Comparison of typical applications of circuit protection devices such as NTC, PTC, TVS, MOV, etc.alarm sensor wholesaler

　　NTC: Negative temperature resistance, the higher the temperature, the smaller the resistance, it is used in the input loop to limit the inrush current of the startup. During normal operation, it generates heat and the resistance decreases, which does not affect the work, but it consumes energy, and the power consumption cannot be ignored. .NTC can also be used for temperature measurement.

　　In order to avoid the inrush current generated at the moment of power-on in the electronic circuit, a power NTC thermistor is connected in series in the power supply circuit, which can effectively suppress the inrush current at the time of power-on, and after completing the function of suppressing the inrush current , Due to the continuous action of its current, the resistance value of the power NTC thermistor will drop to a very small level, the power it consumes can be ignored, and it will not affect the normal working current. Therefore, in the power circuit The use of a power NTC thermistor is the most simple and effective way to suppress the surge when starting up, so as to ensure that the electronic equipment is not damaged.

　　Analysis: When the NTC is not powered on, the resistance value of the NTC is high. When the power is turned on, the resistance value is still high, which limits the inrush current. With the current flowing through the NTC, the temperature increases, and the resistance value drops to a very low level, which can be ignored. During normal operation , the current is small, the resistance value is small, then a sudden surge of current, or the circuit makes the current increase, it will not play a protective role. For example, if the power supply is short-circuited, since the NTC has been turned on, there is nothing to do with it, only the fuse works. Remember that the NTC is only for boot protection.

　　Imagine if the circuit has been powered on normally and the NTC has low resistance, then it is powerless to encounter high voltage NTC. After the power supply works normally for a period of time, frequent switching on and off will cause damage to the power supply, because at this time, due to the rise of the temperature of the NTC and the decline of the resistance value, the ability to suppress the surge is already extremely limited. Therefore, the power supply equipment that uses NTC to suppress the power-on surge cannot be turned on and off frequently. It needs to wait for the NTC to cool down and recover to its cold resistance before turning on again. Otherwise, the meaning of installing the NTC is lost.

　　PTC: positive temperature resistance, stringed in the input loop, also known as: self-recovery fuse. When overcurrent, the resistance increases, and it is equivalent to disconnecting from the input. After cooling down, the resistance decreases, and it can continue to work without replacement. Often used together with varistor and TVS.

　　PTC has many uses, such as the degaussing circuit of color TV, the starting circuit of refrigerator compressor, etc. PTC is sometimes used for over-temperature protection. Both PTC and NTC may be used in series in the loop, but PTC is equivalent to a fuse, and NTC is used to limit the startup current.

　　Varistor: The English name is "Voltage DependentResistor" abbreviated as "VDR", or "Varistor" abbreviated as "VSR". Also called MOV (metal oxide varistors). Parallel connection in the AC side circuit mainly plays the role of "limiting the voltage to be too high". Using the nonlinear characteristics of the varistor, when an overvoltage appears between the two poles of the varistor, the current increases rapidly after the clamping voltage is exceeded (but not It will be short-circuited, which is different from the discharge tube), so as to realize the protection of the subsequent circuit.

　　Application of varistor in solid state relay circuit overvoltage protection circuit When the solid state relay SSR is used to drive an inductive load, a high surge voltage will be generated at the moment when the power is turned on and off. The thyristor will not be damaged or malfunction due to overvoltage, and a varistor MY can be connected in parallel with the AC output end of the SSR. The choice of varistor, if it is a 220V power supply, use 470V; if it is a 380V power supply, use 830V.

　　TVS tube is short for Transient Voltage Suppressor. Its characteristics are: the response speed is very fast (ns level), and the surge resistance is worse than that of the discharge tube and the varistor.

　　When the two poles of the TVS diode are impacted by reverse transient high energy, it can change the high impedance between its two poles into low impedance at a speed of the order of 10 to the minus 12 second, absorbing up to several kilowatts of surge power , so that the voltage between the two poles is clamped at a predetermined value, which effectively protects the precision components in the electronic circuit from damage by various surge pulses. The working principle of the varistor: For example, a "nominal 300V" varistor is working at 220V, and suddenly the 220V rises to 310V. At this time, the varistor is broken down, and after the fuse is blown through a large current, the The circuit behind is protected, and then the varistor returns to its original state.

　　Comparison of TVS tube and varistor: TVS tube and varistor both rely on changing their own resistance characteristics to control electrostatic discharge, transient voltage and surge voltage, and the main difference is the resistance after conduction. TVS has The high-sensitivity N/P junction is used for control, and its on-resistance is very low, while the on-resistance of MOV is much higher than that of TVS, resulting in differences in clamping voltage and clamping ratio. TVS tubes are mainly used in low-voltage circuits, while varistors are mainly used in high-voltage circuits.

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