polyfuse

1. POLY FUSE

2. INTRODUCTION
 A Polymeric positive temperature co-efficiant device.
 A New standard for circuit protection.
 Basically a thermistor.
 Self resettable.
 A series element in the circuit

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.  When the current flowing through the device
(which has a small resistance in the on state)
exceeds the current limit, the PPTC device
warms up above a threshold temperature and
the electrical resistance of the PPTC device
suddenly increases several orders of
magnitude to a "tripped" state where the
resistance will typically be hundreds or
thousands of ohms. The current subsequently
reduces due to the finite voltage of the power
source. The rated trip current can be
anywhere from 20 mA to 100 A.

5. Construction
 A Polymeric PTC device comprises a
non-conductive crystalline organic
polymer matrix that is loaded
with carbon black particles to make it
conductive. While cool, the polymer is in
a crystalline state, with the carbon
forced into the regions between crystals,
forming many conductive chains.

7. Operation
 Since the crystaline-carbon system is conductivet will pass a
given current, called the "hold current". If too much current is
passed through the device, the "trip current", the device will
begin to heat. As the device heats, the polymer will expand,
changing from a crystalline into an amorphous state
 The expansion separates the carbon particles and breaks
the conductive pathways, causing the resistance of the
device to increase. This will cause the device to heat faster
and expand more, further raising the resistance. This
increase in resistance substantially reduces the current in
the circuit

9.  A small current still flows through the device and is
sufficient to maintain the temperature at a level
which will keep it in the high resistance state
 When power is removed, the heating due to the
holding current will stop and the PPTC device will
cool
 As the device cools, it regains its original
crystalline structure and returns to a low
resistance state where it can hold the current as
specified for the device.

10.  The resetting will often not take place even if the
fault alone has been removed with the power still
flowing as the operating current may be above the
holding current of the PPTC
 The resetting will often not take place even if the
fault alone has been removed with the power still
flowing as the operating current may be above the
holding current of the PPTC

11.  it will most likely stabilize at a significantly higher
resistance (up to 4 times initial value).

12. 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.

13.  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

14. Applications
 Poly fuses are used where the replacing of the
fuse is difficult or in the circuit which trips
often
 Protection Device Inside Consumer Electronics
 Protection of Computer Peripherals
 Automobiles, Industrial Control
 Tele Communication Equipment
 Lighting, Security & Fire Alarm System
 Speaker Protection
 Aero space shuttles

15. Advantages
 Low base resistance
 Automatic reset ability
 Lack of arcing during faulty conditions
 Less time to trip
 Compact design and small dimensions
 Internationally approved and standardized
 Lifetime almost ten times longer
 Survive mechanical vibrations and shocks
and comply with safety norms

16. Reference
 http://en.wikipedia.org/wiki/Resettable_f
use
 http://electronics.stackexchange.com/ta
gs/polyfuse/info
 Electronics 4 u

17. END

18. DOUBTS?