In this article we will cover Metal Oxide Varistor its specifications, graph, connections and applications. A Varistor is an electronic component that protects electronics devices from over voltage by the concept of voltage clamping. The word varistor can be stated as variable resistor or a voltage dependent resistor. Its resistance cannot be varied manually like a variable resistor. Varistor changes its resistance automatically with the change in voltage across its terminals.
Physically a varistor looks like a capacitor and is very important part of any electrical or electronic circuit, mostly power supplies.
- Metal oxide varistor is a two terminal semi-conductor device.
- It protect the electrical and electronic devices from voltage transients / surge transients.
Here is a graph as example for Surge Voltage transient, that shows one cycle wave of RMS 230V.
Now what happens is for very short period of time voltage rating goes in kilovolts ( KV ) and causes harm to the devices connected.
This sudden spike in voltage is known as Surge voltage transient.
In this case to protect the devices, we use MOV ( Metal Oxide Varistor ).
Whenever MOV detects any sudden spike in voltage ( may be in Kilovolts for very small instant of time ) it clips or cuts it to the safer range.
Specifications of MOV
- In normal condition, no current flows through varistor.
- It can handle very high current during surge transients.
- It is very fast
- It is of low cost and available in many voltage ratings.
VI Graph for Metal Oxide Varistor
Here Y-axis indicates current and the X-axis indicates voltage.
The graph clearly shows that upto certain voltage, the current remains constant but at the rated voltage current starts increasing and voltage goes constant.
Connections OF MOV
1. For household wiring, we use it between line and neutral.
2. When wiring with earthing, we use it between line ( L ) and neutral (N ), neutral ( N ) and earthing ( E ) and also between line ( L ) and earthing ( E ).
3. In case of three phase industrial supply, we use it between first phase ( R ) and earthing, second phase ( Y ) and earthing and also between third phase ( B )and earthing ( E ).
Now let me make you understand in detail using single phase circuit.
In normal condition, line ( L ) and neutral ( N ) will fetch electricity to device.
But whenever MOV will detect surge voltage transient, its resistance becomes zero ( shorts between line and neutral ) and very high current flows through MOV and blows off the fuse.
This protects the device from getting harmed by surge voltage transient.
( Fuse blows only when surge voltage transient exist for long time or when it happens again and again )
Application of varistor
The applications of varistor are –
- It protects devices and circuits from over voltage.
- Semi conductor components like transistors in electronic circuit are super sensitive to voltages. MOV is used in such cases for protection.
- Surge protection in AC and DC motors.
Following are the disadvantages of MOV
- It cannot protect devices from current surges when they start.
- It cannot provide protection from short circuit current.
Frequently asked questions
- Do metal oxide varistor have polarity ?
- In a Metal Oxide Varistor a zinc layer is sandwiched between two metal electrodes. So they have no polarity.
- What does Varistor stands for ?
- Varistor is a combination of words Variable resistor.
- Can I adjust varistors resistance ?
- No, Varistors resistance cannot be adjusted manually. It automatically adjust its resistance according to change in voltage across its terminals.
- Where do we use Metal Oxide Varistor ?
- A Metal Oxide Varistor is used in the first step of making power supply circuits or directly between the terminals of electrical connections. As shown above in diagrams.
- What is surge Voltage ?
- A sudden spike in voltage for a very short period of time is called surge voltage. It may can happen due to faults or lightening. It is similar to the concept of impulse.
- Do MOV conducts when there is no surge transient ?
- It does not conduct current when there is no surge transient but a small current called leakage current always flows through it. At max leakage current can rise up to 200 µA, that is very small ( negligible ).