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.
Current can be accomplished by using resistors, fuses, switches, circuit breakers
or positive temperature coefficient devices.
Resistors are rarely an
acceptable solution because the high power resistors required are expensive .One
shot fuses can be used but they might fatigue and they must be replaced after
a fault event. Another good solution available is the resettable Ceramic Positive
Temperature Coefficient (CPTC) device. This technology is not widely used because
of its high resistance and power dissipation characteristics. These devices are
also relatively large and vulnerable to cracking as result of shock and vibration.
The preferred solution is the PPTC device, which has a very low resistance in
normal operation and high resistance when exposed to fault. Electrical shorts
and electrically overloaded circuits can cause over current and over temperature
traditional fuses, PPTC devices limit the flow of dangerously high current during
fault condition. Unlike traditional fuses, PPTC devices reset after the fault
is cleared and the power to the circuit is removed. Because a PPTC device does
not usually have to be replaced after it trips and because it is small enough
to be mounted directly into a motor or on a circuit board, it can be located inside
electronic modules, junction boxes and power distribution centers.
Polyfuses are not fuses but Polymeric Positive Temperature Coefficient Thermistors.
For thermistors characterized as positive temperature coefficient, the device
resistance increases with temperature. The PPTC circuit protection devices are
formed from thin sheets of conductive semi-crystalline plastic polymers with electrodes
attached to either side. The conductive plastic is basically a non-conductive
crystalline polymer loaded with a highly conductive carbon to make it conductive.
The electrodes ensure the distribution of power through the circuit.
Polyfuses are usually packaged in radial, axial, surface mount, chip or washer
form. These are available in voltage ratings of 30 to 250 volts and current ratings
of 20 mA to 100A.
PPTC circuit protection devices are formed from a composite of semi-crystalline
polymer and conductive carbon particles. At normal temperature the carbon chains
form low resistance conductive network through the polymer. In case an excessive
current flows through the device, the temperature of the conductive plastic material
rises. When the temperature exceeds the device's switching temperature, the crystallides
in the polymer suddenly melts and become amorphous. The increase in volume during
melting of the crystalline phase cause separation of the conductive particles
and results in a large non-linear increase in the resistance of the device. The
resistance typically increases by 3 or orders of magnitude.
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