What is Varistor-
Varistor are voltage dependent, nonlinear device which have an electrical behavior similar to back-to-back zener diodes.
How varistor suppress transient voltage-
When a transient occurs, the varistor resistance changes from a very high stand-by value to a very low conducting value. The transient is thus absorbed and clamped to a safe level, protecting sensitive circuit components.
When triggered, they will shunt the current created by the high voltage away from sensitive components. A varistor is also known as Voltage Dependent Resistor or VDR. A varistor’s function is to conduct significantly increased current when voltage is excessive.
Only non-ohmic variable resistors are usually called varistors. Other, ohmic types of variable resistor include the potentiometer and the rheostat.
The energy is stored within the varistor as it has some capacitance characteristics, and dissipated gradually.
Equivalent circuit model of Varistor-
A simple equivalent circuit representing a metal oxide varistor as a capacitance in parallel with a voltage dependent resistor is shown in the Equivalent circuit model drawing. Cp and Rp are the capacitance and resistance of the intergranular layer respectively; Rg is the ZnO grain resistance. For low values of applied voltages, Rp behaves as an ohmic loss.
Characteristics curve of varistor:
- A Normal working zone: current is kept as low as possible in order to have low dissipation during
continuous operation (between 10 μA to 300 μA). - B Maximum clamping voltage: the maximum voltage for a given (class) current (peak current based upon
statistical probability determined by standardization authorities). - C Maximum withstanding surge current: the maximum peak current that the varistor can withstand (only)
once in its lifetime.
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