1. Definition of MOV
Varistors, which is also called metal-oxide varistors (MOVs), are used to protect sensitive circuits from a variety of overvoltage conditions. Essentially, these voltage-dependent, nonlinear devices have electrical characteristics similar to back-to-back Zener diodes.
Metal Oxide Varistors (MOVs) are bidirectional and non-linear surge suppressor devices that are widely used for limiting voltage during a surge or transient event in a broad variety of applications. These highly reliable and robust overvoltage protection devices are available in multiple form factors and surge protection ranges.
MOV is simply a variable resistor, but MOVs can adjust resistance depending on the applied voltage, not like potentiometers. The resistance decreases and vice versa if the voltage across it increases. This property is helpful in shielding the circuits from high voltage surges, so they are often used in an electronic network as surge protectors. A basic MOV is shown in the image below which is from UF Capacitors.
2. Working Principle of Varistor
To understand the working of a varistor, we need to understand how voltage surges and spikes get created in a circuit system. Most of the spikes are switching. When we switch off an inductive circuit, a high voltage surge is created due to (L.di/dt.) This surge is created due to the sudden release of energy stored in the inductance. As a thumb rule, it is said that switching on the surge is double the current, and switching off surge is double the voltage. Such surges can also have an adverse effect on all the equipment in the vicinity.
A varistor provides a high resistance path to low voltages and a low resistance path to high voltages. This variation in resistance with voltage is evident from the static resistance curve of the varistor. This characteristic is nonlinear and does not follow Ohm’s law.
3. MOV in Circuit
A Metal Oxide Varistor (MOV) is a voltage suppression device that filters and clamps the transient in an electrical circuit.
Varistors are being used to protect a circuit from high voltage surges. When a high voltage surge is applied to a circuit, the outcome is usually catastrophic to the circuit. A capacitor may be installed across the signal lines. However, this capacitor cannot suppress voltage surges.
Therefore, when circuit protection from voltage surges is required, a varistor is used as a voltage protection device. When a voltage surge exceeding a specified voltage (varistor voltage) is applied, the varistor suppresses the voltage to protect the circuit.
The resistance of the MOV would be very high when the voltage is below the rated limits, and then all the current flows through the circuit and no current flows through the MOV. But when a voltage spike occurs in the main voltage, when it is situated parallel to the AC mains, it appears directly across the MOV. The MOV resistance value would be reduced to a very low value by this high voltage, making it look like a short one.
This forces the flow of a large current through the MOV that would burst the fuse and isolate the circuit from the voltage of the mains. The defective high voltage will return to normal values very quickly during voltage surges, in such situations, the length of the current flow will not be high enough to burst the fuse and when the voltage becomes normal, the circuit returns to normal operation. But each time a spike is observed, the MOV briefly disconnects the circuit by shortening itself and each time damaging itself with a high current. But if you find a MOV damaged in any power circuit, it is probably so several voltage spikes have passed through the circuit.
4. How to Choose the Right MOV for Protection
To choose the right MOV for your device, it’s important to know about the different number of parameters of a MOV. The MOV specification depends on the following info.:
• Maximum working voltage: This is the DC steady-state voltage at which the typical leakage current is lower than the value you specify.
• Surge Current: It’s the max. peak current that can be given to the device without causing any harm to the device; it’s often expressed for a given time in 'current'. The manufacturers suggest removing the system if there is an event of surge current, although the device can handle the surge current.
• Energy Absorption: The maximum amount of energy that can be dissipated during a surge by the MOV for a given peak pulse period of a particular waveform. You may evaluate this value by running all the devices with unique values inside a particular regulated circuit. In standard transient x/y, the energy is normally expressed where x is the transient rise and y is the time to reach its half-peak value.
• Response Time: It’s the time at which the varistor begins to conduct after the surge occurs, there’s no exact response time in certain cases. The standard time of response is always set as 100nS.
• Maximum AC Voltage: It’s the max. RMS line voltage that can be given to the varistor constantly, the max. RMS value should be chosen to be slightly above the actual RMS line voltage. The peak voltage of the sine wave should not overlap with the minimum varistor, if it does, it might reduce the lifetime of the components. In the product description itself, the manufacturers can define the maximum AC voltage we can supply to the system.
5. Applications of MOV
6. Advantages of MOV
7. Disadvantages of MOV
8. Suppliers of MOV
Currently, there are many MOV manufacturers in the industry, like Vishay, Littelfuse, Panasonic, TDK/EPCOS, UF Capacitors.
Simply send RFQ to inquiry@ufcapacitors.com for quote.
There will be experienced engineers do cross reference and quote for you.