Varistor is an ideal protective element with the characteristics of high price, small volume, wide working voltage range, fast response to overvoltage pulse, strong resistance to impulse current, low leakage current (less than a few microamperes to tens of microamperes), small resistance temperature coefficient and so on.
How to design a varistor for a given nominal voltage?
-cm. Designing a varistor for a given nominal varistor voltage, VN , is basically a matter of selecting the device thickness such that the appropriate number of grains, n, are in series between electrodes. In practice, the varistor material is characterized by a voltage gradient measured across its thickness by a specific volts/mm value.
Why choose a varistor for transient suppression?
The symmetrical, sharp breakdown characteristics shown in Figure 1 enable the varistor to provide excellent transient suppression performance. When exposed to high voltage transients the varistor impedance changes many orders of magnitude from a near open circuit to a highly conductive level, thus clamping the transient voltage to a safe level.
What is the maximum pulse current value for a varistor?
It is the maximum pulse current value when the varistor voltage does not exceed ±10% for the specified impulse current waveform and the specified impulse current number at the ambient temperature of 25 ℃.
Functions, characteristics of Varistor – a transient voltage protection component. Varistor are voltage dependent, nonlinear device which have an electrical behavior similar to back-to-back zener diodes. When a transient occurs, the varistor resistance changes from a very high stand-by value to a very low conducting value.
How does a varistor protect the circuit from damage?
The varistors protect the circuit by allowing excess current to flow through them and prevent the circuit from getting damaged.The current- voltage characteristics of varistor also possess nonlinear characteristics. You must be thinking that if varistor shows variable resistance, then it must resemble potentiometer and rheostat.
What is the equivalent circuit model for a metal oxide 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.
What are the Common Grains used in metal oxide varistors?
In the case of metal oxide varistors, grains of zinc oxide or oxides of other metals are used. Generally, 90% of the grains are of zinc oxide, and 10% of grains are of other metals like bismuth, cobalt, manganese.
