PTC is the acronym of English Positive Temperature Coefficient. PTCC self-recovery fuse is made of polymer materials with conductive particles, and its basic principle is energy balance. When the current flows through the element, the generated heat is partly dissipated into the environment and partly increases the temperature of the polymer material. Under the working current, the generated heat and the emitted heat are balanced, and the current can pass normally. When the current is too high, a large amount of heat generated by the element cannot be dissipated in time, which leads to an increase in the temperature of the polymer material. Polymer aggregates and expands, blocking the conductive path composed of conductive particles, which leads to a rapid rise in resistance and restricts the passage of large current, thus playing an overcurrent protection role.
The PTC self-recovery fuse has the following three basic physical laws:
(1) ohm's law: I = u/r.
(2) Joule's law: q = 0.24 * i 2 * r * t.
(3) Electric power: p = u * i = i 2 * r.
According to ohm's law, when the voltage u applied at both ends of the load remains unchanged, the working current I of the whole circuit is inversely proportional to the resistance r of the circuit. The larger the resistance value is, the smaller the current I is.
According to joule's law, the heat q generated by current passing through PPTC is proportional to the square of current i .. As long as the current I of the circuit increases a little, the generated heat Q will increase sharply. Once the accumulated heat in PPTC reaches the temperature required for its operation. The conductive path inside PPTC will melt or break locally, and PPTC will immediately show high resistance value, thus limiting the current of the whole circuit in a very small range.
According to the electric power formula, when PPTC is protected, at this moment:
(1)P input = ＝PRt+ PRL.
(2) where: p input: the input power of the power supply.
(3) PRT: power borne by PTC.
(4)PRL: the power obtained by the load.
(5) Rt>>RL after the protective action of the recoverable fuse occurs.
(6) where: rt: resistance value of pptc.
(7)RL: equivalent resistance value of load (recommended reading: explanation of PTC industry terminology).
(8) Although the currents flowing through them are completely consistent, PRT (= I 2 * RT) > > PRL (= I 2 * RL).
Therefore, the power input by the power supply is almost borne by PPTC, and the load can no longer obtain electric power, thus achieving the purpose of protecting the load.
