Common special resistors include varistors and thermistors, which play a key role in the design and application of AC-DC switching power supplies. Understand the characteristics and specific functions of these two resistors:

Varistor MOV is one of the commonly used devices in the electromagnetic compatibility EMC of the circuit. It is widely used in electronic circuits to protect the circuit from damage due to the transient voltage mutation of the power supply system. Its characteristics are generally understood as when the front-end voltage is higher than the turn-on voltage of the varistor, the varistor is broken down, and the resistance of the varistor decreases and the current is shunted to prevent the subsequent stage from being damaged or interfered by excessive instantaneous voltage. This protects sensitive electronic components. Circuit protection is to use the non-linear characteristics of the varistor. When an overvoltage occurs between the two poles of the varistor, the varistor can clamp the voltage to a relatively fixed voltage value, thus realizing the protection of the subsequent circuit. The main parameters of varistor are: varistor voltage, current capacity, junction capacitance, response time, etc.

However, do not think too much about the role of varistors. Varistors cannot provide complete voltage protection. The energy or power that varistors can withstand is limited and cannot provide continuous overvoltage protection. . Continuous overvoltage will destroy the protective device (varistor) and cause damage to the equipment. The parts that the varistor cannot provide protection include: inrush current when starting up, overcurrent when short-circuited, and voltage dips. These situations require other means of protection.

Thermistor is a temperature-related device, generally divided into two types, NTC is a negative temperature coefficient thermistor, that is, the higher the temperature, the smaller the impedance; PTC is a positive temperature coefficient thermistor, that is, the higher the temperature, the impedance Bigger. The use of impedance's sensitivity to temperature plays an important role in circuit design.

NTC in the circuit mainly suppresses the starting current during the starting of the circuit. When the system is started, due to the power circuit, capacitive and inductive load inside the system, a very large inrush current will appear at the moment of starting. If the instantaneous anti-current capability of the device is not considered in the selection process of the circuit device, then the system will easily cause the device to be broken down during the operation process of multiple startups, and adding NTC to the circuit is equivalent to when the input loop is started. Increase the input impedance to reduce the impact current, and when the system is in a stable state, due to the NTC heating, according to its negative temperature characteristics, the impedance is reduced, so the loss on the NTC is also reduced, reducing the overall loss of the system.

PTC can function as a fuse in the circuit, so it has another name for self-recovery fuse. During the operation of the system, when the circuit is abnormal and causes a large current, if there is a PTC in this part of the circuit, it means that there is a large current flowing in the PTC, and the PTC generates heat. According to its positive temperature characteristics, its The impedance will become very large, so that the impedance of the entire loop becomes larger, so that the current of the loop becomes smaller, which acts as a fuse. According to its positive temperature characteristics, another function of PTC is to realize over-temperature protection in the circuit.