A varistor is a semiconductor device that is used for the clipping ‘noise spikes’ off AC voltage. varistor protects the electrical and electronic device from overvoltage transients it is a two-terminal semiconductor device and its resistance is depended on its input voltage it is a passive two-terminal semiconductor device. Varistor provides overvoltage protection by the voltage clamping. The word varistor is formed by the word VARI-able resistor varistor can change its resistance value automatically with the change in voltage across it. Varistor plays an important role in protecting the electronic circuits from switching spikes and the overvoltage transients. Varistors will be connected in circuits across mains supply and it will be phase to neutral and phase to phase for the AC operation and for the Dc voltage it will be positive to negative.
A varistor is a voltage-dependent resistor the resistance of a varistor is variable and is depended on the voltage applied their resistance decreases when the voltage increases. So in case of voltage spikes their resistance drops and because of this behavior, they are suitable to protect circuits during voltage surges. Voltage surges can be caused by lightning strikes and electrostatic discharge. Varistors will protect circuitry from high energy voltage transients by rapidly changing from high standby resistance to low conducting resistance.
It is created by using crystals of silicon carbide or metal oxide which will be then pressed into a ceramic material. The electrical character of the device is depended on the temperature provided and the atmospheric conditions too for having better electrical contacts the contacts are metalized with silver and copper. Metal oxide varistor is a voltage-dependent resistor and the resistance material is a metallic oxides
Do not use varistors in places whose temperature exceeds their rated operating temperature due to the direct sunlight or heating object
It must not be used in a humid place directly exposed to the weather or stream
Varistors must not be used in places filled with dust, salt-mist or corrosive gas
Do not use solvent such as thinner and acetone which dissolves or make the exterior covering of varistors deteriorate
Do not expose varistors to intense vibrations, shocks or pressure making the exterior covering or the inside element crack
Do not apply the peak current exceeding the rated maximum energy
Varistors must not be used in circuits whose frequency exceeds 1Khz because it may damage their elements by heat generation due to dielectric loss
Do not install varistors In places nearby the flammable substances
Metal oxide varistor (MOV)
Metal oxide varistor is the most common type of varistor it contains a ceramic mass of zinc oxide grains in a matrix of other metal oxides sand-witched between two metal plates. The boundary between each grain and its neighbor forms a diode junction, which allows current to flow in one direction. The mass of randomly originated grains is electrically equivalent to a network of back to back diode pairs, each pair in parallel with many other pairs. When a small voltage is applied across the electrodes, only a tiny current flows and it is caused by the reverse leakage through the diode junctions. When a large voltage is applied the diode junction breaks down due to the combination of thermionic emission and electron tunneling and large current flow. The result of this behavior is a highly non-linear current-voltage characteristic, in which MOV has a high resistance at low voltages and a low resistance at high voltages
Silicon carbide varistor
Before the metal oxide varistor came Into market this was the most commonly used varistor these components utilize silicon carbide. They have been intensively used in high power and high voltage applications. A major disadvantage of this device is that they draw a significant standby current, therefore a series gap is required to limit the standby power consumption
The voltage-dependent resistor has a non-linear varying resistance dependent on the voltage applied. The impedance is high under nominal load conditions, but will sharply decrease to a low value, when a voltage threshold, the breakdown voltage is exceeded. They are used to protect circuits against excessive transient voltages. When the circuit is exposed to high voltage transients, the varistor will start to conduct and clamps the transient voltage to a safe level the energy of the incoming surge is partially conducted and partially absorbed, protecting the circuit. Varistors are useful for short duration protection in case of high transient voltage surges in the order of 1- 1000 microseconds they are however not suitable to handle sustained surges, if transient pulse energy is too high and significantly exceeds the absolute maximum ratings, then they can melt, burn, or explode.
The non-linear characteristics of the varistor make them ideal for use as surge protector devices. Sources of high voltage transients can be lightning strikes, electrostatic discharges, or inductive discharges. They are often used in surge protector power strips. Varistors have a wide variety of applications.
Telephones and other communication line protecting
Surge protector power strips
Power supply protection
Electronics equipment protection
Low voltage board-level protection
Car electronics protection
Industrial high energy AC protection
A transient voltage surge suppressor