PTC Thermistor Overcurrent Protection Experiment

Introduction to Overcurrent Protection Using PTC Thermistors

PTC (Positive Temperature Coefficient) thermistors are used in overcurrent protection circuits where the current needs to be limited after a certain threshold is reached. Unlike NTC thermistors, which decrease resistance as temperature increases, PTC thermistors increase their resistance when they heat up due to excessive current. This behavior makes them ideal for self-resetting overcurrent protection.

In this experiment, we will demonstrate how a PTC thermistor can protect a circuit from overcurrent by cutting off the flow when the current exceeds a safe level, and how it automatically resets after cooling down.

Theory Behind Overcurrent Protection with PTC Thermistors

PTC thermistors have a low resistance under normal operating conditions, allowing the current to flow freely through the circuit. However, when the current exceeds a certain threshold, the thermistor heats up, and its resistance increases sharply, limiting the current. Once the current is reduced and the thermistor cools down, its resistance drops back to normal, allowing the circuit to function again.

This self-resetting feature is why PTC thermistors are commonly used for overcurrent protection in circuits where manual resetting would be inconvenient, such as in power supplies or battery protection systems.

Circuit Diagram

Below is a simple circuit diagram showing the PTC thermistor placed in series with a load:

In this configuration, the PTC thermistor allows the current to flow under normal conditions, but when an overcurrent situation occurs, the thermistor increases its resistance and limits the current flow.

Experiment: Overcurrent Protection Using a PTC Thermistor

To perform this experiment, you will need the following components:

• 1 PTC thermistor (e.g., 60V, 0.5A resettable fuse)
• DC power supply (12V or 5V, depending on your load)
• Resistive load (e.g., a high-power resistor or a small motor)
• Multimeter (to measure current and voltage)
• Breadboard and jumper wires

Steps to Conduct the Experiment:

1. Connect the PTC thermistor in series with your resistive load as shown in the circuit diagram.
2. Apply a DC voltage to the circuit, ensuring that the power supply can handle the expected load current.
3. Measure the current at the moment of power-up using a multimeter.
4. Gradually increase the load to simulate an overcurrent condition and observe how the PTC thermistor reacts as its resistance increases and limits the current.
5. Once the thermistor has activated, reduce the load and allow it to cool down, noting how it resets and allows the current to flow normally again.

In this experiment, you should notice that as the current increases beyond a certain point, the PTC thermistor starts limiting the current by sharply increasing its resistance. After the load is reduced, the thermistor cools and returns to its normal low-resistance state, ready to protect the circuit again.

Applications of PTC Thermistors for Overcurrent Protection

PTC thermistors are used for overcurrent protection in many applications, including:

• Power supplies and converters
• Battery charging circuits
• Telecommunications equipment
• Motors and transformers
• Consumer electronics (e.g., laptops, smartphones)

Conclusion

PTC thermistors are effective components for overcurrent protection. Their self-resetting feature makes them particularly useful in circuits where protection from temporary current surges is needed without requiring manual intervention. By limiting the current flow during an overcurrent event and resetting once the current is reduced, PTC thermistors provide reliable protection in various electronic systems.