Thermistor overcurrent protection type and selection guide

1. Generally there are three types of overcurrent protection:

(1) Current overload: RL1 is the load curve during normal operation. When the load resistance decreases, such as transformer line short circuit, the load curve changes from RL1 to RL2. When it exceeds point B, the PTC thermistor enters the protection state;

(2) Voltage overload: The power supply voltage increases. If the 220V power supply line suddenly rises to 380V, the load curve changes from RL1 to RL2. When it exceeds 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 ABE to A-B1-F, the load curve RL exceeds B1 point, and the PTC thermistor enters the protection state. ;

2. Selection guide

(1) Large working voltage

The PTC thermistor is connected in series in the circuit. Only a small part of the voltage is kept on the PTC thermistor during normal operation. When the PTC thermistor starts to be in a high-impedance state, it must withstand almost all the power supply voltage, so choose When using a PTC thermistor, it is necessary to have a high enough operating voltage, taking into account fluctuations in the supply voltage.

(2) Non-operating current and operating current

In order to have a reliable switching function, the operating current must be at least twice the current. Since the ambient temperature has a great influence on the non-operating current and the operating current, it is necessary to take the bad situation into consideration. For the non-operating current, select the value at the allowable high ambient temperature. For the operating current, select Apply values at lower ambient temperatures.