VDR Voltage Protection Experiment

Objective

The aim of this experiment is to demonstrate how a VDR (Voltage Dependent Resistor), also known as a varistor, protects circuits from overvoltage by clamping high voltages and limiting surge currents.

Components Required

• 1 x Voltage Dependent Resistor (VDR/Varistor)
• 1 x 12V Power Supply
• 1 x Load Resistor (10Ω, 1W)
• 1 x Multimeter (to measure voltage and current)
• 1 x Breadboard
• Connecting wires
• Adjustable DC Power Supply or Signal Generator (optional for testing different voltages)

Theory

A VDR is a passive component that has a non-linear resistance, which changes with the applied voltage. When the voltage across the VDR exceeds a certain threshold (its clamping voltage), the resistance decreases significantly, allowing the VDR to shunt the excess current away from sensitive components. This makes VDRs useful for protecting circuits against voltage surges and spikes.

Circuit Diagram

In this experiment, the VDR is placed in parallel with a load resistor. When the voltage across the circuit exceeds the clamping voltage of the VDR, it will conduct and protect the load by shunting excess current.

Experimental Setup

1. Connect the 12V power supply across the circuit with a load resistor and VDR in parallel.

2. Place the VDR across the load to protect it from overvoltage.

3. Connect the multimeter to measure the voltage across the VDR and the load resistor.

4. Slowly increase the input voltage using the adjustable power supply to observe the clamping effect of the VDR.

Procedure

1. Set up the circuit with the VDR placed in parallel with the load resistor as shown in the circuit diagram.
2. Connect the 12V power supply to the circuit and ensure the multimeter is properly connected to measure the voltage across the VDR.
3. Begin with a low voltage (e.g., 5V) and slowly increase the input voltage while observing the behavior of the VDR and the load resistor.
4. As the voltage increases, note the point at which the VDR starts clamping the voltage (the clamping voltage of the VDR).
5. Observe the voltage across the load resistor as the VDR protects it from overvoltage by shunting excess current.

Results and Observations

As the input voltage increases, the VDR remains in a high-resistance state until the voltage reaches its clamping voltage. At this point, the VDR conducts, lowering the voltage across the load resistor. This protects the load from being exposed to dangerously high voltage levels, demonstrating the overvoltage protection capability of the VDR.

Conclusion

This experiment demonstrates the function of a VDR as a voltage protection device. By clamping high voltages and allowing excess current to flow through it, the VDR protects the load from overvoltage, which could otherwise damage sensitive components. VDRs are commonly used in surge protection devices, ensuring circuits are safeguarded against voltage spikes.