Varistors - Fundamentals and Theory

Varistors: Ideal solution to surge protection

Electromagnetical Compatibility

Michal JankowWroclaw University of Technology

Outline

Varistor The Fundamentals & TheoryMetal Oxide Varistor (MOV)Multi Layer Varistor (MLV)Application examplesManufacture’s technical specificationReferences

Varistor

A varistor is :a variable resistor – a voltage dependent, non-linear device – whose resistance decreases as the voltage applied across the device or terminals increases.

Varistor

Or it can also be described as a device that:

• At low voltages has an extremely high resistance

• At high voltages has a very low resistance

Overvoltage Protection Theory

• Under normal operating conditions, the overvoltage device appears as a high impedance device.

• Under overvoltage operating conditions, the overvoltage device appears as a low impedance device

Varistor – V-I characteristic

The V-I characteristic curve enable varistors to provide fast transient voltage suppression.

I = KVα

V-I characteristic

What varistors don’t do

Varistors provides no equipment protection from:

Inrush current (during equipment start-up)

What varistors don’t do

Varistors provides no equipment protection from:

Inrush current (during equipment startup),

What varistors don’t do

Varistors provides no equipment protection from:

Inrush current (during equipment start-up)

Voltage sags

(also know as a dropout, brownout,

blackout)

What varistors don’t do

Varistors provides no equipment protection from:

Voltage sags (also know as a brownout),

What varistors don’t do

Varistors provides no equipment protection from:

Inrush current

Voltage sags

Overcurrent (created by a short circuit)

Varistors neither senses nor controls such

events

MOV – Metal Oxide Varistor

• MOVs are made of: ZnO, Sik

• Grains form “p-n diode junctions” that allow current to flow in one direction

• Diode junctions they perform like opposing diodes in series.

MLV – Multi Layer Varistor

Application

MOVs: Power Supply

Protection

Application

MOVs: Power Supply

Protection Line Voltage

Protection

Application

MOVs: Power Supply

Protection Line Voltage

Protection Appliacne

Protection

Application

MOVs: Power Supply

Protection Line Voltage

Protection Appliacne

Protection Security System

Protection

Application

MOVs: Power Supply

Protection Line Voltage

Protection Appliacne

Protection Security System

Protection Motor Protection

Application

MOVs: Power Supply

Protection Line Voltage

Protection Appliacne Protection Security System

Protection Motor Protection Data Line Protection

Application

MLVs:• Protecting board

level components from ESD damge

• MVLA Series is ideal for low frequency and signal line application

• MVLB Series is optimal for higher freq. Applications

Technical Specification

THANK YOU

Varistors: Ideal Solutions to Surge Protection

Electromagnetic CompatilibityMichal Jankow

Reference

• Free Encyclopedia www.wikipedia.org• Cooper Bussmann www.cooperbussmann.com• Tyco Electronics www.circuitprotection.com• Google Search Engine www.google.com• Digi-Key Corporation

www.parts.digikey.com/1/parts-kws/epcos-inc-va-tvs-varistors-movs

• Littelfusewww.littelfuse.com/company/news-item.html?NewsID=566