Polyfuse
- 2. Contents Introduction Construction Principle of operation Operating Parameters Types Applications Conclusion
- 3. Introduction Basically a Thermistor It is re-settable by itself. Polymeric positive temperature coefficient thermistors (PPTC). A new standard for circuit protection. A series element in a circuit.
- 4. Construction Made from a composite of semi-crystalline polymer & conductive particles. Comprises thin sheets of conductive plastic with electrodes attached to either side. The device resistance rises with temperature. Fuses provide over current protection & automatic restoration. Voltage rating 30 to 250 volts. Current rating 20mA to 100A.
- 5. Over current protection • Polyfuse is a series element in a circuit. • PPTC device protects the circuit by going from a low-resistance to high- resistance state in response to an over current protection.
- 6. • In normal operation the device has a resistance that is much lower than the remainder of the circuit. • In response to an overcurrent condition, the device increases in resistance (trips) • Reducing the current in the circuit to a value that can be safely carried by any of the circuit elements. • Due to rapid increase in the temperature of the device, caused by I2R heating.
- 7. Practical test on Polyfuse
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- 16. Principle of Operation • In case of excessive current flows through the device, the temperature of conductive material arises. • When temperature exceeds its phase- transformation temperature, the crystalline structure changes into an expanded amorphous state. • Now the conductive particles move apart from each other. • The increase in resistance stops flow of current through the device.
- 17. Operating Parameters • Initial Resistance: It is the resistance of the device as received from the factory of manufacturing. • Operating Voltage: Maximum voltage a device can withstand without damage at the rated current. • Holding Current: Safe current passing through the device under normal operating conditions. • Trip Current: It is the value of current at which the device interrupts the current. • Time to Trip: The time it takes for the device to trip at a given temperature. • Tripped State: Transition from the low resistance state to the high resistance state due to an overload. • Leakage Current: A small value of stray current flowing through the device after it has switched to high resistance mode. • Trip Cycle: The number of trip cycles (at rated voltage & current) the device sustains without failure. • Trip Endurance: The duration of time the device sustains its maximum rated voltage in the tripped state without failure. • Power Dissipation: Power dissipated by the device in its tripped state. • Thermal Duration: Influence of ambient temperature. • Hysteresis: Period b/w the actual beginning of the signaling of the device to trip & the actual tripping of the device.
- 18. Types
- 19. Surface mount re-settable Fuses Radial ledded re-settable fuses Battery strap re-settable fuses
- 21. Applications • Transformer protection • Speaker protection • Battery protection • IC protection • Protection of electrical equipments & computer peripherals • Automobiles • Lighting, Security & Fire alarm system
- 22. Conclusion • Self Resettable device • Low Resistance • Fast trip time • Ideal for low cost over current protection • Ideal for low voltage
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