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.
MOV Specifications
- 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.
- Capacitance
- 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)
- 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).
MOV Performance
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.
23 Comments
dear sir,
can i used mov against Surge protection device(SPD type 2) in solar dc distribution box ? if yes, how i can calculate ratings of MOV for DC side and in which manner i should connect it?
What is functional difference between diode and MOV ?
I have an industrial battery charger, high frequency, high efficiency, 14Amp 480 Volt input, output rating of 24/36/48VDC 0-240Amps. Over the weekend it appears that it had blown two fuses and after I took the cover off and looked at the unit it had a failed MOV between T1 and T3(corresponding fuses on T1 and T3 popped). Wish I could post a picture of it. Simple yet interesting little devices. The manufacturer calls it a bridge MOV and they are mounted on the input side of a rectifier bridge.
what are the use of EMI indutor in the power supply design?
Can I use 2- MOVs in series when the required voltage rating is not available? Eg: For 460v, 2Nos of 230v-varisters in series.
which one is the ideal place in a power supply to connect the MOV for surge protection and why?
Is it on the line and neutral after the fuse before the bridge rectifier or after the EMI magnetics or after the bridge rectifier ?
What is the basic difference between MOV & TVS (Transient voltage suppressor) diode?
I have been approached by a company to do supply side KWH reduction through MOV’s and their proprietary components which is designed to clean up dirty power and reduce kilowatt consumption up to 20%. The claim is that there is no maintenance required with 15 year KWH reduction model promoted. I have 2 questions
1) If MOV’s are used, would there not have to be some component replacement over time?
2) Is supply side kwh reduction legitimate and could you identify companies successfully implementing this approach to power management?
What is difference between MOV and RC snubber functioning?
What will happen is MOV to be replace by RC snubber?
Whats the difference between Metal oxide Varistor & Metal Oxide Lightening Arrestor?
Whats the difference between Metal oxide Varistor & Metal Oxide Lightening Arrestor?
Dear S R Das,
Metal Oxide Varistors (MOV) used in Surge protection device’s (SPD’s) whichever protecting the internal equipments from Transient over voltage/Current. Transient over voltage/Current is generating from lightning.
SPD’s are protecting the equipment from Indirect Risk.
Metal Oxide Lightening Arrestor are used to protect the equipment from Direct Risk. ( What are the equipments are installed outside the structures, like Substation, switchyards, transformers, etc…)
very useful topic for the electronic system designers. thanks.
Excellent piece of information – it is important to study varistors as many are used in SMPS circuits and are usually found faulty in power supplies.
I think the admin of this website is genuinely working hard for his web site, because here every stuff is quality based
information.
When you use MOVs for surge suppression, best connect them in series with a thermal fuse, otherwise a fire hazard may occur.
One has to connect fuse before MOV, which will protect the equipment fast.
sir I use 14d471k move in place of 14d511k