Varistor


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varistor

[və′ris·tər]
(electronics)
A two-electrode semiconductor device having a voltage-dependent nonlinear resistance; its resistance drops as the applied voltage is increased. Also known as voltage-dependent resistor.

Varistor

 

a semiconducting resistor whose electrical resistance (conduction) varies nonlinearly and equally under the influence of both positive and negative voltage. Varistors are made from powdered silicon carbide (a semiconductor) and a binding substance (such as clay, water glass, lacquers, or resins), which are pressed into a mold and baked at a temperature of about 1700° C. The surface of the model is then metallized and leads are soldered to it. The change in electrical conduction of the varistor with voltage buildup across the leads is associated with complex phenomena on the contacts or on the surface of the crystals (the closing of the gaps between the grains of the semiconductor, an increase in the conduction of the surface oxide films in strong electrical fields and the breakdown of the oxides, and an increase in current through p-n junctions that form between grain). Low-voltage varistors are made with an operating voltage of 3 to 200 volts and a current of from 0.1 milliamperes to 1 ampere. High-voltage varistors are made with an operating voltage of up to 20 kilovolts. Varistors have a negative temperature coefficient of resistance. They can tolerate significant electrical overloads; are simple, cheap, and highly reliable; and have low inertia (a maximum operating frequency of up to 500 kilohertz). But they have considerable low-frequency noise and change their parameters with time and with changes in temperature. They are used to stabilize and regulate low-frequency currents and voltages, to perform mathematical operations on known quantities (such as raising to a degree and extracting a root), and to protect contacts from destruction as a result of overloading in electrical circuits (for example, in high-voltage electrical transmission lines, communication lines, and electrical appliances).

REFERENCE

Pasynkov, V. V., L. K. Chirkin, and A. D. Shinkov. Poluprovod-nikovye pribory. Moscow, 1966.

Varistor

Any two-terminal solid-state device in which the electric current I increases considerably faster than the voltage V. This nonlinear effect may occur over all, or only part, of the current-voltage characteristic. It is generally specified as IVn, where n is a number ranging from 3 to 35 depending on the type of varistor. The main use of varistors is to protect electrical and electronic equipment against high-voltage surges by shunting them to ground. See Electric protective devices

One type of varistor comprises a sintered compact of silicon carbide particles with electrical terminals at each end. It has symmetrical characteristics (the same for either polarity of voltage) with n ranging from 3 to 7. These devices are capable of application to very high power levels, for example, lightning arresters. See Lightning and surge protection

Another symmetrical device, the metal-oxide varistor, is made of a ceramiclike material comprising zinc oxide grains and a complex amorphous intergranular material. It has a high resistance (about 109 ohms) at low voltage due to the high resistance of the intergranular phase, which becomes nonlinearly conducting in its control range (100–1000 V) with n > 25.

Semiconductor rectifiers, of either the pn-junction or Schottky barrier (hot carrier) types, are commonly utilized for varistors. A single rectifier has a nonsymmetrical characteristic which makes it useful as a low-voltage varistor when biased in the low-resistance (forward) polarity, and as a high-voltage varistor when biased in the high-resistance (reverse) polarity. Symmetrical rectifier varistors are made by utilizing two rectifiers connected with opposing polarity, in parallel (illus. a) for low-voltage operation and in series (illus. b) for high-voltage use. For the high-voltage semiconductor varistor, n is approximately 35 in its control range, which can be designed to be anywhere from a few volts to several hundred. See Semiconductor rectifier

References in periodicals archive ?
After the BAE-206 circuit breaker was closed, a short circuit current flowed through a column with a experienced preliminary breakdown or shunted by a copper wire varistor with the formation of an electric arc inside the SAp section.
La figura 3 muestra la distribucion de la corriente de impulso por cada varistor, siendo IBR1, IBR2 e IBR3 las corrientes que fluyen por el varistor 1, varistor 2 y varistor 3, donde se puede apreciar que el varistor 2, cuya curva en la figura 2 es la superior y en la figura 3 es la inferior, toma menor corriente debido a su caracteristica V-I mas alta con respecto a los otros dos varistores; situacion contraria a la que ocurre con el varistor 1 cuyo comportamiento ante el impulso de corriente indica que es el que asume mayor corriente de los tres varistores conectados en paralelo, curva superior en la figura 3, y cuyo comportamiento V-I presentado en la figura 2, es el comportamiento inferior.
and Jet Propulsion Laboratory (JPL), because high output power and high efficiency could be achieved compared with the varistors. The operation frequency, output power and conversion efficiency are keep improving for the evolvement of the design method and multiplier process.
Varistores a base de oxido de zinco (ZnO) sao dispositivos ceramicos que apresentam um comportamento eletrico caracterizado pela relacao nao-linear entre a tensao aplicada e a corrente eletrica resultante, bem como elevada capacidade de absorcao de energia.(1,2) A fabricacao industrial destes dispositivos e feita a partir da mistura e homogeneizacao dos pos de ZnO e outros oxidos metalicos que atuam como dopantes, tais como [Bi.sub.2][O.sub.3], [Sb.sub.2][O.sub.3], CoO ou oxidos de terras-raras, e subsequentes etapas de conformacao e sinterizacao tipicas do processamento ceramico.
* Grupo de DPS3: formado por un varistor de oxido de zinc en encapsulado de disco de 10 mm, para funcionamiento en 220 V(ca) y de capacidad de descarga [I.sub.m] (8/20 [micron]s) = 2,5 kA, con el fin de analizar un caso en que se verifique la relacion (9) a partir de no cumplirla, es decir, de la destruccion del dispositivo.
Ti[O.sub.2] is used as pigment, photocatalyst and varistor. In the last years Ti[O.sub.2] nanoparticles have been used for the photocatalytic degradation of organic pollutants in water.
Kyocera and affiliate AVX developed the EMI filters with the varistor function based on AVX advanced material technology.
As Figure 5 illustrates (next page), energy runs up one lead of the component, dissipates through the varistor, and out the other lead using the path of least resistance.
TABLE 1 NEW MODELS SUPPLIED WITH THE ADVANCED MODELING KIT MOSFET MESFET BJT Other BSIM4 Parker-Skellern MEXTRAM Philips JUNCAP HiSIM Angelov (Chalmers) UCSD HBT Thin film transistor MOS EKV Thermal BJTT Varistor MOS-9 MOS-11 References
* MOV protection -- Metal Oxide Varistor circuit that protects the SCRs from excessive transient voltage
Determinar los valores de potencial electrico a traves de cada varistor en el apartarrayo, incluyendo las bridas metalicas.