# Parameters of auto-recovery fuses

Source: Internet
Author: User

IH = hold current: maximum current at which the device will not trip at 25 ℃ still air.

Keep current: when the current of the circuit is smaller than this value, the resistance value of the self-recovery fuse will become very small.

It = Trip current: minimum current at which the device will always trip at 25 ℃ still air.

Threshold Current: when the current in the circuit is greater than this value, the resistance value of the self-recovery fuse will become very large.

IMAX = maximum fault current device can withstand without damage at rated voltage.

Maximum fault current: the maximum current that can be withstand under the rated voltage.

Vmax = maximum voltage device can withstand without damage at rated current.

Maximum voltage: The maximum voltage that can be withstand under the rated current.

Ttrip = maximum time to trip (s) at assigned current.

Maximum recovery time: the maximum recovery time of the specified current. (Personal understanding)

Pdtyp = typical power dissipation: Typical amount of power dissipated by the device when in state air environment.

The power consumption of PTC polymer auto-recovery fuses at a certain ambient temperature.
Rmin = minimum device resistance at 25 ℃ prior to tripping.

The minimum resistance of the device before the mutation at 25 °, which determines the minimum operating current of the polymer auto-recovery fuse.

Rmax specifies the highest resistance for the supplied polymer auto-recovery fuse.

R1max = maximum device resistance measured in the nontripped state 1 hour post reflow.

R1max is the maximum resistance that should be reached after the polymer auto-recovery fuse action, and its resistance determines the maximum holding current of the polymer auto-recovery fuse.

When the polymer fuses itself, the resistance (greater than or equal to rmin and less than or equal to rmax) will rise to less than or equal to r1max.

Principle of auto-recovery Fuse
A self-recovery fuse (fuse) is composed of a polymer resin matrix and conductive particles distributed inside it. Under normal circumstances, conductive particles form a chain-like conductive path in the resin matrix, fuses behave as low impedance (). When there is an overcurrent in the circuit, the heat generated by the large current flowing through the fuse expands the volume of the polymer resin matrix, cut off the chain-like conductive path formed by conductive particles, leading to the rapid increase of the impedance of the fuse, in this way, the circuit is protected over-current (B ). After the fault is rectified, the resin cools down the crystallization again, the volume shrinks, the conductive particles reform the conductive path, and the fuse restores to a low impedance. In the selection process of auto-recovery fuses, the following parameters are involved: 1. Current holding (IH): the highest current that does not trigger a resistance burst in a static air environment at 25 ℃. 2. trigger current (IT): The minimum current for converting PTC polymer auto-recovery fuses from low resistance to high impedance in a static air environment at 25 ℃. 3. Maximum voltage (Vmax): the maximum operating voltage of the PTC polymer auto-recovery fuse. 4. Maximum current (IMAX): the maximum current that the PTC polymer auto-recovery fuse can withstand. 5. Operation Power (pdtyp.): the power consumption of PTC polymer auto-recovery fuse when the ambient temperature is 25 ℃. 6. action time (ttrip): The maximum action time when the current is maintained at 5 times.

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