A varistor is a type of resistor in which its resistance is dependent on the voltage applied. The word is composed of parts of the words “VARiable resISTOR” i.e their resistance varies. The resistance of the varistor decreases when the voltage increases. They are made of semiconductors elements. It can also be called voltage dependent resistor (VDR). The most common type of VDR is the metal oxide varistor (MOV). it has a nonlinear, non-ohmic current–voltage characteristic which is similar to that of a diode .
Types Of Varistors
The most important types of varistors are:
- Metal oxide varistor (MOV)– It is a nonlinear transient suppressor composed of a ceramic mass of zinc oxide (ZnO) oxide grains, in a matrix of other metal oxides (like small amounts of bismuth, cobalt, manganese) sandwiched between two metal plates (the electrodes).
- Silicon carbide varistor – This type of varistor were commonly used before the MOV come into the market. This component uses silicon carbide (SiC). They have been intensively used in high voltage, high voltage applications. One of the disadvantages of this component is that it draws a significant standby current.
Varistor comes in different shapes/packages they are:
- Disc shaped varistor- It is circular in shape.
- Axial leaded varistor- It is spherical.
- Radial leaded varistor- It has a square shape.
- Block shaped varistor- It is the largest of the aforementioned and rectangular. It is used for higher power rating.
Applications And Uses Of Varistors
- They are used as surge protector devices or power strips.
- They are used to protect communication lines. Varistor used for this purpose have low capacitance.
- They are also used for wide variety of applications which include:
- Low voltage board level protection
- Power supply protection
- Radio communication equipment transient suppression
- Cable TV system surge protectors
- Microprocessor protection
- Transient voltage surge suppressor (TVSS)
- Car electronics protection
- Industrial high energy AC protection
- Electronics equipment protection