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A varistor is an electronic component with an electrical resistance that varies with the applied voltage. In contrast to a diode however, it has the same characteristic for both directions of traversing current. Traditionally, varistors were indeed constructed by connecting two rectifiers in anti-parallel configuration. At low voltage the varistor has a high electrical resistance which decreases as the voltage is raised.
Varistors are used as control or compensation elements in circuits either to provide optimal operating conditions or to protect against excessive transient voltages. When varistro as protection devices, they shunt the vaistor created by the excessive voltage away from sensitive components when triggered. The name varistor is a portmanteau of varying resistor. The term is only used for non-ohmic varying resistors. Variable resistorssuch as the potentiometer and the rheostathave ohmic characteristics.
The development of the varistor, in form of a new type of rectifier based on a cuprous oxide layer on copper, originated in the work by L. Grisdale in the early s was used to guard telephone lines from lightning.
The copper-oxide varistor exhibited a varying resistance in dependence on the polarity and magnitude of applied voltage. 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.
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In the s, 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 Type 3B varistor of June below.
Traditional varistor schematic symbol,  used today for the diac. It expresses the diode-like behavior in both directions of current flow. Western Electric Type 44A varistor manufactured inmounted on a U1 telephone receiver element for click suppression.
The Western Electric type telephone set of introduced a dynamic loop equalization varisttor using varistors that shunted relatively high levels varistoor 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.
The most common modern type of varistor is the metal-oxide varistor MOV.
This type contains a ceramic mass of zinc oxide grains, in a matrix of other metal oxides such as small amounts of bismuth, cobalt, manganese 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, 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 tunnelingand 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. 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.
JOYIN Metal Oxide Varistors Series JVR
Varistors can fail for either of two reasons. A catastrophic failure uvr 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 pulse normally measured in joules is too high i.
The probability of catastrophic failure vvaristor 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.
The main parameter affecting varistor life expectancy is its energy Joule rating. Increasing the energy rating raises the number varietor defined maximum size transient pulses that it can accommodate exponentially as well as the cumulative sum varistpr energy from clamping lesser pulses.
Manufacturer’s life-expectancy charts relate current varitsor, severity, and number of transients to make failure predictions jcr on the total energy dissipated over the life of the part. In consumer electronics, particularly surge protectorsthe MOV varistor size employed is small enough that eventually failure is expected. To protect telecommunication lines, transient suppression devices such as 3 mil carbon blocks IEEE C For higher frequencies, such as radio communication equipment, vxristor gas discharge tube GDT may be utilized.
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 UL 3rd edition approval so that catastrophic MOV failure does not create a fire hazard.
MOVs are specified according to the voltage range that they can tolerate without damage.
Vagistor important parameters are the varistor’s energy rating in joules, operating voltage, response time, maximum current, and breakdown clamping voltage. 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 . These low-capacitance varistors are, however, unable to withstand large surge currents simply due to their compact PCB-mount size.
While a MOV is designed to conduct significant power for very short durations about 8 to 20 microsecondssuch 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 varixtor 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 variztor MOV device catching fire. A series connected thermal fuse varistpr 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 suppressors 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 UL in and subsequent revisions in and Properly designed TVSS devices must not fail catastrophically, resulting in the opening of a thermal fuse varitor something equivalent that only disconnects MOV devices.
In particular, a MOV device provides no protection for the connected equipment from sustained over-voltages that may result in damage to that equipment as well as to the protector device.
Other sustained and harmful overvoltages may be lower and therefore ignored by a MOV device. A varistor provides no equipment protection from inrush current surges during equipment startupfrom overcurrent created by a short circuitor from voltage sags also known as a brownout ; it neither senses nor affects such events.
Susceptibility of electronic equipment to these other 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 surge protector 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.
Vzristor 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 that may use air or an inert gas mixture and often, a small amount of radioactive material such as Nito provide a more consistent breakdown voltage and vsristor 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. Multi-layer varistor MLV devices provide electrostatic discharge protection to electronic circuits from low to medium energy transients in vadistor equipment operating at volts dc. They have peak current ratings from about 20 to amperes, and peak energy ratings from 0.
From Wikipedia, the free varitor. Western Electric 3B varistor made in for use as click suppressor in telephone sets. Circuit of the traditional construction of varistors used as click suppressors in telephony.
This section needs expansion. You can help by adding to it. Journal of faristor A. Retrieved 9 April Archived from the original on Rosch Hardware Bible 6th ed. Potentiometer digital Variable capacitor Varicap. Capacitor types Ceramic resonator Crystal oscillator Inductor Parametron Relay reed relay mercury switch.
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