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Handling over-voltage with metal oxide varistors

02 Aug 2013  | Steve Taranovich

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In this article, Steve Taranovich discusses the dangers of over-voltage caused by lightning and the important role of metal oxide varistors in preventing the damage it can do to sensitive electronics.

I love thunderstorms! For some reason, the sound of the heavy rain on the roof almost lulls me to sleep. The sound of distant thunder, caused by occasional flashes of lightning, to me is comforting in a strange way.

As an electronics engineer, I know full well, the devastating damage that a million volt lightning strike can bring down injury or death upon a person, fire and damage upon a dwelling and IC damage on anything electronic. The interruption of power, phone and cable services are inconvenient. The loss of data in my PC can be irreplaceable if I didn't back up that day.

The lightning need not strike that close to do damage to sensitive electronics, a few miles away can still cause local damage. A good circuit designer will implement some sort of shunt device that will be able to clamp a high voltage transient on a power line or phone or cable line. Metal oxide varistors (MOV) are usually the element of choice.

Why and when to use a MOV
The function of the transient suppressor is to limit the maximum instantaneous voltage that can develop across the protected loads in one way or another.

When it's used to protect sensitive circuits, the length of time a transient suppressor requires to begin functioning is extremely important. If the suppressor is slow acting and a fast-rising transient spike appears on the system, the voltage across the protected load can rise to damaging levels before suppression kicks in. On power lines, a Metal Oxide Varistor is usually the best type of suppression device.

Varistors are voltage dependent, non-linear devices that behave electrically similar to back-to back Zener diodes. The symmetrical, sharp breakdown characteristics shown here enable the varistor to provide excellent transient suppression performance.

varistor V-I characteristic

Figure 1: The varistor V-I characteristic displays sharp breakdown characteristics.

When exposed to high voltage transients, the varistor impedance changes many orders of magnitude thereby clamping the transient voltage to a safe level. The potentially destructive energy of the incoming transient pulse is absorbed by the varistor, thereby protecting vulnerable circuit components.

Faster than a speeding bullet
The varistor action depends on a conduction mechanism similar to that of other semiconductor devices. For this reason, conduction occurs very rapidly, with no apparent time lag – even into the nanosecond range. This graph shows a composite photograph of two voltage traces with and without a varistor inserted in a very low inductance impulse generator. The second trace (which is not synchronised with the first but merely superimposed on the oscilloscope) shows that the voltage clamping effect of the varistor occurs in less than one nanosecond.

ZnO varistor

Figure 2: The response of a ZnO varistor to a fast risetime pulse of 500ps shows why varistors are the MOV is the protection device of choice especially on power lines.

Enter the Littelfuse UltraMOV Metal Oxide Varistor Series designed for applications requiring high peak surge current ratings and high energy absorption capability.

These devices can handle a single pulse peak current (ITM), 8x20μs wave, ranging from 1,750 to 10,000 Amps.


Figure 3: Current ratings of the Littelfuse UltraMOV Metal Oxide Varistor Series.

I would want one of these protecting me in a spacecraft on my way to Mars (or even on a power line coming to my house).

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