What are Metal Oxide Varistors?
A varistor, also known as a voltage-dependent resistor, is an electronic component whose function is to protect other devices from high transient voltage. A metal oxide varistor, or MOV, is a specific type of resistor made from a combination of zinc oxide & other types of metal oxides including manganese, cobalt, and others. The material is arranged between two metal plates or electrodes to interact with each other. Varistors of this type are used most commonly in heavy devices to protect them from transient voltages. MOVs are the same as resistors in that they both consist of two leads that don’t have polarity.
MOVs are not able to resist transient voltages above their rating. When they are exposed to higher voltages, the extreme heat causes the device to exhaust and eventually dissolve. To supply better energy-handling capabilities, these varistors are often connected parallel to each other wherever they are used. To supply high voltage ratings, MOVs can also be connected in a series.
An MOV is a type of variable resistor. However, unlike a potentiometer, its resistance will change automatically depending on its voltage. As the voltage across the device increases, the resistance decreases. This is a useful feature in protecting circuits from high voltage spikes. There are many qualities to consider when selecting a MOV. Among the most important are: maximum voltage, varistor voltage, leakage current, capacitance, highest working voltage, highest AC voltage, clamping voltage, surge current, surge shift, response time, and energy absorption. The common features among all MOVs include AC voltage ranges from 130V to 1000V, DC voltage range from 175V to 1200V, insulation resistance of 1000Mohm, and operating temperature range from -55 to +85 °C.
MOVs are commonly found in circuits where they are used in conjunction with a fuse. The devices are connected parallel to the circuit to provide protection. Once the voltage is in the fixed range, the resistance will be very high. As such, the flow of current will be present in the circuit but not within the MOV. However, once a voltage spike occurs in the main voltage, it comes across the varistor because they are located parallel with the AC mains. During a voltage spike, the high faulted voltage will quickly return to regular values. In these scenarios, the duration of current flow will not be enough to damage the fuse and the circuit will return to its normal position as the voltage normalizes. However, when a voltage spike is noticed, the varistor briefly separates the circuit. Over time, many voltage spikes will cause damage to the circuit.
The primary function of an MOV is to work as a surge suppressor. When the voltage across the varistor is lower than the clamping voltage, the varistor will not conduct. The performance of a varistor slows down over time, and the device’s energy rating can also affect its performance. The performance of a varistor can be increased by connecting it parallel to other varistors. The main benefit of MOVs is their response time, as voltage spikes can be shorted in a number of nanoseconds. However, the response time is affected by the mounting design technique & components.
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