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----All Kinds of Capacitors

Varistor is one kinds of transient voltage suppression devices which can be used instead of transient suppression diodes, zener diode and capacitor combination. Varistor IC and other devices can protect the circuit, to prevent electrostatic discharge, surges and other transient current (such as lightning, etc.) the damage caused by them. Simply use varistors and connected to the protected circuit IC or device, when the voltage value above a certain moment, the varistor resistance decreased rapidly, conducting high current, thereby protecting the IC or electrical equipment; when the voltage is lower than the varistor voltage value, the varistor resistance high, almost open, and thus will not affect the device or the normal operation of electrical equipment.

A varistor is an electronic component with a "diode-like" nonlinear current–voltage characteristic. The name is a portmanteau of variable resistor. Varistors are often used to protect circuits against excessive transient voltages by incorporating them into the circuit in such a way that, when triggered, they will shunt the current created by the high voltage away from the sensitive components. A varistor is also known as Voltage Dependent Resistor or VDR. A varistor’s function is to conduct significantly increased current when voltage is excessive.

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A varistor is a type of resistor with a significantly non-ohmic current-voltage characteristic. The name is a portmanteau of variable resistor*, which is misleading since it is not continuously user-variable like a potentiometer or rheostat, and is not a resistor but in fact a capacitor. Varistors are often used to protect circuits against excessive voltage by acting as a spark gap.

The most common type of varistor is the metal oxide varistor, or MOV. This contains a mass of zinc oxide grains, in a matrix of other metal oxides, sandwiched between two metal plates (the electrodes). The boundary between each grain and its neighbour forms a diode junction, which allows current to flow in only one direction. The mass of randomly oriented grains is electrically equivalent to a network of back-to-back diode pairs, each pair in parallel with many other pairs. When a small or moderate voltage is applied across the electrodes, only a tiny current flows, causes by reverse leakage through the diode junctions. When a large voltage is applied, the diode junctions break down because of the avalanche effect, and a large current flows. The result of this behaviour is a highly nonlinear current-voltage characteristic, in which the MOV has a high resistance at low voltages and a low resistance at high voltages.

If the size of the transient pulse (often measured in joules) is too high, the device may melt, or otherwise be damaged. For example, a nearby lightning strike may permanently damage a varistor.

Important parameters for varistors are response time (how long it takes the varistor to break down), maximum current and a well-defined breakdown voltage. When used in communications lines (such as phone lines used for modems), high capacitance is undesirable since it absorbs high frequency signals, thereby reducing the available bandwidth of the line being protected.

Another method for suppressing voltage spikes is the transient voltage suppression diode (TVS). Although diodes do not have as much capacity to conduct large surges as Metal Oxide Varistor, diodes are not degraded by smaller surges and can be implemented with a lower "clamping voltage". Metal Oxide Varistor degrade from repeated exposure to surges and generally have a higher "clamping voltage" so that leakage does not degrade the Metal Oxide Varistor. Both types are available over a wide range of voltages. Metal Oxide Varistor tend to be more suitable for higher voltages, because they can conduct the higher associated energies at less cost.

Another type of transient suppressor is the gas tube suppressor. This is a type of spark gap that may use air or an inert gas mixture and often, a small amount of radioactive material such as Ni-63, to provide a more consistent breakdown voltage and reduce response time. Unfortunately, these devices may have higher breakdown voltages and longer response times than varistors. However, they can handle significantly higher fault currents and withstand multiple high-voltage hits (for example, from lightning) without significant degradation.

A varistor installed at an intermediate point of a power system, downstream from the service entrance, is exposed to a combination of surges: internal surges from load switching within the building, and residual surges from external lightning surges. The frequency of occurrence of these surges has been the subject of many sweys of which a recent publication is representative . In all these surveys, the steep rise in the number of occurrences of surges of lower amplitude is quite remarkable. In other words, a varistor attempting to clamp at lower surge levels will be required to divert a much greater number of surges than a varistor selected with only slightly higher voltage rating. The varistor with the lower clamping voltage will expend its puke rating faster than a varistor with higher clamping voltage.

The most common type of varistor is the metal oxide varistor, or MOV. This contains a mass of zinc oxide grains, in a matrix of other metal oxides, sandwiched between two metal plates (the electrodes). The boundary between each grain and its neighbour forms a diode junction, which allows current to flow in only one direction. The mass of randomly oriented grains is electrically equivalent to a network of back-to-back diode pairs, each pair in parallel with many other pairs. When a small or moderate voltage is applied across the electrodes, only a tiny current flows, causes by reverse leakage through the diode junctions. When a large voltage is applied, the diode junctions break down because of the avalanche effect, and a large current flows. The result of this behaviour is a highly nonlinear current-voltage characteristic, in which the MOV has a high resistance at low voltages and a low resistance at high voltages.

If the size of the transient pulse (often measured in joules) is too high, the device may melt, or otherwise be damaged. For example, a nearby lightning strike may permanently damage a varistor.

Important parameters for varistors are response time (how long it takes the varistor to break down), maximum current and a well-defined breakdown voltage. When used in communications lines (such as phone lines used for modems), high capacitance is undesirable since it absorbs high frequency signals, thereby reducing the available bandwidth of the line being protected.

This property of the metal oxide varistor makes it ideal for use in electrical surge protectors. At normal current levels, the MOV will simply pass along the electricity to the devices plugged into the surge protector. In the event of a power surge, however, the MOV will divert the current into itself, preventing it from reaching the attached devices. This will protect sensitive electronic components, such as computer chips, that can be damaged by excessively large voltages. In the event of a sudden power surge, the MOV can respond much more quickly than similar surge-protector components.

A metal oxide varistor is generally made of zinc oxide or a similar substance. These substances give the varistor its quality of passing along current at normal household levels. When electrical current rises to a certain point, called the breakover point or breakover voltage, the tiny zinc oxide particles begin conducting the current among themselves only. This prevents the high-voltage current from passing into the rest of the surge protector or the devices plugged into it.

A Varistor remains non-conductive as a shunt mode device during normal operation when voltage remains well below its "clamping voltage". If a transient pulse (often measured in joules) is too high, the device may melt, burn, vaporize, or otherwise be damaged or destroyed. This (catastrophic) failure occurs when "Absolute Maximum Ratings" in manufacturer's datasheet are significantly exceeded. Varistor degradation is defined by manufacturer's life expectancy charts using curves that relate current, time, and number of transient pulses. A varistor fully degrades typically when its "clamping voltage" has changed by 10%. A fully degraded varistor remains functional (no catastrophic failure) and is not visibly damaged.

Ballpark number for varistor life expectancy is its energy rating. As MOV joules increase, the number of transient pulses increases and the "clamping voltage" during each transient decreases. The purpose of this shunt mode device is to divert a transient so that pulse energy will be dissipated elsewhere. Some energy is also absorbed by the varistor because a varistor is not a perfect conductor. Less energy is absorbed by a varistor, the varistor is more conductive, and its life expectancy increases exponentially as varistor energy rating is increased. Catastrophic failure can be avoided by significantly increasing varistor energy ratings either by using a varistor of higher joules or by connecting more of these shunt mode devices in parallel

A metal oxide varistor, or MOV, is a voltage dependent, nonlinear device that provides excellent transient voltage suppression. When exposed to high transient voltage, the metal oxide varistor clamps voltage to a safe level. A metal oxide varistor absorbs potentially destructive energy and dissipates it as heat, thus protecting vulnerable circuit components and preventing system damage.

Varistors are non-linear, voltage-dependent resistors whose value of resistance decreases with increasing voltage. Varistors are mainly used for over-voltage protection. Varistors are utilized in low-voltage electronic equipment for absorbing high-voltage pulsative noise while bypass capacitors are utilized for removing low-voltage and high-frequency alternating noise.

An advanced device having both the functions is a ceramic capacitor with varistor performance. Varistors are commonly made by sintering particulate metal oxides, with or without minor amounts of other inorganic materials, as in disk or rod form. Metal oxide varistors (MOV) are semiconductor devices that are fabricated using technology from the ceramic capacitor industry. Zinc oxide varistors are ceramic semiconductor devices based on zinc oxide. Varistors are produced by a ceramic sintering process which gives rise to a structure consisting of conductive zinc oxide grains surrounded by electrically insulating barriers. These barriers are attributed to trap states at grain boundaries induced by additive elements such as bismuth, cobalt, praseodymium, manganese and so forth. A monolithic chip varistor is generally provided with a varistor body including a plurality of varistor layers composed of a zinc oxide-based ceramic material and at least one pair of internal electrodes opposed to each other with one of the varistor layers there between , and a pair of external electrodes.

Suntan metal oxide varistor devices are made primarily of zinc oxide with small amounts of bismuth, cobalt, manganese, and other metal oxides. They can be connected in parallel for increased energy-handling capabilities. MOVs can also be connected in series to provide higher voltage ratings or to provide voltage rating between the standard increments.

Multilayer varistor (MLV) devices provide electrostatic discharge protection to electronic circuits from low to medium energy transients in sensitive equipment operating at 0-120 volts dc. They have peak current ratings from about 20 to 500 amperes, and peak energy ratings from 0.05 to 2.5 joules.

Suntan is with very good support for Multilayer Metal Oxide Chip Varistor. This products is featured with Low firing, Sliver diffusion, Grain resistance, Electrical properties and it is widely used in cellular telephones and automotive electronic subassemblies.