Varistors - definition
Varistors - definition
Varistors are non-linear resistive elements whose resistance varies dynamically as a function of the applied voltage. The main task of a varistor is to protect electronic and electrical systems against overvoltages, such as lightning strokes, switching overvoltages or sudden voltage changes in power networks. The voltage-current characteristic of a varistor is highly non-linear - at normal operating voltage it has a very high resistance, but when a certain voltage threshold is exceeded, its resistance decreases rapidly, allowing a large current to flow and limiting the voltage on the protected object.
The most commonly used varistors are metal-oxide varistors (MOV - Metal-Oxide Varistor), made of fine grains of zinc oxide (ZnO) joined by additional dopants such as bismuth or manganese. The micro-grain structure of the MOV forms numerous grain connectors, which are responsible for the non-linearity effect. The conduction of current in the varistor takes place through the processes of tunneling and thermal emission at the interfaces between the grains, which allows it to absorb surge pulses quickly and efficiently.
Varistors are widely used in switching power supplies, motor control systems, communication systems, Measuring Instruments and in the protection of medium and high-voltage power grids. The selection of a suitable varistor depends on parameters such as clamping voltage, pulse energy, response time and the ability to suppress repeated overvoltages. The lifetime of a varistor is directly related to the number and energy of absorbed pulses, which is why redundant solutions and varistor monitoring systems are used in highly critical applications.
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