A varistor is an

electronic component An electronic component is any basic discrete device or physical entity in an electronic system used to affect electrons or their associated fields. Electronic components are mostly industrial products, available in a singular form and are n ...

with an

electrical resistance The electrical resistance of an object is a measure of its opposition to the flow of electric current. Its reciprocal quantity is , measuring the ease with which an electric current passes. Electrical resistance shares some conceptual parallel ...

that varies with the applied voltage. Also known as a voltage-dependent resistor (VDR), it has a

nonlinear In mathematics and science, a nonlinear system is a system in which the change of the output is not proportional to the change of the input. Nonlinear problems are of interest to engineers, biologists, physicists, mathematicians, and many other ...

, non- ohmic

current–voltage characteristic A current–voltage characteristic or I–V curve (current–voltage curve) is a relationship, typically represented as a chart or graph, between the electric current through a circuit, device, or material, and the corresponding voltage, or ...

that is similar to that of a diode. Unlike a diode however, it has the same characteristic for both directions of traversing current. Traditionally, varistors were indeed constructed by connecting two rectifiers, such as the copper-oxide or germanium-oxide rectifier in antiparallel configuration. At low voltage the varistor has a high electrical resistance which decreases as the voltage is raised. Modern varistors are primarily based on sintered

ceramic A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelai ...

metal-oxide materials which exhibit directional behavior only on a microscopic scale. This type is commonly known as the metal-oxide varistor (MOV). Varistors are used as control or compensation elements in circuits either to provide optimal operating conditions or to protect against excessive transient

voltage Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge t ...

s. When used as protection devices, they shunt the current created by the excessive voltage away from sensitive components when triggered. The name ''varistor'' is a

portmanteau A portmanteau word, or portmanteau (, ) is a blend of wordsVariable resistors, such as the

potentiometer A potentiometer is a three- terminal resistor with a sliding or rotating contact that forms an adjustable voltage divider. If only two terminals are used, one end and the wiper, it acts as a variable resistor or rheostat. The measuring instrum ...

and the

rheostat A potentiometer is a three- terminal resistor with a sliding or rotating contact that forms an adjustable voltage divider. If only two terminals are used, one end and the wiper, it acts as a variable resistor or rheostat. The measuring instrum ...

, have ohmic characteristics.

History

The development of the varistor, in form of a new type of

rectifier A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction. The reverse operation (converting DC to AC) is performed by an inve ...

based on a cuprous oxide layer on copper, originated in the work by L.O. Grondahl and P.H. Geiger in 1927. The copper-oxide varistor exhibited a varying resistance in dependence on the polarity and magnitude of applied voltage.American Telephone & Telegraph; C.F. Myers, L.S.c Crosboy (''eds.''); ''Principles of Electricity applied to Telephone and Telegraph Work'', New York City (November 1938), p.58, 257 It was constructed from a small copper disk, of which one side was formed a layer of cuprous oxide. This arrangement provides low resistance to current flowing from the semiconducting oxide to the copper side, but a high resistance to current in the opposite direction, with the instantaneous resistance varying continuously with the voltage applied. In the 1930s, small multiple-varistor assemblies of a maximum dimension of less than one inch and apparently indefinite useful lifetime found application in replacing bulky electron tube circuits as modulators and demodulators in carrier current systems for telephonic transmission. Other applications for varistors in the telephone plant included protection of circuits from voltage spikes and noise, as well as click suppression on receiver (''ear-piece'') elements to protect users' ears from popping noises when switching circuits. These varistors were constructed by layering an even number of rectifier disks in a stack and connecting the terminal ends and the center in an anti-parallel configuration, as shown in the photo of a

Western Electric The Western Electric Company was an American electrical engineering and manufacturing company officially founded in 1869. A wholly owned subsidiary of American Telephone & Telegraph for most of its lifespan, it served as the primary equipment ma ...

Type 3B varistor of June 1952 (below). File:Western Electric Type 3B varistor manufactured in June 1952.jpg, Western Electric 3B varistor made in 1952 for use as click suppressor in telephone sets File:Varistor circuit historical construction.png, Circuit of the traditional construction of varistors used as click suppressors in telephony File:Diac.svg, Traditional varistor schematic symbol, used today for the diac. It expresses the diode-like behavior in both directions of current flow. File:Western Electric Type 44A varistor on U1 receiver.jpg, Western Electric Type 44A varistor for click suppression, mounted on a U1 telephone receiver element manufactured in 1958. The Western Electric type 500 telephone set of 1949 introduced a dynamic loop equalization circuit using varistors that shunted relatively high levels of loop current on short central office loops to adjust the transmission and receiving signal levels automatically. On long loops, the varistors maintained a relatively high resistance and did not alter the signals significantly. Another type of varistor was made from

silicon carbide Silicon carbide (SiC), also known as carborundum (), is a hard chemical compound containing silicon and carbon. A semiconductor, it occurs in nature as the extremely rare mineral moissanite, but has been mass-produced as a powder and crystal ...

by R. O. Grisdale in the early 1930s. It was used to guard telephone lines from lightning. In the early 1970s, Japanese researchers recognized the semiconducting electronic properties of zinc oxide (ZnO) as being useful as a new varistor type in a

sintering process, which exhibited a voltage-current function similar to that of a pair of back-to-back

Zener diode A Zener diode is a special type of diode designed to reliably allow current to flow "backwards" (inverted polarity) when a certain set reverse voltage, known as the ''Zener voltage'', is reached. Zener diodes are manufactured with a great var ...

s. This type of device became the preferred method for protecting circuits from power surges and other destructive electric disturbances, and became known generally as the metal-oxide varistor (MOV). The randomness of orientation of ZnO grains in the bulk of this material provided the same voltage-current characteristics for both directions of current flow.

Composition, properties, and operation of the metal-oxide varistor

The most common modern type of varistor is the metal-oxide varistor (MOV). This type contains a

mass of

zinc oxide Zinc oxide is an inorganic compound with the Chemical formula, formula . It is a white powder that is insoluble in water. ZnO is used as an additive in numerous materials and products including cosmetics, food supplements, rubbers, plastics, ceram ...

(ZnO) grains, in a matrix of other metal oxides, such as small amounts of bismuth, cobalt, manganese oxides, sandwiched between two metal plates, which constitute the electrodes of the device. The boundary between each grain and a neighbor forms a diode junction, which allows current to flow in only one direction. The accumulation 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 voltage is applied across the electrodes, only a tiny current flows, caused by reverse leakage through the diode junctions. When a large voltage is applied, the diode junction breaks down due to a combination of

thermionic emission Thermionic emission is the liberation of electrons from an electrode by virtue of its temperature (releasing of energy supplied by heat). This occurs because the thermal energy given to the charge carrier overcomes the work function of the mate ...

and electron tunneling, resulting in a large current flow. The result of this behavior is a nonlinear current-voltage characteristic, in which the MOV has a high resistance at low voltages and a low resistance at high voltages.

Electrical characteristics

A varistor remains non-conductive as a shunt-mode device during normal operation when the voltage across it remains well below its "clamping voltage", thus varistors are typically used for suppressing line voltage surges. Varistors can fail for either of two reasons. A catastrophic failure occurs from not successfully limiting a very large surge from an event like a

lightning Lightning is a naturally occurring electrostatic discharge during which two electrically charged regions, both in the atmosphere or with one on the ground, temporarily neutralize themselves, causing the instantaneous release of an average ...

strike, where the energy involved is many orders of magnitude greater than the varistor can handle. Follow-through current resulting from a strike may melt, burn, or even vaporize the varistor. This

thermal runaway Thermal runaway describes a process that is accelerated by increased temperature, in turn releasing energy that further increases temperature. Thermal runaway occurs in situations where an increase in temperature changes the conditions in a way t ...

is due to a lack of conformity in individual grain-boundary junctions, which leads to the failure of dominant current paths under thermal stress when the energy in a

transient ECHELON, originally a secret government code name, is a surveillance program ( signals intelligence/SIGINT collection and analysis network) operated by the five signatory states to the UKUSA Security Agreement:Given the 5 dialects that ...

pulse (normally measured in

joule The joule ( , ; symbol: J) is the unit of energy in the International System of Units (SI). It is equal to the amount of work done when a force of 1 newton displaces a mass through a distance of 1 metre in the direction of the force applie ...

s) is too high (i.e. significantly exceeds the manufacture's "Absolute Maximum Ratings"). The probability of catastrophic failure can be reduced by increasing the rating, or using specially selected MOVs in parallel. Cumulative degradation occurs as more surges happen. For historical reasons, many MOVs have been incorrectly specified allowing frequent swells to also degrade capacity. In this condition the varistor is not visibly damaged and outwardly appears functional (no catastrophic failure), but it no longer offers protection. Eventually, it proceeds into a shorted circuit condition as the energy discharges create a conductive channel through the oxides. The main parameter affecting varistor life expectancy is its energy (Joule) rating. Increasing the energy rating raises the number of (defined maximum size) transient pulses that it can accommodate exponentially as well as the cumulative sum of energy from clamping lesser pulses. As these pulses occur, the "clamping voltage" it provides during each event decreases, and a varistor is typically deemed to be functionally degraded when its "clamping voltage" has changed by 10%. Manufacturer's life-expectancy charts relate current, severity, and number of transients to make failure predictions based on the total energy dissipated over the life of the part. In consumer electronics, particularly

surge protector A 'surge protector'' (or spike suppressor, surge suppressor, surge diverter, surge protection device (SPD) or transient voltage surge suppressor (TVSS) is an appliance or device intended to protect electrical devices from voltage spikes in al ...

s, the MOV varistor size employed is small enough that eventually failure is expected. Other applications, such as power transmission, use VDRs of different construction in multiple configurations engineered for long life span.

Voltage rating

MOVs are specified according to the voltage range that they can tolerate without damage. Other important

parameter A parameter (), generally, is any characteristic that can help in defining or classifying a particular system (meaning an event, project, object, situation, etc.). That is, a parameter is an element of a system that is useful, or critical, when ...

s are the varistor's energy rating in joules, operating voltage, response time, maximum current, and breakdown (clamping) voltage. Energy rating is often defined using standardized transients such as 8/20 microseconds or 10/1000 microseconds, where 8 microseconds is the transient's front time and 20 microseconds is the time to half value.

Capacitance

Typical capacitance for consumer-sized (7–20 mm diameter) varistors are in the range of 100–2,500 pF. Smaller, lower-capacitance varistors are available with capacitance of ~1 pF for microelectronic protection, such as in cellular phones. These low-capacitance varistors are, however, unable to withstand large surge currents simply due to their compact PCB-mount size.

Response time

The response time of the MOV is not standardized. The sub-nanosecond MOV response claim is based on the material's intrinsic response time, but will be slowed down by other factors such as the inductance of component leads and the mounting method. That response time is also qualified as insignificant when compared to a transient having an 8 µs rise-time, thereby allowing ample time for the device to slowly turn-on. When subjected to a very fast, <1 ns rise-time transient, response times for the MOV are in the 40–60 ns range.

Applications

To protect

telecommunication Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than tha ...

lines, transient suppression devices such as 3 mil carbon blocks (IEEE C62.32), ultra-low capacitance varistors, and

avalanche diode In electronics, an avalanche diode is a diode (made from silicon or other semiconductor) that is designed to experience avalanche breakdown at a specified reverse bias voltage. The junction of an avalanche diode is designed to prevent current c ...

s are used. For higher frequencies, such as radio communication equipment, a gas discharge tube (GDT) may be utilized. A typical

power strip is built using MOVs. Low-cost versions may use only one varistor, from the hot (live, active) to the neutral conductor. A better protector contains at least three varistors; one across each of the three pairs of conductors. In the United States, a power strip protector should have an

Underwriters Laboratories The UL enterprise is a global safety science company headquartered in Northbrook, Illinois, composed of three organizations, UL Research Institutes, UL Standards & Engagement and UL Solutions. Established in 1894, the UL enterprise was founded as ...

(UL) 1449 3rd edition approval so that catastrophic MOV failure does not create a fire hazard. Surge protector plug circuitry back & front

Hazards

While a MOV is designed to conduct significant power for very short durations (about 8 to 20 microseconds), such as caused by lightning strikes, it typically does not have the capacity to conduct sustained energy. Under normal utility voltage conditions, this is not a problem. However, certain types of faults on the utility power grid can result in sustained over-voltage conditions. Examples include a loss of a neutral conductor or shorted lines on the high voltage system. Application of sustained over-voltage to a MOV can cause high dissipation, potentially resulting in the MOV device catching fire. The

National Fire Protection Association The National Fire Protection Association (NFPA) is an international nonprofit organization devoted to eliminating death, injury, property and economic loss due to fire, electrical and related hazards. As of 2018, the NFPA claims to have 50,000 mem ...

(NFPA) has documented many cases of catastrophic fires that have been caused by MOV devices in surge suppressors, and has issued bulletins on the issue. Varistorfail full

A series connected thermal fuse is one solution to catastrophic MOV failure. Varistors with internal thermal protection are also available. There are several issues to be noted regarding behavior of

transient voltage surge suppressor A 'surge protector'' (or spike suppressor, surge suppressor, surge diverter, surge protection device (SPD) or transient voltage surge suppressor (TVSS) is an appliance or device intended to protect electrical devices from voltage spikes in a ...

s (TVSS) incorporating MOVs under over-voltage conditions. Depending on the level of conducted current, dissipated heat may be insufficient to cause failure, but may degrade the MOV device and reduce its life expectancy. If excessive current is conducted by a MOV, it may fail catastrophically, keeping the load connected, but now without any surge protection. A user may have no indication when the surge suppressor has failed. Under the right conditions of over-voltage and line impedance, it may be possible to cause the MOV to burst into flames, the root cause of many fires and the main reason for NFPA's concern resulting in UL1449 in 1986 and subsequent revisions in 1998 and 2009. Properly designed TVSS devices must not fail catastrophically, resulting in the opening of a thermal fuse or something equivalent that only disconnects MOV devices.

Limitations

A MOV inside a transient voltage

surge suppressor A 'surge protector'' (or spike suppressor, surge suppressor, surge diverter, surge protection device (SPD) or transient voltage surge suppressor (TVSS) is an appliance or device intended to protect electrical devices from voltage spikes in alt ...

(TVSS) does not provide complete protection for electrical equipment. In particular, it provides no protection from sustained over-voltages that may result in damage to that equipment as well as to the protector device. Other sustained and harmful over-voltages may be lower and therefore ignored by a MOV device. A varistor provides no equipment protection from inrush current surges (during equipment startup), from

overcurrent In an electric power system, overcurrent or excess current is a situation where a larger than intended electric current exists through a conductor, leading to excessive generation of heat, and the risk of fire or damage to equipment. Possible cause ...

(created by a short circuit), or from

voltage sag Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to ...

s ( brownouts); it neither senses nor affects such events. Susceptibility of electronic equipment to these other electric power disturbances is defined by other aspects of the system design, either inside the equipment itself or externally by means such as a UPS, a voltage regulator or a

with built-in overvoltage protection (which typically consists of a voltage-sensing circuit and a relay for disconnecting the AC input when the voltage reaches a danger threshold).

Comparison to other transient suppressors

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 MOVs, diodes are not degraded by smaller surges and can be implemented with a lower "clamping voltage". MOVs degrade from repeated exposure to surges and generally have a higher "clamping voltage" so that leakage does not degrade the MOV. Both types are available over a wide range of voltages. MOVs 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 A spark gap consists of an arrangement of two conducting electrodes separated by a gap usually filled with a gas such as air, designed to allow an electric spark to pass between the conductors. When the potential difference between the condu ...

that may use air or an

inert gas An inert gas is a gas that does not readily undergo chemical reactions with other chemical substances and therefore does not readily form chemical compounds. The noble gases often do not react with many substances and were historically referred to ...

mixture and often, a small amount of

radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consi ...

material such as

Ni-63 Naturally occurring nickel (28Ni) is composed of five stable isotopes; , , , and , with being the most abundant (68.077% natural abundance). 26 radioisotopes have been characterised with the most stable being with a half-life of 76,000 years, ...

, 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

) without significant degradation.

Multi-layer varistor

Multi-layer varistor (MLV) devices provide

electrostatic discharge Electrostatic discharge (ESD) is a sudden and momentary flow of electric current between two electrically charged objects caused by contact, an electrical short or dielectric breakdown. A buildup of static electricity can be caused by tribochar ...

protection to

electronic circuit An electronic circuit is composed of individual electronic components, such as resistors, transistors, capacitors, inductors and diodes, connected by conductive wires or traces through which electric current can flow. It is a type of electri ...

s 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.

References

External links

''The ABCs of MOVs''
nbsp;— application notes from Littelfuse company
Varistor testing
from Littelfuse company {{Authority control Electrical components Resistive components