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Metal Oxide Varistor (MOV) Overview: Working and Application

Author: Apogeeweb
Date: 16 Jan 2021
 1126
How does MOV work

Ⅰ Introduction

The blue or orange-colored circular part that you can typically spot on the AC input side of any Power Supply Circuit is a Metal Oxide Varistor or MOV. It is possible to consider the Metal Oxide Varistor as another form of variable resistor that can change its resistance depending on the voltage applied across it. As a high current travels through a MOV, it decreases its resistance value and functions as a short circuit. In order to protect circuits from high voltage spikes, MOVs are therefore usually used in combination with a fuse. In this post, we will learn more about working with MOV and how to use it to protect your circuits from voltage spikes in your designs. We will also learn about the MOV's electrical properties and how to pick a MOV according to your requirements for the design, so let's get started.

Catalog

Ⅰ Introduction

Ⅱ Definition of MOV

Ⅲ The Working of MOV

Ⅳ The Using of MOV in Circuit

Ⅴ Construction of MOV

Ⅵ Electrical Characteristics of MOV

Ⅶ How to Choose the Right MOV for Protection

Ⅷ Applications of MOV

Ⅸ Protection Circuit of MOV

Ⅹ FAQ

Ⅱ Definition of MOV

MOV is simply a variable resistor, but MOVs can adjust their resistance depending on the applied voltage, unlike 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.

MOV Definition

Ⅲ The Working of MOV

The MOV resistance would be strong under normal operating conditions and they will draw very little current, however when there is a spike in the network, the voltage will increase above the knee or clamping voltage and they will draw more current, dissipating the surge and protecting the equipment.

 

The MOVs can only be used to defend against short surges, they can not cope with prolonged surges. Their properties can slightly degrade if the MOVs are exposed to repeated surges. Whenever they encounter a surge, the clamping voltage drops slightly lower, which may also lead to their destruction after some time. MOVs are often linked in series with a thermal switch/fuse that could be triggered if a high current is drawn to prevent these types of risks. Let's talk more about how the MOV in a circuit operates.

Ⅳ The Using of MOV in Circuit

Parallel to the circuit that is to be covered, MOV a.k.a varistors are widely used along with fuse. The picture below illustrates how to use MOV in the circuit for electronics.

 

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.

Ⅴ Construction of MOV

The Metal Oxide Varistor is a voltage-dependent resistor made of metal oxides ceramic powders such as zinc oxide and some of the other metal oxides such as cobalt oxides, manganese, bismuth, etc. A MOV consists of around 90% zinc oxide and a limited quantity of other metal oxides. Between two metal plates known as the electrodes, the ceramic powders of the metal oxides are kept intact.

 

A diode junction between each immediate neighbor is created by the grains of metal oxides. So, a MOV is a large number of series-linked diodes. A reverse leakage current occurs through the junctions when you add a small voltage to the electrodes. The produced current will initially be small, but due to electron tunneling and avalanche breakdown, the diode border junctions break down when a high voltage is applied to the MOV. In the picture below, the internal structure of a MOV is shown.

MOV construction

When a particular voltage is applied through the connecting leads, the MOV varistor begins conducting and ceases conducting when the voltage falls below the threshold voltage. MOVs are available in different formats, such as disk formats, devices with axial lead, blocks and screw terminals, and devices with radial lead. For increased power handling capacity, the MOVs should always be connected in parallel and you should link it in series if you want to get a higher voltage rating.

Ⅵ Electrical Characteristics of MOV

To better understand the MOV properties, let's look at the various electrical characteristics of MOV.

Static Resistance

The MOV static resistance curve is plotted with the resistance value of the X-axis MOV and the Y-axis voltage value.

Static resistance of MOV

The above curve is a MOV's voltage and resistance curve; the resistance is at its highest at the standard voltage, but the varistor's resistance decreases as the voltage rises. This curve can be used to understand how much resistance at various voltage levels would be in your MOV.

 

V-I Characteristics 

The V-I characteristic curve of a linear resistor is always a straight line, according to Ohms law, but in terms of a variable resistor, we can not assume the same. 

VI characteristic of MOV

The MOV can work in both directions, so it has bi-directional, symmetrical characteristics. The curve would look identical to the characteristic curve of two back-to-back connected Zener diodes. The curve has a linear relationship when the MOV does not work, where the current flowing through the varistor is almost zero, with a high resistance up to a certain voltage, say 0-200Volts. The resistance decreases as we increase the applied voltage in the range of 200-250V, and the varistor starts conducting and a few current micro-amperes begin to flow, which does not make much difference in the curve.

 

The varistor becomes highly conductive once the rising voltage exceeds the rated or clamping voltage (250V), about 1mA of current begins to flow through the varistor. The resistance of the varistor becomes small when the transient voltage across the varistor is equal to or greater than the clamping voltage, which transforms it into a conductor due to the semiconductor material's avalanche effect.

 

Capacitance of MOV

As we have already recognized that the MOV is built with two electrodes, it operates as a dielectric medium and has capacitor effects that, if not taken into account, could influence the system's functioning. Depending on the region that is also inversely dependent on its thickness, each semiconductor varistor will have a capacitance value.

 

When it comes to a DC circuit, the capacitance value is not a big deal, because the capacitance will remain almost constant until the device's voltage exceeds the clamping voltage. When the voltage exceeds the clamping voltage, there will be no capacitance effect as the varistor begins its normal work.

 

The capacitance of the MOV could affect the overall body resistance of the MOV that causes the leakage current when it comes to AC circuits. The leakage resistance of the varistor decreases rapidly as the frequency increases as the varistor is connected parallel to the system to be covered. The MOV reactance value can be determined using the formula

 

Xc=1/2πfC

 

Where Xc is the capacitive reactance, and the AC supply frequency is f. The leakage current, as seen in the non-conducting leakage region of the V-I characteristics curve described above, will also increase if the frequency increases.

Ⅶ How to Choose the Right MOV for Protection

To pick the right unit for your pieces of equipment, you should know about the different number of parameters of a MOV. The MOV specification depends on the following information:

• Maximum working voltage: This is the DC steady-state voltage at which the typical leakage current is lower than the value you specify.

• Clamping voltage: It is the voltage at which the surge current begins to be conducted and dissipated by the MOV.

• Surge Current: It is the maximum peak current that can be given to the device without causing any harm to the device; it is 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.

• Surge Shift: If the system experiences a spike, the rated clamping voltage decreases, the surge shift is called the variation in the voltage after the surge.

• 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 is the time at which the varistor begins to conduct after the surge occurs, there is no exact response time in certain cases. The standard time of response is always set as 100nS.

• Maximum AC Voltage: It is the maximum RMS line voltage that can be given to the varistor constantly, the maximum 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.

• Leakage Current: When the varistor works below the clamping voltage, it is the amount of current that the varistor draws when there is no surge in the network. The leakage current will usually be defined across the system at a given operating voltage.

Ⅷ Applications of MOV

MOVs may be used to shield different kinds of equipment from various types of faults. They can be used in AC/DC electrical circuits for single-phase line-to-line protection and single-phase line-to-line & line-to-ground protection. They can be used in motor-operated devices for semiconductor switching protection in transistors, MOSFETs, or Thyristor and Contact arcing protection.

Application of MOV

The MOVs can be used in circuits anywhere there is a chance of a surge or voltage spikes when it comes to implementation. MOVs are often used in surge-protected adapters and strips, mains-connected power supplies, telephone and other contact lines, industrial high-energy AC line protection, data or power networks, general electronic device protection such as mobile phones, digital cameras, digital personal assistants, MP3 players and laptop computers.

 

In certain cases, MOVs are also used, such as microwave mixers for modulation, detection and frequency conversion, which are not the most common MOV applications.

Ⅸ Protection Circuit of MOV

Now that we've talked about what a MOV is and how it's used to protect your circuit from voltage spikes, let's conclude the article with a few design tips that will be useful when designing your circuit.

 

You have to choose the varistor with the highest AC or DC voltage that matches or slightly higher than the applied voltage. The first step in selecting a MOV is to decide the continuous working voltage that will be given through the varistor. It is normal to choose a varistor that has a maximum rated voltage 10-15 percent higher than the actual line voltage as the supply lines often have tolerance of voltage variance. In some situations, if you prefer to achieve an exceptionally low leakage current despite the lowest safety level available, you can use the varistor with a higher operating voltage. This ratio would be included in their voltage values.

 

• Find out the amount of energy that the varistor consumes in the event of a wave. This can be calculated by using all of the varistor's absolute maximum load during an environmental surge and the requirements given in the datasheet. You can choose the varistor that can dissipate more energy that is equal to or slightly greater than the energy dissipation that the circuit can generate during the surge.

• Using the varistor to measure the peak transient current or the surge current. In order to ensure that it works properly, you should pick the varistor that has the surge current rating equal to or slightly greater than the current rating required by an event that the circuit will cause.

• You can also decide the power dissipation needed and select the varistor that has a power rating equal to or preferably exceeds the power handling required by the event that the circuit can generate. Similar to all the above properties.

• If you are unsure about the factors of the case, the prudent thing to do is to choose the system with higher strength, surge current and energy ratings. The power, surge current and energy ratings are often chosen in a way that is greater than the predicted event.

• Selecting the model that can provide the necessary clamping voltage is the final and most significant step of all. Based on the estimated peak voltage value, you can choose the clamping voltage that will allow the input or output of your circuit to be seen during a case. This will be the maximum voltage your circuit down line will feel, you should make sure that your circuit will be able to withstand this voltage.

Ⅹ FAQ

1. What is a metal oxide varistor?

The Metal Oxide Varistor or MOV is a voltage-dependent, nonlinear device that provides excellent transient voltage suppression. The Metal Oxide Varistor is designed to protect various types of electronic devices and semiconductor elements from switching and induced lightning surges.

 

2. How do metal-oxide varistors (MOV) work?

They are designed to dead short themselves when the voltage exceeds a certain limit. Basically, they’re a voltage clamp. A Varistor basically acts like a 'variable resistor' depending upon the voltage level.

 

They are used in parallel with the line voltage so that they cause the fuse or circuit breaker before them to be blown/tripped. This is known as 'crowbarring' because it’s like throwing a crowbar across the hot & neutral AC line.

 

Multiple MOV’s are typically used on cheap plug strips to protect equipment from voltage spikes. As each MOV stops a spike, it is up to the next MOV to stop the next voltage spike.

 

3. What does the voltage rating on a metal oxide varistor mean?

It is the maximum working voltage in AC volts that the part is designed for. For example, a part rated at 125 or 130 volts is designed to be connected from line to neutral and or line to the ground of a 120-volt ac supply. MOVs are devices that conduct very little until you reach a voltage where they rather quickly start conducting.

 

They are symmetrical and act kind of like anti-parallel Zener diodes. Since the peak voltage of a 120 VAC sine wave is about 170 volts, the MOV rated to work across a 120 line has to have a breakdown voltage above 170 volts, with some considerable safety margin. For example, a 125 VAC MOV clamps transients at about 220 volts.

 

4. What does a metal oxide varistor do?

Varistors, 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.

 

5. What does a MOV do in an electrical circuit?

A Metal Oxide Varistor (MOV) is a voltage suppression device that filters and clamps the transient in an electrical circuit.

 

6. How do you select a metal oxide varistor?

For example, selecting a MOV or silicon varistor for that matter, for voltage, its maximum continuous RMS voltage rating should be just above the highest expected supply voltage, say 130 volts RMS for a 120 volt supply, and 260 volts RMS for a 230 volt supply.

 

7. How do you calculate MOV?

The current rating of the MOV could be twice that of the SMPS rating, meaning if the SMPS wattage is rated at 24 watts at the secondary, then the primary could be calculated as 24/285 = 0.084 amps, therefore the MOV current could be anywhere above 0.084 x 2 = 0.168 amps or 200mA.

 

8. How does a metal oxide varistor fail?

This failure mode can be caused by long-duration overvoltages, such as switching from a reactive load or thermal runaway of the MOV connected to the ac mains. Open circuit failures are possible if a MOV is operated at steady-state conditions above its voltage rating.

 

9. What is a MOV surge protector?

In the most common type of surge protector, a component called a metal oxide varistor, or MOV diverts the extra voltage. In this way, the MOV only diverts the surge current, while allowing the standard current to continue powering whatever machines are connected to the surge protector.

 

10. What happens when a varistor fails?

Varistors need to absorb the energy deposited by temporary overvoltage, switching surges, or lightning impulses. The energy absorption capability can be divided into thermal energy absorption capability and impulse energy absorption capability.

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