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
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.
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.
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
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.
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.
PARAMETERS FOR POLYFUSES
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
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
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