Now widely used "zinc oxide" (ZnO) varistor, its main material is composed of bivalent element zinc (Zn) and hexvalent element oxygen (O). So from the point of view of material, zinc oxide varistor is a kind of "group Ⅱ-Ⅵ oxide semiconductor". In Taiwan, a varistor is called a "surge absorber" or sometimes an "electrical shock (surge) suppressor (absorber)".
Varistor is a voltage limiting protection device. Using the nonlinear characteristics of the varistor, when the overvoltage appears between the poles of the varistor, the varistor can clamp the voltage to a relatively fixed voltage value, so as to realize the protection of the later stage circuit. The main parameters of varistor are: pressure sensitive voltage, current capacity, junction capacitance, response time and so on.
The response time of varistor is ns, faster than gas discharge tube and slightly slower than TVS tube. Generally, the response speed of varistor can meet the requirements for overvoltage protection used in electronic circuits. The junction capacitance of varistor is generally in the order of several hundred to several thousand Pf range. In many cases, it is not suitable to be directly applied in the protection of high frequency signal lines. When applied in the protection of AC circuits, its junction capacitance is larger than that of AC circuits, which will increase the leakage current. Varistors have larger through-flow capacity than gas discharge tubes. Varistor (VDR) is a kind of non - linear over - voltage protection semiconductor device sensitive to voltage.
When the voltage applied to a varistor falls below its threshold, the current flowing through it is so small that it is equivalent to a resistor of infinite resistance. That is, when the voltage applied to it falls below its threshold, it acts as an off switch.
When the voltage applied to a varistor exceeds its threshold, the current flowing through it surges and corresponds to a resistance of infinitesimal resistance. That is, when the voltage applied to it is higher than its threshold, it is equivalent to a switch in the closed state.