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.
Surface Mount Resettable Fuses
This surface mount polyfuse family of polymer of polymer
based resettable fuses provides reliable over current protection for a wide
range of products such as computer motherboards, USB hubs and ports, CD/DVD
drives , digital cameras and battery packs. Each of these polyfuse series
features low voltage drops and fast trip times while offering full
resettability. This makes each an ideal choice for protection in datacom and
battery powered applications where momentary surges may occur during
interchange of batteries or plug and play operations.
Abstract
A fuse is a one time over current protection device
employing fusible link that melts after the current exceeds a certain level for
a certain length of time. Typically, a wire or chemical compound breaks the
circuit when the current exceeds the rated value.
EDGES OVER CONVENTIONAL
FUSES
v Over
current protection
v Low
base resistance
v Latching
operation
v Automatic
resettability
v Short
time to trip
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.
Battery
Strap Resettable Fuses
This type profile strap type polyfuse family of resettable
fuses provides thermal and over charge protection for rechargeable battery
packs commonly used in portable electronics such as mobile phones, notebook
computers and camcorders.
Applications
for Resettable Circuit Protection in Automotive Electronics
The conventional solution groups similar circuits together
and protects them all with a single fuse. The fuse must be sized to carry the
sum of the currents drawn by each of the protected loads; and, to limit risk of
damage and fire, the wires feeding from the fuse to each load must be chosen
according to the fuse size selected. This design practice often results in
oversized wires with high current-carrying capability feeding loads that
require relatively low currents. Using heavy-gauge wire also requires use of
larger terminals and connectors, which further increases cost, size, and
weight. It also increases harness weight, and the weight of the automobile,
which has an effect on fuel efficiency.
Introduction
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.
Conclusion
Polymeric Positive Temperature Coefficient device provide
net cost savings through reduced component count and reduction in wire size.
They can help provide protection against short circuits in wire traces and
electronic components. The low resistance, relatively fast time to trip and low
profile of these devices improve reliability.
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