The most common special resistors are varistor and thermistor, which play a key role in the design and application of AC-DC switching power supply
Varistor mov is one of the most commonly used devices in circuit electromagnetic compatibility (EMC). It is widely used in electronic circuits. Its characteristic is that when the front-end voltage is higher than the opening voltage of varistor, the varistor will be broken down, and the resistance value of varistor will be reduced to shunt the current, so as to prevent the later stage from being damaged or interfered by excessive instantaneous voltage, so as to protect the sensitive electricity Subcomponents. The use is the nonlinear characteristics of the varistor, when the overvoltage occurs between the two poles of the varistor, the varistor can clamp the voltage to a relatively fixed value, so as to realize the protection of the later stage circuit. The main parameters of varistor are: varistor voltage, current capacity, junction capacitance, response time, etc.
Thermistor is a kind of temperature related device, generally divided into two kinds, NTC is negative temperature coefficient thermistor, that is, the higher the temperature, the smaller the impedance, TC is positive temperature coefficient thermistor, that is, the higher the temperature, the greater the impedance. The sensitivity of impedance to temperature plays an important role in circuit design.
NTC in the circuit is mainly to suppress the starting current in the starting process of the circuit. When the system starts up, because there are power circuits, capacitive and inductive loads in the system, there will be a very large impact current at the starting moment. If the instantaneous anti current ability of the device is not considered in the process of circuit device selection, it is easy to cause the device to be broken down and damaged in the process of multiple start-up operation. Adding NTC in the circuit is equal to increasing the input impedance and reducing the impact current when the input circuit is started. When the system is in a stable state, due to NTC heating, according to its negative temperature characteristics, the impedance is reduced So the loss on NTC is also reduced, and the overall loss of the system is reduced.
PTC can act as a fuse in the circuit, so it has another name for self recovery fuse. In the process of system operation, when the circuit is abnormal, leading to a large current, if there is a PTC in the circuit, it means that there is a large current flowing through the PTC, PTC heating, according to its positive temperature characteristics, its impedance will become large, making the impedance of the whole circuit larger, thus making the current of the circuit smaller, playing the role of a fuse. According to its positive temperature characteristics, another function of PTC is to realize over temperature protection in the circuit.
