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Polyfuses is a new standard for circuit protection .It is re-settable by itself. Many manufactures also call it as Polyswitch or Multifuse. Polyfuses are not fuses but Polymeric Positive temperature Coefficient Thermistors (PPTC).We can use several circuit protection schemes in power supplies to provide protection against fault condition and the resultant over current and over temperature damage.

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  2. 2. INTRODUCTION  Basically a Thermistor Polymeric Positive Temperature Coefficient (PPTC)  A new standard for circuit protection  Self Resettable A series element in a circuit
  3. 3. Over Current Protection Polyfuse is a series element in a circuit. The PPTC device protects the circuit by going from a low-resistance to a high-resistance state in response to an over-current condition In normal operation the device has a resistance that is much lower than the remainder of the circuit. In response to an over-current 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.
  4. 4. Construction Polyfuse circuit protection devices are made from a composite of semi-crystalline polymer and conductive particles. At normal temperature, the conductive particles form low-resistance networks in the polymer. However, if the temperature rises above the device’s switching temperature (Tsw) either from high current through the part or from an increase in the ambient temperature, the crystallites in the polymer melt and become amorphous. The increase in volume during melting of the crystalline phase separates the conductive particles resulting in a large non-linear increase in the resistance of the device.
  5. 5. Principle of Operation
  6. 6. Operating Characteristics Typical pair of operating curves for a PolySwitch device in still air at 0°C and 75°C. The curves are different because the heat required to trip the device comes both from electrical I2R heating and from the device environment. At 75°C the heat input from the environment is substantially greater than it is at 0°C, so the additional I2R needed to trip the device is correspondingly less, resulting in a lower trip current at a given trip time (or a faster trip at given trip current).
  7. 7. Typical Resistance Recovery after a Trip Event Figure shows typical behavior of a PolySwitch device that is tripped and then allowed to cool. This figure illustrates how, even after a no.of hours, the device resistance is still greater than the initial resistance. Over an extended period of time, device resistance will continue to fall and will eventually approach initial resistance. when PolySwitch devices are chosen R1MAX should be taken into consideration when determining hold current. R1MAX is the resistance of the device one hour after the thermal event.
  8. 8. Operating Parameters  Initial Resistance: The resistance of the device as received from the factory  Operating Voltage: The maximum voltage a device can withstand without damage at rated current  Holding Current: Safe current through the device.  Trip Current: The current at which the interrupts the current  Time to Trip: The time it takes for the device to trip at a given temperature and current  Tripped State: Transition from low resistance state to 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.
  9. 9. Operating Parameters Trip Cycle: The number of trip cycles 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 the tripped state. Thermal Duration: Influence of ambient temperature. Hysteresis: The period between the actual beginning of the signaling of the device to trip and the actual tripping of the device.
  10. 10. Hold and Trip Current as a Function of Temperature
  11. 11. Hold and Trip Current as a Function of Temperature Region A describes the combinations of current and temperature at which the PolySwitch device will trip (go into the high-resistance state) and protect the circuit. Region B describes the combinations of current and temperature at which the PolySwitch device will allow for normal operation of the circuit. Region C, it is possible for the device to either trip or remain in the low-resistance state.
  12. 12. Types of Polyfuses Surface Mount Resettable Fuses Radial-Leaded Resettable Fuses Battery Strap Resettable Fuses
  13. 13. Edges Over Conventional Fuses Over current protection Low base resistance Latching operation Automatic resettability Short time to trip No arching during faulty situations Small dimensions and compact designs Internationally standardized and approved. No accidental hot plugging Withstand mechanical shocks and vibrations Life time- up to 10 times longer
  14. 14. Comparison between PPTC and Other Types of Fuses Item PPTC CPTC Bi-Metal Fuse Resettable Yes Yes Yes No Size Small Medium Large Large Warranty Low Low High High Cost Power Loss Low High Low Low Resistance Low High Low Low
  15. 15. Applications of Polyfuse Speaker Protection Battery Protection IC Protection Protection of Electrical Equipments Protection Device Inside Consumer Electronics Protection of Computer Peripherals Automobiles, Industrial Control Medical Electronics Lighting, Security & Fire Alarm System Tele Communication Equipment
  16. 16. Conclusion Self Resettable Device Compact Low Resistance Fast Trip Time Ideal for low cost over current protection Ideal for Low Voltage A.C & D.C
  17. 17. References Electronics For You, Edition-September, 2004 Raychem circuit protection products- Tyco Electronics http://www.circuitprotection.com http://www.wikipedia.com http://www.inter-technical.com
  18. 18. Queries ?
  19. 19. Thank You !!!