In this paper, the principle of zinc oxide varistors, features, issues such as the correct selection of profiles, and provide some application circuit examples for your reference.
ZnO varistor voltage characteristics is actually a sensitive non-linear components, under normal voltage conditions, which is equivalent to a small capacitor, which occurred when the circuit voltage, its resistance sharply and quickly conduction, the operating current increases several orders of magnitude, thus effectively protecting the other components in the circuit will not be over-voltage damage, and its voltage characteristic is symmetric, as shown in Figure (1) a below. This device was made of ceramics, its internal micro-structure shown in Figure (1) b below. Micro-structure, including zinc oxide around the grains and grain boundary layer. The resistivity of ZnO grains is very low, while the grain boundary layer resistance rate is high, the match between the two grains forming the equivalent of a zener diode barrier, which is a varistor unit breakdown voltage of each cell is about 3.5V, if many of these units to be constituted a series and parallel varistor matrix. The more cells in series, and its high breakdown voltage, the greater the cross sectional area of the substrate, the greater the flow capacity. Varistor at work, each varistor unit to withstand surges in electric energy, rather than as a Zener diode junction under electric power only, which is why the ratio of varistor Zener diode can withstand much greater electric energy reasons.

Varistor in the circuit is normally connected to the protection of electrical input, as shown in Figure (2) below.

Zv varistor impedance of the circuit (including the surge source impedance Zs) constitute a voltage divider, so the restrictions varistor voltage V = VsZv / (Zs + Zv). Zv of the resistance can be reduced to normal levels when a few megohm Europe, or even less than 1Ω. This shows that in an instant flow Zv large current, over voltage most of the land in the Zs, while the input voltage is compared with the electrical stability, and therefore can play a protective role. Figure (3) shows characteristic curve can explain the protective principle. Straight line is the total impedance Zs, curve varistor characteristic curve, the two intersect at the point Q, the protection points, corresponding to the limit voltage V, it is the use of varistors added work on the use of electrical appliances voltage. Vs for the surge voltage, it exceeds the value of using electrical voltage VL, coupled with pressure-sensitive resistor, the operating voltage with the electrical voltage V is less than the value of VL, which effectively protects the use of electrical appliances. Different line impedances have different protection features, from the protective effect of view, Zs bigger, the better the protection, if Zs = 0, the circuit impedance is zero, varistors to afford protection role. Figure (4) the curve described by Zs and protection features can explain the relationship between.