What is Varistor
Varistor
are voltage dependent, nonlinear device which have an electrical behavior
similar to back-to-back zener diodes.
Fig: A Varistor
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.
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| Fig: Varistor Sysmbol |
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
Fig: Varistor
equivalent circuit model
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
Fig: Varistor characteristics curve
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.
Fig: Varistor current voltage characteristics
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