Varistor

Varistor – Symbol, Working, Types and Applications The small package of resistance, the resistor is used in so many circuits, in so many forms, that it is almost an omnipresent electric component. From the very basic fixed resistors, where the resistance remains unchanged, to different types of variable resistors, whose resistances change according to different factors. The variable resistors are of different types; there are ones in which the effective length of the resistive strip plays role in changing the resistors like the potentiometers and rheostats and then there are other set of variable resistors where a manual change in resistance is not possible, rather they are sensitive to physical factors such as temperature, voltage, magnetic field etc. We have already discussed the variable resistor where the resistance can be changed manually (like potentiometer and rheostats) in our previous articles. This article will guide you through the world of a resistor that is dependent on voltage, known as the Varistors. What is a Varistor? A Varistor is a varying resistor whose resistance depends on the applied voltage. The name has been coined by linguistic blend of words; “varying” and “resistor”. They are also known by the name VDR [voltage dependent resistor] and have non-ohmic characteristics. Therefore, they come under non linear type of resistors. Unlike the potentiometers and rheostats, where the resistance changes from a minimum value to a maximum value, here in Varistor the resistance changes automatically with a change in applied voltage. This Varistor has two semiconductor elements, and provides over voltage protection in a circuit, similar to a Zener diode. So how does the change in applied voltage change its resistance? Well, the answer lies in its composition. Since it is made of semiconductor material, its resistance dips as the voltage across it increases. When there is an excessive increase in voltage, the resistance across it decreases manifold. This behaviour makes them a good choice for over-voltage protection in sensitive circuits.   Image Credit A real life Varistor are shown in the above figure. You may confuse them with capacitors. However Varistors and capacitors have nothing more in common than their size and design. A Varistor is used to suppress voltage, whereas a capacitor cannot perform such functions. Varistor Symbol In its early days, the Varistor was represented as two diodes placed anti-parallel to each other, as shown in figure, due to its diode like behaviour in both directions of current flow. However, now that symbol is used for a DIAC. In modern circuits, the symbol for Varistor is shown below.       You may wonder on how es a Varistor helps in suppressing the voltage transients in a circuit? To understand this, let us first understand what the source of voltage transient is. The origin of voltage Transient in electrical circuits and sources are regardless of whether they operate from an AC or DC source, since their origin is from the circuit itself or they are transmitted from any external sources. These transients result in an increase in voltage to several thousand volts, which can prove to catastrophic for the circuit. Hence these voltage transients need to be suppressed. The L(di/dt) effect which is caused by the switching of inductive coils, transformer magnetizing currents and other DC motor switching applications, is the most common source of voltage transient. The figure below shows the waveform of an AC transient.   The connection os a Varistor in a circuit can be done as follows: In AC circuits: Phase to neutral or Phase to Phase In DC circuits: Positive to negative terminal. Now what about the resistance offered...
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