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Resistive Products

Technical Note

Varistors Introduction

TECHNICAL NOTE

Revision: 04-Sep-13

Document Number: 29079

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GENERAL

Varistors provide reliable and economical protection against

high voltage transients and surges which may be produced,

for example, by lightning, switching or electrical noise on AC

or DC power lines. They have the advantage over transient

suppressor diodes in as much as they can absorb much

higher transient energies and can suppress positive and

negative transients.

When a transient occurs, the varistor resistance changes

from a very high stand-by value to a very low conducting

value. The transient is thus absorbed and clamped to a safe

level, protecting sensitive circuit components.

Varistors are manufactured from a non-homogeneous

material, giving a rectifying action at the contact points of

two particles. Many series and parallel connections

determine the voltage rating and the current capability of the

varistor.

Manufacturing process flow chart

FEATURES

• Wide voltage range selection - from 14 V

RMS

to 680 V

RMS

This allows easy selection of the correct component for

the specific application.

• High energy absorption capability with respect to size of

component.

• Response time of less than 20 ns, clamping the transient

the instant it occurs.

• Low stand-by power - virtually no current is used in the

stand-by condition.

• Low capacitance values, making the varistors suitable for

the protection of digital switching circuitry.

• High body insulation - an ochre coating provides

protection up to 2500 V, preventing short circuits to

adjacent components or tracks.

• Available on tape with accurately defined dimensional

tolerances, making the varistors ideal for automatic

insertion.

• Approved to UL 1449 edition 3 (file number: E332800) and

manufactured using UL approved flame retardant

materials.

• Completely non flammable, in accordance with IEC, even

under severe loading conditions.

• Non porous lacquer making the varistors safe for use in

humid or toxic environments. The lacquer is also resistant

to cleaning solvents in accordance with IEC 60068-2-45.

VARISTORS MANUFACTURING PROCESS

In order to guarantee top performance and maximum

reliability, close in-line control is maintained over the

automated manufacturing techniques. The manufacturing

process flow chart shows each step of the manufacturing

process, clearly indicating the emphasis on in-line control.

Each major step in the manufacturing process shown in the

Manufacturing process flow chart is described in the

following sections:

MILLING AND MIXING

Incoming materials are checked, weighed, milled and mixed

for several hours to make a homogeneous mixture.

GRANULATION

A binder is added to produce larger granules for processing.

PRESSING

The surface area and thickness of the disc help to determine

the final electrical characteristics of the varistor, therefore

pressing is a very important stage in the manufacturing

process. The granulated powder is fed into dies and formed

into discs using a high speed rotary press.

Summary of content (12 pages)

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VISHAY BCCOMPONENTS www.vishay.com Resistive Products Technical Note Varistors Introduction GENERAL FEATURES Varistors provide reliable and economical protection against high voltage transients and surges which may be produced, for example, by lightning, switching or electrical noise on AC or DC power lines. They have the advantage over transient suppressor diodes in as much as they can absorb much higher transient energies and can suppress positive and negative transients.
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Technical Note www.vishay.com Vishay BCcomponents Varistors Introduction FIRING QUALITY The pressed products are first pre-fired to burn out the binder. They are then fired for a controlled period and temperature until the required electrical characteristics are obtained. Regular visual and electrical checks are made on the fired batch. APPROVALS • UL 1449 ed.
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Technical Note www.vishay.com Vishay BCcomponents Varistors Introduction DEFINITIONS In order to calculate the energy dissipated during a pulse, reference is generally made to a standardized wave of current. The wave prescribed by IEC 60 060-2 section 6 has a shape which increases from zero to a peak value in a short time, and thereafter decreases to zero either at an approximate exponential rate, or in the manner of a heavily damped sinusoidal curve.
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Technical Note www.vishay.com Vishay BCcomponents Varistors Introduction SPCIFICATION OF A VARISTOR CURVE log V max. leakage current region β = 1 = fixed resistor max. clamping voltage region up-turn region log V β = 0.4 (SiC) β = 0.03 (ZnO) + 10 % max. leakage current β = 0 = ideal varistor max.
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Technical Note www.vishay.com Vishay BCcomponents Varistors Introduction log V normal working condition (no spike) protection zone A B Given the small value of  (0.03 to 0.05), it is evident that the modification of C x I will be very small compared to the variation of R x I when VI is increased to VI + VI. A large increase of VI will induce a large increase of VR and a small increase of VO. C Examples The varistor is a typical component of the series VDR05C275 (C = 520;  = 0.
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Technical Note www.vishay.com Vishay BCcomponents Varistors Introduction Nevertheless, due to the structural characteristic of the zinc oxide varistors, the capacitance itself decreases slightly with an increase in frequency. This phenomenon is emphasized when the frequency reaches approximately 100 kHz. See the effect of HF alternating current on the varistor type VDRS14T250; C = 480 pF drawing.
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Technical Note www.vishay.com Vishay BCcomponents Varistors Introduction Examples Example of calculation of energy for a VDRS07H250 type, at the maximum peak current (33 A) for a duration t2 = 1000 μs (K = 1.4) Pulse life time rating of VDRS07H060, 60 V type. Energy capability: E = K x Vp x Ip x t2 1 pulse; 8 µs to 20 µs: 1200 A = 1 x 8 J Ipeak 10 pulses; 8 µs to 20 µs: 300 A = 10 x 1.45 J I (A) 1 pulse; 10 µs to 1000 µs: 33A = 1 x 8.3 J 10 pulses; 10 µs to 1000 µs: 11 A = 10 x 2.
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Technical Note www.vishay.com Vishay BCcomponents Varistors Introduction For suppression of internally generated spikes in electronic circuits, see Varistor used across a transistor or coil in a television circuit and Varistor used across a switch or coil drawings.
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Technical Note www.vishay.com Vishay BCcomponents Varistors Introduction SELECTION OF THE CORRECT VARISTOR TYPE In order to select a ZnO varistor for a specific application, the following points must first be considered: To ensure correct selection of varistor type, two multi choice selection charts have been prepared, see charts below. 1. The normal operating conditions of the apparatus or system, AC or DC voltage? The first chart determines the necessary steady state voltage rating (i.e.
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