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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