Selection of overcurrent overload PTC thermistor

According to the temperature coefficient, it is divided into positive temperature coefficient thermistor (PTC) and negative temperature coefficient thermistor (NTC). Overcurrent overload protection PTC thermistor selection mainly has the following aspects:

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 large enough operating voltage and also to consider electric power.

2. No action current and action current

For a reliable switching function, the operating current must be at least twice the inactive current.

3. High current allowed at operating voltage

When the PTC thermistor is required to perform the protection function, it is necessary to check whether there is a condition in the circuit that exceeds the allowable large current, which generally means that the user has a possibility of generating a short circuit. The specification has given a large current value that, beyond this value, can cause damage or early failure of the PTC thermistor.

4. Switching temperature (Curie temperature)

The non-operating current depends on the Curie temperature and the diameter of the PTC thermistor chip. In terms of cost reduction, high Curie temperature and small size components should be used. On the other hand, the PC thermistor thus selected will be considered. Whether the higher surface temperature will cause bad effects in the line. Under normal circumstances, the Curie temperature should exceed the high ambient temperature of 20~40C.

5. The impact of the use environment

When contacting chemical reagents or when using infusion materials or fillers, special care must be taken to reduce the PTC thermistor effect caused by the reduction of barium titanate ceramics, and the change in thermal conduction conditions due to perfusion may cause local overheating of the PTC thermistor. And damaged.