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Varistor
“Varistor” is a resistor device with nonlinear volt-ampere characteristics, mainly used for voltage clamping when the circuit is subjected to overvoltage, absorbing excess current to protect sensitive devices. The resistor body material of the varistor is a semiconductor, so it is a variety of semiconductor resistors.
The currently widely used “zinc oxide” (ZnO) varistors are composed of divalent element zinc (Zn) and hexavalent element oxygen (O) as their main materials. Therefore, from the perspective of materials, zinc oxide varistors are a type of “Ⅱ-Ⅵ group oxide semiconductors”. In Taiwan, China, varistors are called “surge absorbers” and sometimes “electric shock (surge) suppressors (absorbers)”.
A varistor is a voltage-limiting protection device. Using the nonlinear characteristics of the varistor, when an overvoltage appears between the two poles of the varistor, the varistor can clamp the voltage to a relatively fixed value, thereby realizing the protection of the subsequent circuit. The main parameters of the varistor include: varistor voltage, current capacity, junction capacitance, response time, etc.
The response time of the varistor is at the ns level, which is faster than that of the gas discharge tube and slightly slower than that of the TVS tube. Generally speaking, its response speed can meet the requirements for overvoltage protection of electronic circuits. The junction capacitance of the varistor is generally in the order of hundreds to thousands of Pf. In many cases, it is not suitable to be directly used in the protection of high-frequency signal lines. When used in the protection of AC circuits, its large junction capacitance will increase the leakage current, which needs to be fully considered when designing the protection circuit. The current capacity of the varistor is relatively large, but smaller than that of the gas discharge tube. Varistor, referred to as VDR for short, is a voltage-sensitive nonlinear overvoltage protection semiconductor component.
When the voltage applied to the varistor is lower than its threshold value, the current flowing through it is extremely small, and it is equivalent to a resistor with infinite resistance. That is to say, when the voltage applied to it is lower than its threshold value, it is equivalent to an open switch.
When the voltage applied to the varistor exceeds its threshold value, the current flowing through it surges, and it is equivalent to a resistor with infinite small resistance. That is to say, when the voltage applied to it is higher than its threshold value, it is equivalent to a closed switch.