Metal Oxide Varistor (MOV)
Basics of Varistor
A varistor/voltage dependent resistor (VDR) is a component which has a voltage – current characteristics that is very much similar to that of a diode. This component is used to protect electrical devices from high transient voltages. They are planted in the devices in such a manner that it will short itself when a high current is produced due to the high voltage. Thus the current dependent components in the device will remain safe from the sudden surge.
I have already explained in detail the working and applications of a variable resistor [varistor]. To know more about it, check the link given below.
TAKE A LOOK : VARIABLE RESISTORS – WORKING AND APPLICATIONS
It should also be noted that VDR’s are mainly non-ohmic variable resistors. In the case of ohmic variable resistors, potentiometers and rheostat are commonly used.
To know more, check the link given below.
TAKE A LOOK : POTENTIOMETER AND RHEOSTAT – WORKING AND COMPARISON
Metal Oxide Varistor – Basics
MOV is the most commonly used type of varistor. It is called so as the component is made from a mixture of zinc oxide and other metal oxides like cobalt, manganese and so on and is kept intact between two electrodes which are basically metal plates. MOV’s are the most used component to protect heavy devices from transient voltages. A diode junction is formed between each border of the grain and its immediate neighbour. Thus an MOV is basically a huge number of diodes that are connected parallel to each other. They are designed to be in the parallel mode as it will have better energy handling ability. But, if the component is meant for providing better voltage rating, it is better to connect them in series.
A reverse leakage current appears across the diode junctions of each border when an external tiny voltage is applied across the electrodes. The current produced will also be very small. But, when a large voltage is applied across the electrodes, the diode border junction breaks down as a result of the combination of electron tunnelling and avalanche breakdown. Thus the device is said to show a high level of non-linear voltage – current characteristics. From the characteristics, it should also be noted that the component will have low amount of resistance at high voltages and high resistance at low voltages.
The only problem with this component is that they cannot withstand the transient voltage more than the exceeded rating. They tend to deteriorate after a certain level. If so, they will have to be replaced at times. When they absorb the transient voltage they tend to dissipate it as heat. When this process continues repetitively for some time, the device begins to wear out due to the excessive heat.
They can be connected in parallel for increased energy-handling capabilities. MOVs can also be connected in series to provide higher voltage ratings or to provide voltage rating between the standard increments.
- Maximum working voltage is the maximum steady-state, DC voltage. In this case, the value of the typical leakage current will be lesser than a specified value.
- Varistor voltage
- Maximum clamping voltage is obtained when a certain pulse current is applied to the component to obtain a maximum peak voltage.
- Surge current
- Surge shift refers to the variation in voltage after a surge current is given.
- Energy absorption refers to the maximum energy that is dissipated for a certain waveform without many problems.
- Leakage current
- Response time
- Maximum AC RMS voltage refers to the maximum amount of RMS voltage that can be delivered to the component.
Working of Metal Oxide Varistor (MOV)
The working of a MOV is shown in the figure above.
The resistance of the MOV is very high. First, let us consider the component to have an open-circuit as shown in figure 1(a). The component starts conducting as soon as the voltage across it reaches the threshold voltage. When it exceeds the threshold voltage, the resistance in the MOV makes a huge drop and reaches zero. This is shown in the figure 1(b). As the device has very small impedance at this time due to the heavy voltage across it, all the current will pass through the metal oxide varistor itself. The component has to be connected in parallel to the load. The maximum voltage that will pass through the load will be the sum of the voltage that appears across the wiring and disconnect given for the device. The clamp voltage across the MOV will also be added. After the transient voltage passes through the component, the MOV will again wait for the next transient voltage. This is shown in the figure 1(c).
The varistor is mainly used to perform as a line voltage surge suppressor. The device does not conduct when the voltage across it is below the clamping voltage. But, if a high surge (lighting) that is higher in rate that a varistor can handle is passed through it, the component will not perform. The resulting current will be so high that it will damage the MOV.
The performance of the varistor will slow down with time even if small surges pass through it. The life of a MOV will be explained through the manufacturers chart. The chart will have graphs and readings between the current, time and also the number of transient pulses that passes through the varistor.
Another main reason that affects the performance of a MOV is the energy rating. When there is an increase in the energy rating, there will be an exponential change in the life of the varistor. Thus, there will be a change in the transient pulses that the device can manage. This increases the clamping voltage when each transient breaks down.
The performance can be increased by connecting more varistors in parallel. An increase in rating will also help in the process.
One of the best features of the MOV is its response time. The spikes are shorted through the device within nanoseconds. But the response time can be affected by the mounting design method and inductance of component leads.