Polyfuse

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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.

 

THE BASICS

 

??????????? 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.

 PRINCIPLE OF OPERATION

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.

The principle of operation and increase in resistance in shown in the Fig.1. The increase in resistance protects the equipment in the circuit by reducing the amount of current that can flow under the fault condition to a low steady state level. The device will remain in its latched (high resistance state)until the fault is cleared, providing continuous protection to the circuit. At this time the conductive polymer particles cool and recrystallises restoring the PPTC to a low resistance state within few seconds. The circuit and the affected equipment return to the normal operating condition.

Thus a polyfuse acts like a self-resetting solid-state circuit breaker, which makes it suitable for providing low cost over current protection. The resistance of polyfuse at room temperature is in the order of few ohms and increases rapidly above 110? C.

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