TRANSIENT VOLTAGE SUPPRESSOR-VARISTOR EXPLAINED

TRANSIENT VOLTAGE SUPPRESSOR- VARISTOR'S IN PRACTICAL CIRCUITS

A transient is a change in the steady-state condition of voltage, current, or both. if its for short duration then it is refereed as spikes or transient voltages. if the time duration is longer then it is called surge voltages.

In order to suppress this transient voltage changes some components are used in the electrical/electronic  industry . some of them are .Varistor,TVS diodes ,GDT,Spark gaps etc.

Here in this post , the Varistor is explained.

VARISTOR

Varistor's are voltage dependent, non-linear devices that behave electrically similar to back-to-back Zener diodes.When exposed to high voltage transients, the varistor impedance changes many orders of magnitude—from a near open-circuit to a highly conductive level and thereby clamping the transient voltage to a safe level.

SCHEMATIC SYMBOL

Varistor is connected in parallel with the circuit or device which is to protected from transient voltages. when the voltages raises above its breakdown voltage , the resistance of the varistor will reduces and there by clamping the voltage to a safer level by shunts down the current. The power hence generated will be dissipated by heat.Thus the potentially destructive energy of the incoming transient pulse is absorbed by the varistor, thereby protecting vulnerable circuit components.

Compared with the other clamping and crowbar technologies that are used for voltage

suppression, varistor technology is still one of the most cost-effective ways to protect against high energy surges on these 12VDC–96VDC lines.

Varistor's can be connected in parallel if the power requirement is high and it can be connected in series if there is higher voltage handling capability required. 

VARISTOR IN CIRCUIT BOARD

For selecting the MOV , the rating should be selected in such way that the tolerance is more than 20 percent of the required voltage level .

ie :- I/P voltage * 1.2

The disadvantage of varistor's is , it can't handle transient currents and 

will be destroyed in too many surges.

Ratings of MOV:

●Energy measured in Joules

●Breakdown/Clamping Voltage

●Capacitance Response time

Unlike GDT it wont get short circuited in tripped conditions, but instead it reduces its impedance. MOVs are very fast, reacting in the nanosecond range. Imagine GDT using in the INPUT of power supply ! it causes short circuit in the Input during transients which will lead to more severe damages.damages has higher load capacitance which will causes problems in high frequency circuits 

On power lines, a Metal Oxide Varistor is usually the best type of suppression device. TVS Diodes and Gas Discharge Tubes are also used occasionally.