Lab Report: Determination of Firing Angle Using DIAC-TRIAC Phase-Controlled Circuit

1. Objective:

To determine the firing angle (α\alpha) and output power in a DIAC-TRIAC phase-controlled circuit under different load conditions, such as lamp, motor, and heater.

2. Apparatus Required:

  • DIAC (e.g., DB3)
  • TRIAC (e.g., BT136)
  • Resistors (100Ω to 10kΩ)
  • Capacitors (0.1µF to 100µF)
  • AC power supply (230V, 50Hz)
  • Loads:
    • Incandescent lamp (60W)
    • AC motor (fan)
    • Heater (up to 200W)
  • CRO (Cathode Ray Oscilloscope)
  • Multimeter
  • Breadboard and connecting wires

3. Theory:

DIAC-TRIAC Phase Control:

  • The DIAC acts as a bidirectional trigger device, firing the TRIAC at specific phases of the AC cycle.
  • By adjusting the RC network, the firing angle can be controlled, affecting the output voltage and power delivered to the load.

Firing Angle (α\alpha) Calculation:

The firing angle is measured in degrees from the start of each half-cycle of the AC waveform.

Power Delivered to the Load:

Pavg=Vm22Ï€R⋅(1+cos⁡α)P_{avg} = \frac{V_m^2}{2\pi R} \cdot (1 + \cos \alpha)

Where:

  • VmV_m = Peak voltage
  • RR = Load resistance
  • α\alpha = Firing angle

4. Circuit Diagram:

  • Show a typical DIAC-TRIAC circuit with an RC network for phase control.
  • Indicate connections to various loads (lamp, motor, and heater).

5. Procedure:

Step 1: Circuit Setup

  1. Connect the DIAC and TRIAC circuit as per the diagram, with an RC network for phase control.
  2. Connect the load (lamp, motor, or heater) across the output terminals.
  3. Connect the CRO across the load to observe the output waveform.

Step 2: Firing Angle Measurement

  1. Apply AC power and observe the waveform on the CRO.
  2. Measure the phase angle delay (α\alpha) from the start of the positive half-cycle to the point where the TRIAC conducts.
  3. Adjust the resistor or capacitor in the RC network to vary the firing angle.
  4. Record the output voltage and corresponding firing angle for each setting.

Step 3: Load Testing

  1. Repeat the measurements for different loads (lamp, motor, and heater).
  2. Observe the changes in waveform shape and load behavior.
  3. Measure the power delivered to each load using a multimeter.

6. Observations:

Load Type Firing Angle (α\alpha) Output Voltage (V) Power Delivered (W)
Lamp
Motor
Heater

7. Results:

  • The firing angle was successfully controlled using the DIAC-TRIAC circuit.
  • Output power varied significantly with changes in the firing angle and load type.
  • Motor performance showed smoother control compared to lamp and heater.

8. Conclusion:

The experiment demonstrated the use of a DIAC-TRIAC phase-controlled circuit to adjust the output power for different loads. The firing angle directly influenced the power delivered to the load, showcasing efficient control for resistive and inductive loads.