Applied Principle     

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    The traditional fuse being used for overcurrent protection, only can protect one time, once being blown out it must be replaced. But the resettable fuse the new type overcurrent protection products with the double functions of overcurrent protection and automatic reset.
PPTC (polymeric positive temperature coefficient)

Overcurrent  protection 

     PPTC (Polymeric positive temperature coefficient ) element is serially connected to the circuit, under normal circumstances it presents low resistance state and can ensure the normal work of circuit: in case of occurring short circuit or unusually high current, the self-heating of PPTC may make its impedance increasing to limit very low current pass through, so playing the role for protecting current.

Automatic  restoration

     Once the fault resulting in overcurrent being removed, PPTC element will automatically return to its low resistance state, which not only avoid maintenance or replacement but also avoid continuously circulating switching state which can cause circuit damage.  PPTC resettable fuse is manufactured by mixing high polymer with conducting materials. Under normal circumstances, conducting materials through polymeric materials forms three-dimensional passage, so PPTC's resistance value is very low. In case of passing unusually high current, PPTC element's temperature will rapidly rise, and accordingly polymer materials will expand, and lead to conducting passage breaking, thus resulting the impedance dramatically rising, the passing current turning low and circuit being like break off, and achieving the goal of protection. When unusually high current disappears, the self-heating of PPTC will be not enough to maintain its high resistance state, so its impedance will turn to low resistance state again. Compare to the traditional fuse, PPTC possesses the advantages of resettable, little volume and solidness.

Operating Principle 

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     The operating principle of PPTC Resettable Fuse is based on the dynamic equilibrium of energy, with part of or all the head brought about by the current passing PPTC component due to the relation with PPTC sending forth into the ambience while the heat not sending out raising the temperature of PPTC component.

     Temperature is rather low during normally working, while a dynamic equilibrium between the heat produced the that sent forth being reached. When PPTC component being at low resistance state, PPTC doesn't operate. And though the current passing PPTC component increases and the ambient temperature rises, but if the heat produced and that sent forth reaching a equilibratory state, PPTC will still not operate. When current and ambient temperature further raising, PPTC will come to a rather high temperature. If right now current and ambient temperature continue to further rise, then the quantity of heat produced will be larger than that sent forth, thus causing the temperature of  PPTC component abruptly rising. In this stage, the least change of temperature will cause resistance value substantially increasing, and PPTC component will be in the high resistance protection state. The increase of impedance will limit current, which will dramatically fall within a very short time, thus protecting circuit unit from damage. So provided for the quantity of heat produced by the applied voltage is enough to matc that sent forth by PPTC component, the PPTC component under changing state will always remain in the operating state (high resistance). Once the applied voltage disappears, PPTC will automatically reset itself.

Characteristics of Restoring Time 

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     Within some seconds after the current passing PPTC component disappearing, the temperature of PPTC component will drop soon and it will rapidly reset itself into the low resistance state.

Effect of Ambient Temperature 

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     1. Increase of ambient temperature over 20°C will cause current passing the resettable fuse dropping swiftly.

     2. When ambient temperature being at 20°C, 100 percent of current on the line will pass through the resettable fuse, but if there being such much current as over doubling the rated value produced, the resettable fuse will operate.

     3. The higher the ambient temperature, the larger the passing current and the shorter the operating time. The reduction rate of ambient temperature and current value is shown as following table:

Methods of Selection 

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     4. Then make selection according to step two, and the TH needed should larger than or equivalent to the value calculated in step three.