A varistor is an electrical device having an electrical resistance that differs with the potential applied to it. A varistor is popularly known as Voltage Dependent Resistor (VDR). It possesses a non-ohmic and non-linear Current-Voltage characteristic which is equivalent to a diode. Unlike the diode, it has a similar characteristic for the flow of both directions of current. Customarily, varistors were made by joining 2 rectifiers i.e. either the germanium-oxide or the copper-oxide rectifier in a configuration that is anti-parallel. The electrical resistance of the varistor is high at low voltage but as the voltage increases the resistance decreases. The varistors that are used presently are fundamentally dependent on sintered ceramic metal-oxide materials. On a microscopic scale, it shows directional behavior. This category is popularly known as MOV or Metal Oxide Varistor.
Varistor serves as a compensation and a control element in the electrical circuit either to defend against excess voltage or to offer optimal operating conditions. When utilized as defending devices, they shunt the current established by the exorbitant voltage away from touchy segments when activated.
Composition
As said earlier, the newest form of varistor that is used is the Metal Oxide Varistor. This kind of varistor is constituted by a ceramic mass of zinc oxide grains with metal oxides (like a small quantity of cobalt, bismuth, manganese oxides) placed in the middle of 2 metal plates, which comprise the device’s electrodes. A diode junction is formed between each grain and its neighbor.
When the electrodes are given a small potential difference then only a small amount of current flows through the diode junction which is a result of reverse leakage. On the other hand when a large voltage is applied on the electrodes a large amount of current flows, as the diode junction separates itself because of the combination of electron tunneling and thermionic emission. The consequence of the above-said behavior is a non-linear Current-Voltage characteristic where the MOV possesses a low resistance at high voltage and high resistance at low voltage.
Voltage Rating
MOVs are categorized by the range of voltage that they can endure without damage. The other parameter is the varistor’s breakdown voltage, maximum current, energy rating in joules, response time and operating voltage.
The rating of energy is regularly characterized utilizing institutionalized transients like 10/1000 microseconds or 8/20 microseconds. Here 10 microseconds are the front time of the transient and 1000 microseconds is the time of half value.
Capacitance
The range of capacitance for varistors which are consumer-sized i.e. 7 to 20mm in diameter is from 100pF to 2,500pF. Varistors that are used in micro electric protection have low capacitance like around 1pF. You can get this kind in cellular phone. These varistors having low capacitance cannot withstand the flow of a large amount of current as the PCB mount size is compact.
Response time
The response time of the MOV isn't institutionalized.
Hope you liked the article on Varistors. Brands that make them are Littelfuse, EPCOS, Panasonic. You can refer to them using the links as provided. Thank you for reading.
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