Dec. 22, 2020
A varistor is a voltage-dependent resistor(VDR). The resistance of a varistor is variable and depends on the voltage applied. The word is derived from the "variable resistor. As the voltage increases, their resistance decreases. If the voltage is too high, its resistance will drop sharply. This behavior makes them suitable for protecting circuits during surges. The causes of the surge may include lightning and electrostatic discharge. The most common type of VDR is metal oxide varistor or MOV.
The varistor is a nonlinear binary semiconductor whose resistance decreases with the increase of voltage. Voltage-dependent resistors are commonly used as surge suppressors for sensitive circuits.
A resistor having a nonlinear change, depending on the voltage applied. Under nominal load conditions, the impedance is very high, but when the voltage threshold (the breakdown voltage) is exceeded, the impedance drops sharply to a low value. They are usually used to protect circuits from excessive transient voltages. When the circuit is exposed to high voltage transients, the varistor initiates conduction and the transient voltage clamp is brought to a safe level. The energy from the incoming surge is partially transmitted and partially absorbed, thereby protecting the circuit.
The most common type is MOV or metal oxide varistor. They consist of sintered substrates of zinc oxide (ZnO) grains. Grain boundaries provide PN junction semiconductor characteristics, similar to diode junctions. The matrix of randomly oriented grains can be compared with large diode networks in series and parallel. When a low voltage is applied, the current due to the reverse leakage through the junction is very small. However, when a high voltage is applied that exceeds the breakdown voltage, an avalanche breakdown occurs and a large current flows through the junction. This behavior leads to nonlinear current-voltage characteristics.
A varistor is formed when a crystal of silicon carbide or metal oxide is pressed into a ceramic material.
The material is then dried and sintered at a high temperature. The electrical properties of the equipment depend on the temperature and atmospheric conditions provided.
For good electrical contact, the contact point of the material is metalized with silver or copper. Then lead is then welded to the contact and the varistor is supplied and coded.
The nonlinear characteristics of varistors make them ideal for use as surge protectors. The source of high voltage transients may be an electrostatic or inductive discharge from a motor or transformer, such as a lightning strike. For example, they are commonly used for surge protectors on power panels. A special type of low capacitance protects communication lines. These VDRS are available for a variety of applications, including:
Telephone and other communication line protection
Transient suppression of radio communication equipment
Surge protector power panel
Surge protector for the cable TV system
Electronic equipment protection
Low-pressure plate level protection
Transient voltage surge suppressor (TVSS)
Automobile electronic protection
Industrial high energy exchange protection
It provides excellent overpressure protection.
Because it does not show polarity effects, it is easy to achieve bidirectionally.
This is expensive equipment.
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