MOV Metal oxide Varistor

Varistor Introduction

The varistor is a passive two-terminal solid state semiconductor device that is used to provide protection to electrical and electronic circuits. Unlike the fuse or circuit breaker which offers over-current protection, the varistor provides over-voltage protection by means of voltage-clamping in a similar way to the zener diode.

The word “Varistor” is an acronym, and is a combination of the words VARI-able resi-STOR used to describe their mode of operation way back in their early days of development which is a bit misleading since a varistor can not be manually varied like a potentiometer or rheostat.

A Varistor

But unlike a variable resistor whose resistance value can be manually varied between its minimum and maximum values, the varistor changes its resistance value automatically with the change in voltage across it making it a voltage-dependant, nonlinear resistor or VDR for short.

Nowadays the resistive body of a varistor is made from

Semiconductor Principles and ApplicationsList Price: $114.95Current Price: $17.99" href=" ">Semiconductor Material making it a type of semiconductor resistor with a non-ohmic symmetrical voltage and current characteristics suitable for both AC and DC voltage applications.

In many ways the varistor looks similar in size and design to a capacitor and is often confused as being one. However, a capacitor cannot suppress voltage surges in the same way a varistor can. When a high voltage surge is applied to a circuit, the outcome is usually catastrophic to the circuit, therefore the varistor plays an important role in the protection of delicate electronic circuits from switching spikes and over voltage transients.

Transient surges originate from a variety of electrical circuits and sources regardless of whether they operate from an AC or DC supply as they are often generated within the circuit itself or transmitted into the circuit from external sources. Transients within a circuit can rise rapidly increasing the voltage to several thousand volts, and it is these voltage spikes which must be prevented from appearing across delicate electronic circuits and components.

One of the most common sources of voltage transients is the L(di/dt) effect caused by the switching of inductive coils and transformer magnetizing currents, DC motor switching applications and surges from the switching-on of fluorescent lighting circuits or other supply surges.

AC Waveform Transients

Varistors are connected in circuits across a mains supply either phase-to-neutral, phase-to-phase for AC operation, or positive-to-negative for DC operation and have a voltage rating to suit their application. A varistor can also be used for DC voltage stabilization and especially for electronic circuit protection against over voltage pulses.

Varistor Static Resistance

Under normal operation the varistor has a very high resistance, hence part of its name, operating in a similar way to the zener diode by allowing lower threshold voltages to pass unaffected.

However, when the voltage across the varistor (either polarity) exceeds the varistors rated value, its effective resistance decreases strongly with an increasing voltage as shown.

We know from Ohms Law that the current-voltage (I-V) characteristics of a fixed resistor is a straight line provided that R is kept constant. Then the current is directly proportional to the potential difference across the ends of the resistor.