Thermistors can be divided into positive temperature coefficient and negative temperature coefficient according to the relationship between their resistance value and temperature change. The so-called positive temperature coefficient means that the resistance value of thermistor decreases with the increase of ambient temperature.

The nominal resistance value of thermistor refers to the resistance value when the environment is 25 degree . Therefore, when measuring the resistance value of thermistor, we need to pay attention to the influence of environmental temperature on its resistance value. When the ambient temperature is 25 degree , the resistance value of thermistor measured by multimeter is its nominal resistance value. If the ambient temperature is not 25 degree , it is normal that the measured resistance value does not match the nominal resistance value of thermistor.

A: The bridge stack essentially encapsulates a bridge rectifier circuit in a plastic shell, which can not only reduce the area occupied by the diode on the printed circuit board, but also simplify the circuit. The shape and internal circuit of the bridge stack are shown in Figure 3-12. Generally, the symbols of AC voltage input and DC voltage output are marked on the outer surfaces of the corresponding pins of the bridge stack.