Understanding Firing Delay and Conduction Angle in Silicon Controlled Rectifiers (SCR)

Introduction:

When discussing the operation of Silicon Controlled Rectifiers (SCRs), two critical parameters often come into play: the firing delay (firing angle) and the conduction angle. These factors are essential for efficient power control in various electrical applications.

Firing Delay and Conduction Angle: Key Concepts

Firing Delay (Firing Angle α)

Definition: The firing delay, also known as the firing angle (α), refers to the time delay between the moment the SCR is triggered and the point in the AC cycle when the SCR begins to conduct electricity.

Measurement: Firing delay is typically measured in degrees (°) of the AC waveform. For instance, if the SCR is triggered at 90° in the AC cycle, the firing angle is 90°.

Impact: The firing delay determines the portion of the AC waveform used for power delivery. A larger firing angle indicates that the SCR starts conducting later in the cycle, resulting in reduced average output voltage and power.

Conduction Angle

Definition: The conduction angle, also known as the conduction period, is the angular duration over which the SCR remains in the conducting state after it has been triggered.

Measurement: This angle is also measured in degrees (°). For example, if the SCR conducts from 90° to 180°, the conduction angle is 90°.

Impact: The conduction angle affects the overall power delivered to the load. A larger conduction angle allows more of the AC waveform to contribute to the output, increasing the average output voltage and power.

Interconnectedness of Firing Delay and Conduction Angle

The firing delay and conduction angle are intimately related and interconnected. The total angle of one complete AC cycle is 360°. If the firing delay is increased, the conduction angle may decrease, as the SCR is turned off at the end of the half-cycle. Conversely, a shorter firing delay can allow for a larger conduction angle.

A common application is when the SCR is triggered at a firing angle of α and conducts until the end of the half-cycle. In such a scenario, the conduction angle would be 180° - α for a single half-cycle.

Conclusion

Understanding these parameters is crucial for designing circuits intended for precise power control and efficiency in various applications, such as phase control, motor speed control, and light dimming.

Keywords:

firing delay conduction angle silicon controlled rectifier (SCR)

Article Resources:

For further reading and deeper insights into the topic, please refer to the relevant technical manuals and online resources focusing on power electronics and semiconductor devices.