Question

What is the firing angle?

Answer 1

The firing angle, often referred to as the **triggering angle** or **delay angle**, is a term used primarily in the context of power electronics and thyristor circuits. It describes the angle at which a gate or trigger signal is applied to a thyristor or silicon-controlled rectifier (SCR) to turn it on during each half-cycle of the AC waveform.

### Key Points About Firing Angle:

1. **Definition and Context**:
   - The firing angle is the angle measured from the zero crossing of the AC waveform at which the thyristor is triggered to conduct.
   - In a typical phase-controlled rectifier circuit, controlling the firing angle allows for regulation of the output voltage.

2. **Role in Control**:
   - By adjusting the firing angle, you can control the amount of power delivered to the load. A smaller firing angle means that the thyristor is turned on earlier in the AC cycle, allowing more of the waveform to contribute to the output, thus increasing the power delivered.
   - Conversely, a larger firing angle delays the conduction, reducing the average power delivered to the load.

3. **Application**:
   - Firing angle control is commonly used in phase-control applications, such as light dimmers, motor speed controls, and temperature regulators.
   - For example, in a light dimmer, varying the firing angle adjusts the brightness of the light by changing the effective voltage applied to the lamp.

4. **Calculation and Measurement**:
   - The firing angle is typically measured in degrees and ranges from 0° to 180° for a half-cycle of the AC waveform.
   - In practical circuits, it can be adjusted using a variable resistor or a digital controller.

5. **Typical Circuit Components**:
   - The circuit that controls the firing angle usually includes a phase-locked loop (PLL) or a trigger circuit that generates the gate pulses for the thyristor at the desired firing angle.

### Example:

In a simple single-phase AC circuit with a resistive load:
- If the firing angle is set to 0°, the thyristor is triggered at the start of each half-cycle, and the output voltage is nearly the full RMS value of the AC supply.
- If the firing angle is increased to 90°, the thyristor will conduct only during the latter half of each cycle, reducing the effective power delivered to the load.

Adjusting the firing angle is crucial for applications requiring precise control of electrical power, such as in heating elements or motor drives.

Answer 2

The firing angle is a concept primarily associated with controlled rectifiers, which are devices used to convert alternating current (AC) to direct current (DC) while allowing for control of the output voltage and current. This concept is crucial in the operation of devices such as phase-controlled rectifiers, which include thyristors (like SCRs or silicon-controlled rectifiers) and triacs.

### Detailed Explanation

#### 1. **Basics of Controlled Rectifiers**
   Controlled rectifiers, like those using thyristors, can be used to regulate the amount of power delivered to a load. Unlike uncontrolled rectifiers, which provide a constant output based solely on the input AC voltage, controlled rectifiers allow for adjustment of the output by varying the time at which the rectifier starts conducting within each AC cycle.

#### 2. **Understanding the Firing Angle**
   - **Definition**: The firing angle, often denoted as \( \alpha \), is the angle in the AC waveform at which the controlled rectifier is triggered or fired to start conducting. It is measured from the point where the AC waveform crosses zero (the zero-crossing point).

   - **Purpose**: By adjusting the firing angle, the effective output voltage and current can be controlled. This adjustment is done by delaying the conduction of the rectifier until a specified angle after the zero-crossing point. The later the firing angle, the less time the rectifier conducts within each AC cycle, leading to a lower average output voltage and current.

   - **Impact on Output**:
     - When the firing angle is small (close to zero degrees), the rectifier conducts for a larger portion of each AC cycle, resulting in a higher average output voltage and current.
     - As the firing angle increases, the conduction period of the rectifier decreases, which results in a lower average output voltage and current.

#### 3. **Applications**
   - **Phase Control**: In phase-controlled rectifiers, such as those used in light dimmers, motor speed controllers, and temperature controllers, the firing angle is adjusted to control the power delivered to the load.
   - **Power Control**: For instance, in a light dimmer, increasing the firing angle decreases the amount of power delivered to the light bulb, thereby dimming the light.

#### 4. **Practical Considerations**
   - **Triggering Mechanisms**: The firing angle is typically controlled by a trigger circuit, which might use components like timers, microcontrollers, or phase-locked loops to determine the precise moment to fire the rectifier.
   - **Phase Delay**: The control circuit ensures that the rectifier is triggered at the correct phase delay relative to the AC voltage waveform. This requires accurate timing to achieve the desired control.

### Example

Consider a single-phase AC waveform where the rectifier is triggered at a firing angle of 90 degrees. In this case, the rectifier will start conducting when the AC voltage waveform is at its peak. If the firing angle were 0 degrees, the rectifier would start conducting immediately at the zero-crossing point. By adjusting this angle, you control how much of the AC cycle contributes to the output DC voltage.

In summary, the firing angle is a critical parameter in the operation of controlled rectifiers, allowing precise control over the power delivered to a load by adjusting when the rectifier begins to conduct in each AC cycle.