Varistors (variable resistors) are voltage-dependent resistors with a symmetrical V/I characteristic curve which resistance decreases with increasing voltage. Connected in parallel with the electronic device or circuit that is to be guarded, they form a low-resistance shunt when voltage increases and thus prevent any further rise in the surge overvoltage. The most common type of varistor is the metal-oxide varistor (MOV).

Sintering Zinc oxide together with other metal oxide additives under specific conditions produces a polycristalline ceramic whose resistance exhibits a pronounced dependence on voltage. This phenomenon is called the varistor effect. The figure shows the conduction mechanism in a varistor element in simplified form. The zinc oxide grains themselves are highly conductive, while the intergranular boundary formed of other oxide is highly resistive. Only at those points where zinc oxide grains meet does sintering produce "microvaristors", comparable to symmetrical zener diodes (protection level approx. 3.5V). The electrical behaviour of the metal oxide varistor, as indicated by figure, results from the number of microvaristors connected.
The series and parallel connection of the individual microvaristors in the sintered body of a SIOV also explains its high electrical load capacity compared to semiconductors. While the power in semiconductors is dissipated almost entirely in one thin p-n junction area, in a SIOV it is distributed all over the microvaristors, i.e. uniformly throughout the component's volume. Each microvaristor is provided with energy absorbers in the form of zinc oxide grains with optimum thermal contact. This permits high absorption of energy and thus exceptionally high surge current handling capability.
TDK-EPCOS varistors a re wildely applied in industrial equipment and home plliance:

• For protection of semiconductor devices: thyristors, bidirectional thyristors, transistors, diodes, stabilitrons;
• For input protection;
• For protection of electromagnetic splashing in power inductive elements;

Typical value of varistor's operation time at voltage overload is up to 25 ns. But for protection of some equipment types this value is unsufficient (for electrostatic protection the operating time must be no more than 1 ns). In such a way the improving of varistors production technology is directed on increasing of their response rete. For example, thanks to wide application of varistors wth multilayer structure( SIOV-CN and SMD-performance) “TDK-Epcos” created components with operating time less than 0,5 ns.In disk varistors the operating time is increased up to a few nanoseconds. .
Such important characteristics as high reliability, perfect electrical features in a wide temperature range and small sizes are the main advantages of Epcos varistors.

EPCOS Leaded varistors are applied in devices requiring large energy dissipation. They are been produced with a compact case covered by epoxy resin. Leads are made of tinned copper wire that provides qualitative and easy mounting at the Circuit Board.

Fields of application:

• Industrial equipment;
• Power supplies;
• Fotoelectrical devices;
• Home electronics;
• Telecommunications;
• Inverters

Series of leaded varistors created specially for application in varistors created especially for application in automobile electronics. They are suitable for operating at voltage 16…48 V and working temperature up to plus125°C.

Fields of application:

• autoelectronics;
• motor run

Series of varistors designed especially for automated line using SMT-technology. Electroctechnical characteristics are equivalent of EPCOS SIOV S05-S07.

Fields of application:

• power supplies;
• fotoelectrical devices;
• home appliance;
• mobile devices;
• telecommunications.

Varistors in hermetic package with screw terminals. Such construction makes the mounting easier.

Fields of application:

• industrial equipment;
• power supplies;
• fotoelectrical devices;
• wind machines;
• inverters.

EPCOS disk varistors inspecial plastic case. They unite all advantages of TDK-EPCOS leaded varistors and also provide a perfect protection of circuits due to a fuze.

Fields of application:

• Industrial equipment;
• POwer supplies;
• Power meters;
• Inverters.

SIOV metal oxide varistors in the E series are designed to be used as active elements in gapless surge arresters for protection of medium and high voltage AC power utility distribution systems against overvoltages. Glass collar passivation makes this series suitable for a broad range of arrester designs such as porcelain housed arresters, or polymer housed arresters with a hollow insulator as well as for molded polymer arresters. The broad range of diameters supports the different class requirements according IEC and ANSI.

Fields of application:

• Arrester blocks for distribution class
• Arrester blocks for station class

This series of EPCOS is created for application in circuits with large current capability. They withstand maximal discharging current from 40 кА ( LS40) to 75 кА ( LS50). features of construction: Bolt-holed strap terminals for screw fixing or soldering, Bent or straight strap terminals.

fields of application:

• Power supplies;
• Fotoelectronic converters;
• Wind-powered generator;
• The protection form voltage overload.