Varistors - Fundamentals And Theory

Varistors: Ideal solution to surge protection

Electromagnetical Compatibility Michal Jankow Wroclaw University of Technology

Outline Varistor The Fundamentals & Theory Metal Oxide Varistor (MOV) Multi Layer Varistor (MLV) Application examples Manufacture’s technical specification References

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

Outline

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

Outline

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

Outline

Technical Specification

Varistors: Ideal Solutions to Surge Protection Electromagnetic Compatilibity Michal Jankow

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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• Littelfuse www.littelfuse.com/company/news-item.html?News