Polyfuse is a new standard for circuit protection. It is resettable. Many manufacturers also call it polyswitch or multifuse. Polyfuses are not fuses but polymeric positive temperature co-efficient (PPTC) thermistors.
Current limiting can be accomplished by using resistors , fuses , switches or positive temperature co-efficient devices. Resistors are rarely an acceptable solution because the high power resistors that are usually required are expensive. One-shot fuses can be used, but they might fatigue, and they must be replaced after a fault event. Ceramic PTC devices tends to have high resistance and power dissipation characteristics.
The preferred solution is a PPTC device which has low resistance in normal operation and high resistance when exposed to a fault. Electrical shorts or electrically over-loaded circuits can cause over-current and over temperature damage.
Like traditional fuses, PPTC devices limit the flow of dangerously high current during fault conditions. Unlike traditional fuses, PPTC devices reset after the fault is cleared and the power to the circuit is removed.
2. THE BASICS
Technically, polyfuses are not fuses but polymeric positive temperature co-efficient (PPTC) thermistors. For thermistors characterized as positive temperature co-efficient, the device resistance increases with temperature. These comprise thin sheets of conductive plastic 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 even distribution of power throughout the device.
Polyfuses are usually packaged in radial, axial, surface- mount, chip, disk or washer form, these are available in voltage ratings of 30 to 250 volts and current ratings of 20Ma to 100 amps.
3. PPTC PRINCIPLE OF OPERATION
PPTC circuit protection devices are formed from a composite of semi crystalline polymer and conductive particles. At normal temperatures, the conductive particles form low resistance networks in the polymer. However if temperature rises above the device's switching temperature, 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 crystalline phase causes separation of conductive particles and results in a large non linear increase in resistance of the device.
The resistance typically increases by three or more orders of magnitude as shown in figure 1. The increased 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) position until the fault is cleared and power to the circuit is removed at which time the conductive composite cools and recrystallises , restoring the PPTC to a low resistance state and the circuit and the circuit and the affected equipment to normal operating conditions.
Thus a polyfuse acts like a self-resetting solid state circuit breaker, which makes it suitable for providing low-cost over-protection device. The resistance of polyfuse (expressed on log scale) at room temperature is a few ohms and rapidly increases above 110 degree Celsius.
PPTC devices protect the circuit by going from a low steady state to a high resistance state in response to an over-current or over temperature condition.