POLYFUSE SATISH SUKUMARAN 1DS08EC407
Nov 18, 2014
POLYFUSE
SATISH SUKUMARAN1DS08EC407
INTRODUCTION
Basically a Thermistor
Polymeric Positive Temperature Coefficient
(PPTC)
A new standard for circuit protection
Self Resettable
A series element in a circuit
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.
Construction
Polyfuse circuit protection devices are made from a compositeof semi-crystalline polymer and conductive particles. At normaltemperature, the conductive particles form low-resistancenetworks 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.
Principle of Operation
Operating CharacteristicsTypical pair of operating
curves for a PolySwitchdevice 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 iscorrespondingly less, resulting in a lower trip current at a giventrip time (or a faster trip at given trip current).
Typical Resistance Recovery after a Trip Event
Figure shows typical behavior of a PolySwitch device that istripped 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.
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.
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.
Hold and Trip Current as a Function of Temperature
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.
Types of Polyfuses
Surface Mount Resettable Fuses
Radial-Leaded Resettable Fuses
Battery Strap Resettable Fuses
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
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
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
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
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
Queries ?
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