Objective
This experiment aims to demonstrate the function of a Metal Oxide Varistor (MOV) as a surge protection device in an electrical circuit. The MOV will be tested to show how it responds to voltage spikes and protects sensitive components from overvoltage conditions.
Materials Needed
- AC power supply or DC power supply (adjustable)
- Metal Oxide Varistor (MOV) suitable for your application
- Resistor (for load testing)
- Multimeter (for measuring voltage and current)
- Oscilloscope (optional, for observing voltage spikes)
- LED or small load (to visualize circuit activity)
- Connection wires and breadboard
Theory
A Metal Oxide Varistor (MOV) is a voltage-dependent resistor that is commonly used for surge protection in electrical circuits. It has a nonlinear resistance characteristic, meaning its resistance decreases significantly when the voltage across it exceeds a certain threshold (clamping voltage). When a voltage spike occurs, the MOV provides a low-resistance path for excess current, diverting it away from sensitive components and preventing damage.
The clamping behavior of an MOV can be described by the following relationship:
V_clamp = V_rated + (I * R)
Where **V_clamp** is the clamped voltage across the MOV, **V_rated** is the rated voltage, **I** is the current through the MOV, and **R** is the resistance of the MOV when in conduction mode.
Steps
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Set Up the Circuit
Connect the MOV in parallel with the load resistor across the power supply. This arrangement allows the MOV to protect the load from voltage spikes. Use a breadboard for easy connections, and ensure the multimeter is connected to measure the voltage across the load.
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Normal Operation
Turn on the power supply and allow the circuit to operate under normal voltage conditions. Measure and record the voltage across the load and the current flowing through the circuit. At this stage, the MOV should not conduct, and the voltage should be within normal operating limits.
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Introduce a Voltage Spike
Using a variable transformer or a controlled power supply, increase the voltage to create a spike (ensure you do this cautiously to avoid damaging components). Observe the voltage across the MOV and the load. If you have an oscilloscope, connect it to observe the voltage waveform during this spike. Record the peak voltage and current during this condition.
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Observe MOV Operation
During the voltage spike, the MOV should clamp the voltage and prevent it from exceeding its rated voltage. Measure the voltage across the MOV and verify that it is within the clamping specifications of the device. Note the behavior of the load—should remain unaffected by the spike.
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Reset and Review
After the voltage spike, allow the circuit to return to normal operation. Disconnect the power supply, and check the MOV for any signs of damage. It should remain functional. Document your observations and any changes in the behavior of the circuit components.
Data and Calculations
Record the voltage across the MOV and the load during normal operation and the surge event. Below is a sample data table:
| Condition | Voltage Across Load (V) | Voltage Across MOV (V) | Current Through Load (A) |
|---|---|---|---|
| Normal Operation | 120 | 0 | 0.5 |
| During Surge | 30 | 90 | 0.2 |
In this example, the MOV successfully clamped the voltage to protect the load during the surge event, allowing only 30V to reach the load.
Conclusion
The experiment demonstrates the effective use of Metal Oxide Varistors (MOVs) for surge protection in electrical circuits. By clamping excessive voltage spikes, MOVs prevent damage to sensitive components, ensuring the reliability and longevity of electronic devices. MOVs are essential in applications where voltage surges are common, such as power supplies, telecommunication equipment, and household appliances.