A Polymer-Resettable-Fuse device comprises a polymer matrix that is loaded with carbon black particles to make it conductive. Since it is conductive it will pass a given amount of current. If too much current is passed through the device, the device will begin to heat by I2R effect. As the device heats it will expand. As it expands, the carbon particles will separate and the resistance of the device will increase. This will cause the device to heat faster and expand more, further raising the resistance. When the internal temperature of the device reaches 125°C, the change in resistance increases dramatically. This increase in resistance is sufficient to substantially reduce the current in the circuit. A small amount of current will still flow through the device sufficient to maintain the temperature of the device and keep the Polymer-Resettable-Fuse device at the high resistance level. When the power is cycled off and the fault removed, the Polymer-Resettable-Fuse device is allowed to cool. As the device cools, it contracts to its original shape and reconnects the carbon particles thus lowering the resistance of the device to a level where it can hold the current as specified for the device. This cycled can be repeated multiple times.

Ih(A): Hold Current, Ih is defined as the maximum current at which a device will not trip at temperature (20°C to 25°C depending on product family) in still air. In other words, this is the expected maximum operational current at room temperature.

It(A): Trip Current, It is defined as the minimum current at which the device will always trip at temperature (20°C to 25°C) depending on product family) in still air. In other words, this is the expected minimum fault condition that is expected at room temperature.
Tt (S): Maximum time to trip(s) at specified current (generally at 5 IH), Tt is defined as the time it takes for the voltage drop across the thermistor to rise to greater than 80 percent of the voltage of the power source, or when

the resistance of the device increases substantially relative to the load resistance.

Vmax (V): Maximum Interrupt Voltage, Vmax is defined as the highest voltage that can safely be dropped across a PPTC thermistor in its tripped state under specified fault conditions.

Imax(A): Maximum fault Current , Imax is defined as the highest fault current that can safely be used to trip a PPTC thermistor under specified conditions. Typically the lower the voltage dropped across the PolySwitch device in its tripped state, the higher the maximum interrupt current.

Pdtyp (W): Typical power dissipation, Pdtyp is defined as typical amount of power dissipated by the device when in tripped state air enviroment.

Ro(O): Minimum-Maximum device resistance at 25? prior to tripping

R1max(O): The maximum resistance of a PPTC thermistor at room temperature one hour after being tripped or after reflow soldering.

Step1: Determine the circuit’s operating parameters. Include Vmax, Ih, Temperature of working.
Step2: Select the device that will accommodate with the circuits size, configuration, and maximum voltage ratings.
Step3:Compare the current Ih in relevant “T-I De-rating Form” at maximum operating temperature, find out the most suitable model.
Step4: Evaluate the suitability of the selected device by experiment.