preventing unintended conduction and maintaining the intended circuit behavior.

Examples of Firing and Delay Angles

Imagine an AC supply waveform with a frequency of 60 Hz. In this case, the AC waveform completes one cycle in 1/60th of a second, or 16.67 milliseconds. If the firing angle is set to 30 degrees, the SCR will turn on 30 degrees after the start of the positive half-cycle. This corresponds to an electrical delay of 5 milliseconds (since 30 degrees represents 30/360 1/12 of a cycle, and 16.67 milliseconds / 12 1.389 milliseconds). The delay angle, say 90 degrees, would mean the SCR turns off 90 degrees after the start of the negative half-cycle, or 25 millisecond after the start of the positive half-cycle, placing it at 41.67 milliseconds into the cycle.

Conclusion

Understanding the firing and delay angles in SCR thyristors is crucial for effective control of power electronics systems. By carefully adjusting these angles, engineers can precisely control the operation of devices such as inverters, AC-to-DC converters, and motor controllers. These parameters enable the design of efficient and precise power electronic systems, making them indispensable in modern technology.

Keywords

Keywords: firing angle, delay angle, thyristor

This article is designed to be informative and provide a comprehensive understanding of firing and delay angles in SCR thyristors, making it suitable for those seeking detailed knowledge in electronic control systems.

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