Triac Control Circuit Experiment

Introduction

In this experiment, we will design a circuit to control the power delivered to an AC load using a Triac. Triacs are semiconductor devices capable of conducting current in both directions, making them ideal for controlling AC power in applications such as dimmers, motor speed controllers, and other systems requiring adjustable power delivery.

Objective

The objective is to build a Triac-based control circuit, observe how phase-angle control adjusts power delivery to an AC load, and understand the principle behind applications like dimming lights or controlling motor speed.

Components Required

• Triac (e.g., BT136 or equivalent)
• Diac (e.g., DB3 for triggering)
• Resistors (e.g., 10kΩ, 100Ω)
• Potentiometer (e.g., 500kΩ)
• Capacitor (e.g., 0.1 µF or 0.22 µF for timing)
• AC Lamp (or other low-power AC load)
• Breadboard and connecting wires
• Isolation Transformer (essential for safety when working with AC)
• Oscilloscope (optional, to observe waveforms)

Circuit Diagram

Note: The circuit consists of a triac connected in series with the AC load, a diac for triggering, and an RC network to control the phase angle.

Procedure

1. Assemble the Circuit: Connect the triac in series with the AC load (e.g., a lamp). Use an RC network (resistor, capacitor, and potentiometer) to control the charging of the capacitor, and place a diac between the RC network and the triac's gate for triggering.
2. Adjust the Triggering Circuit: Use the potentiometer to vary the phase angle of the capacitor's charging, which controls when the triac is triggered during the AC cycle.
3. Power the Circuit: Power the circuit with an isolated AC source. Take necessary safety precautions when handling high-voltage AC.
4. Observe Output: Monitor the AC load (e.g., the lamp) while adjusting the potentiometer. Notice the changes in brightness or motor speed as the phase angle varies.
5. Optional - Waveform Observation: Use an oscilloscope (if available) to observe how the AC waveform changes as the phase angle is adjusted. The “cutoff” point in the waveform will shift accordingly.

Observations and Analysis

• Adjusting the potentiometer changes the capacitor's charging rate, altering the phase angle and the triggering point of the triac.
• At earlier phase angles (faster triggering), more power is delivered to the load, resulting in brighter light or faster motor speed.
• At later phase angles (delayed triggering), less power is delivered, dimming the light or slowing down the motor.

Conclusion

This experiment demonstrates the principle of phase-angle control using a triac. By adjusting the triggering phase, the average power delivered to an AC load can be controlled. This technique is widely used in practical applications like dimmers, motor speed controllers, and AC power regulators.