Each thermistor has parameters such as "withstand voltage", "withstand current", "maintain current" and "action time". You can choose the parameters of the product according to the requirements of the specific circuit. The specific methods are as follows:

1. First determine the maximum ambient temperature of the protected circuit during normal operation, the operating current in the circuit, the maximum voltage to be withstood after the thermistor is operated, and the required operating time;

2. Select different shapes of thermistors such as "chip type", "radial lead-out type", "axial lead-out type" or "surface mount type" according to the characteristics of the protected circuit or product;

3. According to the maximum working voltage, select the product series whose "withstand voltage" level is greater than or equal to the maximum working voltage;

4. According to the maximum ambient temperature and the working current in the circuit, select the product specification that “maintaining current” is greater than the working current;

5. Confirm that the operating time of the thermistor of this specification is less than the time required by the protection circuit;

6. Check the size of the thermistor of this specification to meet the requirements according to the data provided in the specification.
For example, a control circuit requires overcurrent protection, its operating voltage is 48 volts, the current is 450 mA when the circuit is operating normally, and the ambient temperature of the circuit is 50 °C. When the current in the circuit is required to be 5 amps, the current in the circuit should be reduced to less than 500 mA within 2 seconds.

According to its working voltage of 48 volts, we can first select KT60-B series thermistors with a voltage rating of 60 volts, as shown in Table 1;
Then select the products of KT60-0750B or KT60-0900B according to the list of maintenance current and temperature of the series of thermistors, as shown in Table 2;
According to the relationship between the action time and the current, it is found that the operation time of KT60-0750B is about 1 second at 5 amps and the operation time of KT60-0900B is about 2 seconds, as shown in Fig. 1;
Therefore, the KT60-0750B size thermistor should be selected.
The current in the circuit after the thermistor of this specification is less than 30 mA, thus meeting the requirements of overcurrent protection.