Metal Oxide Varistor (MOV)
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A Metal Oxide Varistor (MOV) is a protection component used in power supply circuits that is powered directly from AC mains. It is used to protect the circuit from high voltage spikes by varying its resistance.
Metal Oxide Varistors are similar to resistors and has only two leads. There is no polarity for these leads and hence can be connected in both directions.
- AC voltage range: 130 to 1000V
- DC Voltage range: 175 to 1200V
- Insulation resistance: 1000Mohm
- Operating temperature: -55 to +85 °C
Note: The above features are applicable to all the LA Varistor series of Littlefuse. Refer to the datasheet linked below to select the appropriate model number according to your application.
How does a MOV work?
The term MOV stands for “Metal Oxide Varistor”. As the name Varistor suggests, it is a variable resistor. But unlike a potentiometer, the resistance of the MOV changes automatically based on the voltage across. If the voltage across it increases the resistance decreases and vice versa. This property is useful in protecting the circuits from high voltage spikes.
How to use a MOV in your circuit?
MOV is commonly used along with fuse in parallel to the circuit that is to be protected as shown on the image below.
When the voltage is within the rated limits the resistance of the MOV will be very high and hence all the current flows through the circuit and no current flows through the MOV.
But when a voltage spike occurs in the main voltage, it appears directly across the MOV since it is placed in parallel to AC mains. This high voltage will decrease the resistance value of the MOV to a very low value making it appear like a short.
This forces a large current to flow through the MOV which would flow the fuse and disconnect the circuit from the mains voltage. During voltage spikes the faulted high voltage will return to normal values very soon, in those cases the duration of the current flow will not be high enough to blow the fuse and the circuit returns to normal operation when the voltage becomes normal. But, every time a spike is detected the MOV disconnects the circuit momentarily by shorting itself and damaging itself with high current each time. So if you find a MOV damaged in any power circuit it is possibly because the circuit went through many voltage spikes.
- Over voltage protection
- Voltage spike protection
- Line to Line protection
- Switching protection
- Arching protection.
MOVs come in many different sizes and shapes. 7mm,10mm,14mm and 20mm are the standard notations. The dimensions of all can be found from the table below.