PTC Thermistor for Overcurrent and Overload Protection

General

MZO series of PTC thermistor are protective component which automatically protect and restore from abnormal temperature and current, also called “auto restore fuse” and “ten-thousand-time fuse”. It can replace traditional fuse, widely applied in overcurrent and overload protection of motor, transformer, electronic circuit. PTC thermistor decreases its residual current through resistance sharp increasing, restraining consumption. Unlike traditional fuse, PTC thermsitor can restore its protective function after the trouble was removed.

    In selecting PTC thermistor as overload protection component, first, make sure of the maximum operating current in the circuit (non-trip current of PTC thermistor), and maximum ambient temperature in installation position, second, protective current (trip current of PTC thermistor), maximum operating voltage, rated zero power resistance, and also the outlook and dimension should be also taken into consideration.

Main parameter

Non-trip current Int（mA）
Trip current It（mA）
Maximum current Imax(A)
Maximum operating voltage Vmax (V)
Ambient temperature （℃）
Rated zero power resistance R25（Ω）

Operating principle

        When a circuit is in the normal status, the current through PTC is lower than the rated current and PTC is in the normal state with small resistance value, which will not affect the normal operation of the protected circuit. In case of some trouble in the circuit and the current is greater than the rated current, the PTC thermistor will become hot quickly and present a high resistance state, which sets the circuit relatively “off” to protect the circuit from damage. After the trouble is removed, PTC will automatically restore its low resistance state and the circuit will resume normal operation.

        Fig. 2 is Voltage-Current Curve and Load Curve in normal operating state. From point A to point B, the voltage on PTC thermistor increases gradually, current through PTC thermistor also increases linearly, it indicates that PTC thermistor resistance keeps almost unchanged, in low resistance state; From point B to E, the voltage increases gradually, PTC thermistor resistance increases sharply due to heating, current through PTC thermistor decreases rapidly, it indicates that PTC thermistor in protection state. Normal load curve lower than point B, PTC thermistor won’t enter into protection state.

Generally there are 3 types of overcurrent and overload protection.

        Current overload (Fig. 3):RL1 is the loading current in normal operating state, When overloading resistance decreases, such as transformer circuit short circuit, loading curve changes from RL1to RL2 , exceeding point B, PTC thermistor will enter into protection state.

        Voltage overload (Fig. 4): Power voltage increases, such as increases abruptly from 220V to 380V, loading curve changes from RL1to RL2 , exceeding point B, PTC thermistor will enter into protection state.

        Temperature protection (Fig. 5): When ambient temperature increases exceeding certain temperature, PTC thermsitor voltage-resistance curve changes from A-B-E to A-B1-F, loading curve RL , exceeding point B1, PTC thermistor will enter into protection state.

Application examples