Varistores

Introduction
Vishay BCcomponents

Varistors
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 suppresspositive 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 andparallel connections determine the voltage rating and the current capability of the varistor.

FEATURES
• Wide voltage range selection - from 14 VRMS to 680 VRMS. 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. • Lowstand-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 thevaristors 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 withIEC 60068-2-45.

VARISTOR 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 theManufacturing 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.

Manufacturing process flow chart

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Document Number: 29079 Revision: 17-Sep-09

Introduction
Varistors

Vishay BCcomponents

FIRING
The pressedproducts 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.

QUALITY APPROVALS
• UL 1449 ed. 3 according file E332800 • VDE following IEC 61051-1/2 according file 40002622 or 40013495 • CSA file 219883 and cUL accoring fileE332800 The term ‘QUALITY ASSESSMENT’ is defined as the continuous surveillance by the manufacturer of a product to ensure that it conforms to the requirements to which it was made.

METALLIZATION
The fired ceramic discs are metallized on both sides with a silver content layer to produce good low resisitive electrical contacts. Metallization is achieved by screen printing. Visual checks are...