The principle and function of varistor

1. Principle of varistor

A varistor means "the resistance value varies with voltage within a certain current-voltage range" or a resistor whose resistance value is voltage-sensitive. The corresponding English name is “VoltageDependentResistor” and is abbreviated as “VDR”.

As the voltage applied to it increases, its resistance can change from MΩ (megaohms) to mΩ (milliohms). When the voltage is low, the varistor works in the leakage current region, exhibits a large resistance, and the leakage current is small. When the voltage rises into the nonlinear region, the current changes within a relatively large range, and the voltage does not change much. It exhibits better voltage limiting characteristics; the voltage rises again, and the varistor enters the saturation region, exhibiting a small linear resistance. Due to the large current, the varistor will overheat and burn or even burst due to the long time. In normal use, the varistor is in the leakage current area. When it is subjected to a surge, it enters the nonlinear region to discharge the surge current, and generally cannot enter the saturation region.

The resistor material of the varistor is a semiconductor, so it is a variety of semiconductor resistors. A "zinc oxide" (ZnO) varistor, which is currently used in large quantities, has a host material composed of a divalent element (Zn) and a hexavalent element oxygen (O). Therefore, from the material point of view, the zinc oxide varistor is a "II-VI oxide semiconductor".

2. The role of varistor

The biggest characteristic of a varistor is that when the voltage applied to it is lower than its threshold "UN", the current flowing through it is extremely small, equivalent to a shut-off valve. When the voltage exceeds UN, it flows. Its current surges, which is equivalent to the valve opening. With this function, it is possible to suppress abnormal overvoltages that often occur in circuits and protect circuits from overvoltages.

A varistor is a component with transient voltage suppression that can be used in place of a combination of a transient suppression diode, a Zener diode, and a capacitor. Varistors protect circuits in ICs and other devices from electrostatic discharges, surges, and other transients such as lightning strikes. When using, it is only necessary to connect the varistor to the protected IC or device circuit. When the voltage is higher than a certain value, the resistance of the varistor decreases rapidly, and a large current is turned on, thereby protecting the IC or the electric appliance. Equipment; when the voltage is lower than the working voltage of the varistor, the varistor has a very high resistance value and is almost open, so it does not affect the normal operation of the device or electrical equipment.