Metal Oxide Varistor
- metal oxide varistors, MOV, are widely used for protection on electronic circuits as they are able to provide protection from voltage spikes and surges.
Varistors are electronic components for which the resistivity changes with the applied voltage. The most common type of varistor uses a metal oxide and hence they are often known as metal oxide varistors, MOV.
As a result of their voltage dependent nature, these devices are also known as voltage dependent resistors.
The name varistor, comes from the fact that they are 'variable-resistors', the name, varistor being a contraction of the two words.
The metal oxide varistor, MOV, has a characteristic where for low voltages it has a high electrical resistance, and for higher voltages the resistance falls. Unlike diodes, the metal oxide varistor exhibits in both polarities
There are two main types of varistor:
- Ceramic / metal oxide varistor: When the term varistor is used, this is generally what is being referred to. The varistor is bidirectional and is based around a ceramic or metal oxide.
- Diode varistor: This type of structure utilises a diode whose turn on characteristic provides the variable resistance. If only a single diode is used, then it only acts in one direction, but obviously back to back diodes can be used to provide bidirectional variable resistance properties. Normally when diodes are used for protection they are not normally referred to as varistors, although this may sometimes be the case.
Metal oxide varistor basics
The metal oxide varistor, MOV, is the most widely used for of varistor. The metal oxide varistor is generally manufactured from a material such as zinc oxide, ZnO, although silicon carbide, SiC provides similar properties.
In manufacture, the ceramic powder, ZnO or SiC is compressed, typically into a disc shape, and then sintered at a high temperature, often around 1200°C. Electrodes / connections and leads are added, and then the device is encapsulated.
Varistors are available in many formats: disc format, axially leaded devices; blocks with screw terminals (for high power devices); radial leaded devices.
The characteristic of a metal oxide varistor or a silicon carbide varistor can be expressed in the format:
I = current through the device
k = a constant for the component
V = applied voltage
n = value for the device style
Typically for silicon carbide the value of n is between about 3 - 7, but for zinc oxide device the value can be in the region of 20 - 50 making the characteristic much sharper.
The metal oxide varistor, MOV, and silicon carbide varistor operate because the grain boundaries between the grains of the material act as small PN junctions. The whole component acts as if it is a large mass of small diodes in series and parallel. When a low voltage is applied, very little current flows because the junctions are reverse biased and the only current is the leakage current. When a surge appears across the device that exceeds the clamping voltage, the diodes experience avalanche breakdown and a large current is able to flow through the device.
Varistors are used in many areas, typically for surge protection in many areas where they are placed across the lines to be protected, or down to ground from the line. Under normal circumstances they draw little current, but when a surge comes, the voltage rises to above the knee or clamping voltage and they draw current, thereby dissipating the surge and protecting the equipment. The actual surge is part absorbed by the varistor and part conducted away.
Varistors are only suitable for short duration pulses, and they are not suitable to handle sustained surges. Exceeding the rated period or voltage can cause the devices to burn out or in extreme cases when the energy they are required to dissipate is much too high they can explode. It is therefore important to operate them within their ratings.
A further point to watch is that metal oxide varistors, MOVs, that are exposed to repeated surges can change their properties slightly and degrade. After they experience surges the clamping voltage moves a little lower and eventually this can lead to their destruction.
As a result of this failure mode, MOVs are often connected in series with a thermal switch / fuse that will activate if too much current is drawn.
When choosing a varistor for a given application there are a number of parameters that need to be considered. Some of the key varistor specifications are listed below:
- Clamping voltage: This is the voltage at which the varistor starts to show significant conduction.
- Peak current: This si the maximum current that the device can handle. It may be expressed as a current for a given time.
- Maximum pulse energy: This is the maximum energy of a pulse, expressed in Joules that the device can dissipate. The energy rating for the varistor is often defined using standardised transients. The transient is expressed in the format x/y where x is the time for the transient rise and y is the time to reach its half peak value. Typical formats are 8/20 and 10/1000.
- Rated voltage: This voltage, either stated as AC or DC is the maximum voltage at which the device can be used. It is normally best to have a good margin between the rated voltage and the operating voltage, although this will need to be balanced against the clamping voltage and the level of protection required.
- Response time This is the time for the varistor to start conduction after the pulse is applied. In many instances this is not an issue. Typical values are sub 100nS.
- Standby current : The standby current is the level of current that is drawn by the varistor when it is operating below the clamping voltage. Normally this current will be specified at a given operating voltage across the device.
- Capacitance: The metal oxide varistor has a relatively high capacitance across the device. Although for low frequency applications, this may not be an issue, it may present problems when it is used with lines carrying data, etc. It is therefore necessary to check the value of the capacitance across the device for any circuit where this could be an issue. Typical metal oxide varistors may have capacitance levels between 100 and 1000 pF, although low capacitance versions are available.
The varistor circuit symbol is very similar to that of a thermistor, but has a U underneath to mark it out.
It can be seen that the varistor circuit symbol indicates the non-linear characteristic of the component.
Typical areas where varistors are used include:
- Surge protected power adaptors and strips
- Telephone and other communication lines
- Power supplies - typically those connected to mains power lines
- General electronics equipment protection
- Automotive electronics - car electrics are notorious for having many spikes on the power lines
- Industrial high energy AC line protection
Varistors are also used in some circumstances as microwave mixers for modulation, detection and also frequency conversion, although this is not a standard application.
By Ian Poole
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