Polyfuse

1. www.studymafia.org
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www.studymafia.org www.studymafia.org
Seminar
On
Polyfuse

2. Content
 Introduction
 Over Current Protection
 Construction
 Principle of Operation
 Types of Polyfuses
 Edges Over Conventional Fuses
 Applications of Polyfuse
 Conclusion
 References

3. INTRODUCTION
 Basically a Thermistor
Polymeric Positive Temperature Coefficient (PPTC)
 A new standard for circuit protection
 Self Resettable
A series element in a circuit

4. 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.

5. 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.

6. Principle of Operation

7. 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).

8. 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.

9. 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.

10. 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.

11. Hold and Trip Current as a Function of
Temperature

12. 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.

13. Types of Polyfuses
Surface Mount Resettable Fuses
Radial-Leaded Resettable Fuses
Battery Strap Resettable Fuses

14. 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

15. Item PPTC CPTC Bi-Metal Fuse
Resettable Yes Yes Yes No
Size Small Medium Large Large
Warranty
Cost
Low Low High High
Power Loss Low High Low Low
Resistance Low High Low Low
Comparison between PPTC and Other Types of Fuses

16. 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

17. 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

18. References
 www.google.com
 www.wikipedia.org
 www.studymafia.org

19. Thanks