Objective
This experiment aims to demonstrate the functionality of a polyfuse (resettable fuse) as an overcurrent protection device in a DC circuit. The experiment will show how the polyfuse reacts to overcurrent conditions and how it automatically resets once the fault is cleared.
Introduction
Overcurrent protection is essential in electronic circuits to prevent damage caused by excessive current. Polyfuses, or resettable fuses, are reliable and reusable devices that protect circuits from overcurrent situations. Unlike traditional fuses, they can reset themselves, providing long-term protection without the need for replacements. This experiment introduces the basic working principles of polyfuses and showcases their behavior under different conditions.
Materials Needed
- DC power supply (adjustable)
- Polyfuse (PPTC, resettable fuse, rated for your circuit)
- Resistor (for overcurrent testing)
- Multimeter (for measuring current and voltage)
- Switch (optional, for creating a load)
- LED or other small load (to visualize circuit activity)
- Connection wires and breadboard
Theory
A polyfuse, also known as a polymeric positive temperature coefficient (PPTC) device, is a resettable fuse designed to protect circuits from overcurrent conditions. Unlike traditional fuses that need to be replaced after they blow, polyfuses can reset themselves once the overcurrent condition is removed. Polyfuses rely on the change in material resistance as they heat up. Under normal current conditions, the polyfuse has low resistance. When overcurrent flows through the fuse, it heats up and increases its resistance dramatically, limiting the current. Once the overcurrent event is cleared, the polyfuse cools down and returns to its low-resistance state, allowing current to flow normally.
The relationship between resistance and temperature is a key factor in understanding polyfuses:
R ∝ T
Where R is the resistance and T is the temperature of the polyfuse. This characteristic makes polyfuses ideal for resettable overcurrent protection.
Circuit Diagram
The basic setup for this experiment is as follows:
- The polyfuse is connected in series with the DC power supply and load.
- A multimeter is placed in series to measure current through the circuit.
- An optional LED can be added parallel to the load to visually indicate the circuit status.
Below is a simplified diagram:
[Power Supply] --- [Polyfuse] --- [Load] --- [Ground]
|
[Multimeter]
Steps
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Set Up the Circuit
Place the polyfuse in series with the power supply and load (an LED or small resistor). Use a breadboard for easy connections. The multimeter should be placed in series with the polyfuse to measure the current flowing through the circuit. If you have an adjustable power supply, set the voltage to a level suitable for your load, ensuring it doesn't exceed the polyfuse's rated voltage.
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Normal Operation
Turn on the power supply and allow normal current to flow through the polyfuse. Measure and record the voltage across the load and the current through the circuit. At this stage, the polyfuse should exhibit low resistance, allowing the current to flow normally.
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Overcurrent Condition
Now, increase the current in the circuit by either lowering the resistance of the load or increasing the voltage from the power supply. Once the current exceeds the rated threshold of the polyfuse, the fuse should 'trip,' increasing its resistance and limiting the current. Observe the reduction in current and voltage drop across the polyfuse. Record these values.
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Automatic Reset
After tripping, remove the overcurrent condition by either turning off the power or reducing the load current. Allow the polyfuse to cool down. Once the fuse cools, it will reset automatically, and the current will flow normally again. Record the time taken for the polyfuse to reset, and verify that the circuit returns to normal operation.
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Optional: Testing with an Oscilloscope
If available, use an oscilloscope to observe the voltage drop across the polyfuse before, during, and after the overcurrent condition. This will provide a visual representation of how the polyfuse responds to current overload and resets.
Data and Calculations
In this experiment, you should record values such as current, voltage, and the time taken for the polyfuse to reset after tripping. Below is a sample data table:
| Condition | Voltage (V) | Current (A) | Time to Reset (seconds) |
|---|---|---|---|
| Normal Operation | 5 | 0.5 | -- |
| Overcurrent Tripping | 5 | 1.2 | -- |
| Reset After Tripping | 5 | 0.5 | 8 |
In this example, the polyfuse tripped when the current exceeded 1.2A, and it took approximately 8 seconds to reset after the overcurrent condition was cleared.
Conclusion
The experiment demonstrates how polyfuses provide resettable overcurrent protection in circuits. When an overcurrent condition occurs, the polyfuse increases its resistance, limiting the current and protecting the circuit. Once the fault is removed, the polyfuse resets and allows the circuit to resume normal operation. Polyfuses are an effective, reusable alternative to traditional fuses in applications where protection from short circuits or overcurrent conditions is necessary.
Applications
Polyfuses are used in various applications:
- Power supplies and chargers
- Battery packs and portable devices
- LED lighting systems
- IoT devices and embedded systems