Thermistor Overcurrent and Thermal Protection Types and Selection Guide

There are generally three types of overcurrent and overheating protection:

1. Current overload: RL1 is the load curve during normal operation. When the load resistance value is reduced, such as the transformer circuit is short-circuited, the load curve changes from RL1 to RL2, exceeding point B, the PTC thermistor enters the protection state;

2. Voltage overload: If the power supply voltage increases, such as the 220V power line suddenly rises to 380V, the load curve changes from RL1 to RL2, exceeding point B, the PTC thermistor enters the protection state;

3. Temperature overheating: When the ambient temperature rises above a certain limit, the volt-ampere characteristic curve of the PTC thermistor changes from A-B-E to A-B1-F, the load curve RL exceeds the B1 point, and the PTC thermistor enters a protection state;

Selection guide:

Large working voltage

The PTC thermistor is connected in series in the circuit, and only a small part of the voltage is maintained on the PTC thermistor during normal operation. When the PTC thermistor starts up in a high-impedance state, it must bear almost all of the power supply voltage, so choose For PTC thermistors, there must be a sufficiently large working voltage, and at the same time, the fluctuation of the power supply voltage must be considered.

2. Non-operating current and operating current

In order to have a reliable switching function, the operating current must be at least twice the non-operating current. Since the ambient temperature has a great influence on the non-operating current and the operating current, it is necessary to consider the bad situation. For the non-operating current, choose the value applied at the allowed high ambient temperature. For the operating current, select Values applied at lower ambient temperatures.