The firing angle and conduction angle are two key concepts in the operation of an SCR (Silicon Controlled Rectifier) and understanding their differences is crucial for controlling AC voltage using SCRs. Here’s a detailed explanation:

Firing Angle

Definition: The firing angle of SCR (α) is the angle in the AC cycle at which the gate pulse is applied to the SCR, causing it to start conducting.

Control: By adjusting the firing angle, you control the point in the AC waveform where the SCR begins to conduct. This is crucial in applications like phase control for power regulation.

Range: The firing angle can range from 0 degrees to 180 degrees in a half-cycle of the AC waveform. A firing angle of 0 degrees means the SCR is triggered at the start of the cycle, while 180 degrees means it is triggered at the very end.

Impact on Power: A smaller firing angle means the SCR conducts for a larger portion of the AC cycle, delivering more power to the load. Conversely, a larger firing angle reduces the conduction period and the power delivered.

Conduction Angle

Definition: The conduction angle (θ) is the duration, measured in degrees of the AC cycle, during which the SCR remains in the conducting state after being triggered.

Relation to Firing Angle: The conduction angle is inversely related to the firing angle. If the firing angle increases, the conduction angle decreases.

**Calculation: **In a half-cycle (180 degrees), the conduction angle is given by
θ=180°−α,
where
α is the firing angle.

Power Delivery: The conduction angle directly determines the amount of power delivered to the load. A longer conduction angle means more power is transferred to the load during each AC cycle.

Summary of Differences

Timing:

Firing Angle: Refers to the specific point (in degrees) within the AC cycle when the SCR is triggered.
Conduction Angle: Refers to the duration (in degrees) within the AC cycle during which the SCR conducts current.

Control:

Firing Angle: Controlled by the timing of the gate pulse.
Conduction Angle: Determined by the difference between 180 degrees and the firing angle.

Effect:

Firing Angle: Directly affects when the SCR starts conducting within each AC cycle.
**Conduction Angle: **Directly affects how long the SCR conducts and, consequently, the power delivered to the load.

Practical Example

Consider an AC waveform:

If the firing angle (α) is set to 30 degrees, the SCR is triggered 30 degrees after the start of each half-cycle.
The conduction angle (θ) in this case will be
180°−30°=150°, meaning the SCR will conduct for 150 degrees of each half-cycle.
By understanding and manipulating the firing and conduction angles, you can precisely control the power delivered to a load in applications such as light dimmers, motor speed controllers, and heating elements.