SCR Phase Control Experiment

Introduction

In this experiment, we’ll explore how to control the power delivered to an AC load using an SCR (Silicon Controlled Rectifier). This method, known as phase-angle control, is commonly employed in applications like dimming lights, controlling motor speed, and managing other adjustable AC power systems.

Objective

The goal is to construct an SCR-based phase control circuit, adjust the phase angle, and observe how this impacts the power delivered to an AC load.

Components Required

• SCR (e.g., T106 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)
• AC Load (e.g., incandescent bulb or small motor)
• Breadboard and connecting wires
• Isolation Transformer (to ensure safety when working with AC)
• Oscilloscope (optional, for waveform observation)

Circuit Diagram

Note: The circuit includes an SCR in series with the AC load, with a diac and RC network controlling the triggering on the SCR’s gate to adjust the phase angle.

Procedure

1. Build the Circuit: Connect the SCR in series with the AC load (e.g., a lamp). Design the RC network (resistor, capacitor, and potentiometer) to control the phase angle and attach it to the SCR's gate via the diac.
2. Adjust the Triggering Circuit: Use the potentiometer to vary the capacitor’s charging time. This affects the phase angle at which the SCR is triggered.
3. Power the Circuit: Use an isolation transformer for safety when powering the circuit with AC voltage. Follow all standard precautions when working with high-voltage AC circuits.
4. Observe Output: Adjust the potentiometer and monitor the AC load (e.g., the brightness of the bulb or the motor’s speed). The load’s performance should change as the phase angle varies.
5. Optional - Waveform Analysis: If an oscilloscope is available, observe how the AC waveform changes. You’ll notice the waveform’s cutoff point shifting with adjustments to the potentiometer.

Observations and Analysis

• Lower phase angles (early triggering in the AC cycle) deliver more power to the load, increasing brightness (or motor speed).
• Higher phase angles (late triggering) reduce the power delivered to the load, causing dimmer light (or slower motor speed).
• The SCR conducts for the rest of each half-cycle after being triggered until the AC waveform crosses zero.

Conclusion

This experiment demonstrates the practical application of SCR phase-angle control. By adjusting the triggering phase, we can regulate the power delivered to an AC load. This technique is highly efficient and is widely used in industrial and domestic power control systems.