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.
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.
OPERATING PARAMETERS FOR POLYFUSES
1) INITIAL RESISTANCE:- The resistance of the device as received from the factory of manufacturing.
2) OPERATING VOLTAGE:- The maximum voltage a device can withstand without damage at the rated current.
3) HOLDING CURRENT:- Safe current through the device.
4) TRIP CURRENT:- Where the device interrupts the current.
5) TIME TO TRIP:- The time it takes for the device to trip at a given temperature.
6) TRIPPED STATE:- Transition from the low resistance state to the high resistance state due to an overload.
7) LEAKAGE CURRENT:- A small value of stray current flowing through the device after it has switched to high resistance mode.
8) TRIP CYCLE:- The number of trip cycles (at rated voltage and current) the device sustains without failure.
9) TRIP ENDURANCE:- The duration of time the device sustains its maximum rated voltage in the tripped state without failure.
10) POWER DISSIPATION:- Power dissipated by the device in its tripped state.
11) THERMAL DURATION:- Influence of ambient temperature.
12) HYSTERESIS:- The period between the actual beginning of the signaling of the device to trip and the actual tripping of the device.