Editing Varistor (section)

== Composition, properties, and operation of the metal-oxide varistor ==
[[File:Typische Varistorkennlinien.svg|thumb|300px|Varistor current vs voltage for zinc oxide (ZnO) and silicon carbide (SiC) devices]]
The most common modern type of varistor is the metal-oxide varistor (MOV). This type contains a [[ceramic]] mass of [[zinc oxide]] (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.<ref>[http://www.powerguru.org/2012/06/05/introduction-to-metal-oxide-varistors/ ''Introduction to Metal Oxide Varistors''], www.powerguru.org</ref>

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]] 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 (electrical)|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]] 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]] 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 (oscillation)|transient]] pulse (normally measured in [[joule]]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.<ref>{{cite web|url=https://www.littelfuse.com/~/media/electronics_technical/application_notes/varistors/littelfuse_the_abcs_of_movs_application_note.pdf|title=The ABCs of MOVs|date=1999|publisher=Littelfuse, Inc.|access-date=2022-08-09|url-status=live|archive-date=2022-05-14|archive-url=https://web.archive.org/web/20220514123022/https://www.littelfuse.com/~/media/electronics_technical/application_notes/varistors/littelfuse_the_abcs_of_movs_application_note.pdf}}</ref>

Cumulative degradation occurs as more surges happen.  For historical reasons, many MOVs have been incorrectly specified allowing frequent swells to also degrade capacity.<ref>{{cite web|url=https://www.nist.gov/pml/div684/upload/Lower_not_better.pdf |title=Lower not better |website=www.nist.gov |format=PDF |date= |access-date=2021-12-11}}</ref>  In this condition the varistor is not visibly damaged and outwardly appears functional (no catastrophic failure), but it no longer offers protection.<ref>{{cite web|url=https://www.research.usf.edu/dpl/content/data/PDF/05B127.pdf|title=MOV Failure Mode Identification|date=n.d.|publisher=University of South Florida|access-date=2022-08-09|url-status=live|archive-date=2021-02-25|archive-url=https://web.archive.org/web/20210225100556/https://www.research.usf.edu/dpl/content/data/PDF/05B127.pdf}}</ref> 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 [[electric current|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]]s, the MOV varistor size employed is small enough that eventually failure is expected.<ref>{{cite web|url=http://www.circuitstoday.com/metal-oxide-varistor-mov|title=Metal Oxide Varistor (MOV) – Electronic Circuits and Diagram-Electronics Projects and Design|date=23 March 2011}}</ref> Other applications, such as power transmission, use VDRs of different construction in multiple configurations engineered for long life span.<ref>{{cite web|url=https://www.gegridsolutions.com/products/brochures/arrestertranquellpolyporceint.pdf|title=GE TRANQUELLTM Surge Arresters|date=2013|publisher=GE Grid Solutions|access-date=2022-08-09|url-status=live|archive-date=2021-11-23|archive-url=https://web.archive.org/web/20211123000903/https://www.gegridsolutions.com/products/brochures/arrestertranquellpolyporceint.pdf}}</ref>

=== Voltage rating ===
[[File:Varistor.jpg|thumb|upright|High voltage varistor]]
MOVs are specified according to the voltage range that they can tolerate without damage. Other important [[parameter]]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 [[Transient (oscillation)|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&nbsp;mm diameter) varistors are in the range of 100–2,500&nbsp;pF.  Smaller, lower-capacitance varistors are available with capacitance of ~1&nbsp;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.<ref>D. Månsson, R. Thottappillil, "Comments on ‘Linear and nonlinear filters suppressing UWB pulses’", IEEE Transactions on Electromagnetic Compatibility, vol. 47, no. 3, pp. 671–672, Aug. 2005.</ref> That response time is also qualified as insignificant when compared to a transient having an 8&nbsp;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&nbsp;ns range.<ref>{{cite web | url = http://www.lightningtalks.com/LightningProtection.htm | archive-url = https://web.archive.org/web/20101105072142/http://www.lightningtalks.com/LightningProtection.htm | archive-date = 2010-11-05 | title = Detailed Comparison of Surge Suppression Devices }}</ref>