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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
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.
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.
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
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
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