Abstract :
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 damage.
Like 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.
Technically 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.
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