metal oxide varistors application

Traditional Metal oxide varistors schematic symbol. It expresses the diode-like behavior in both directions of current flow.
Modern varistor schematic symbol.
A metal oxide varistors application is an electronic component with an electrical resistance that varies with the applied voltage. Also known as a voltage-dependent resistor (VDR), it has a nonlinear, non-ohmic current–voltage characteristic that is similar to that of a diode. In contrast to a diode however, it has the same characteristic for both directions of traversing current. At low voltage it 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 used as protection devices, they shunt the current created by the excessive voltage away from sensitive components when triggered.
The development of the varistor, in the form of a new type of rectifier (copper oxide), originated in the work by L.O. Grondahl and P.H. Geiger in 1927. The name varistor is a portmanteau of varying resistor. The term is only used for non-ohmic varying resistors. Variable resistors, such as the potentiometer and the rheostat, have ohmic characteristics.

Metal Oxide Varistors (MOV)

While an Metal Oxide Varistors (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 metal oxide varistor can cause high dissipation, potentially resulting in the MOV device catching fire. The National Fire Protection Association (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.

A 130 volt, 150 J MOV that has undergone catastrophic failure, apparently as a result of a lightning strike, showing evidence of heat and smoke. The 3 amp fast-blow fuse immediately in front of the varistor blew during the same event.
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 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,[5] the root cause of many fires[6] 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.

the Specifications of metal oxide varistor application

Voltage rating
MOVs are specified according to the voltage range that they can tolerate without damage. Other important parameters are the metal oxide varistor application 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.
Response time
The response time of the varistors 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.
Capacitance

Typical capacitance for consumer-sized (7–20 mm diameter) varistors are in the range of 100–1,000 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.

the introduction of metal oxide varistors application

Traditional Metal oxide varistors schematic symbol. It expresses the diode-like behavior in both directions of current flow.
Modern varistor schematic symbol.
A metal oxide varistors application is an electronic component with an electrical resistance that varies with the applied voltage. Also known as a voltage-dependent resistor (VDR), it has a nonlinear, non-ohmic current–voltage characteristic that is similar to that of a diode. In contrast to a diode however, it has the same characteristic for both directions of traversing current. At low voltage it 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 used as protection devices, they shunt the current created by the excessive voltage away from sensitive components when triggered.
The development of the varistor, in the form of a new type of rectifier (copper oxide), originated in the work by L.O. Grondahl and P.H. Geiger in 1927. The name varistor is a portmanteau of varying resistor. The term is only used for non-ohmic varying resistors. Variable resistors, such as the potentiometer and the rheostat, have ohmic characteristics.

Mov metal oxide varistor

The structural characteristics of the varistor varistor metal oxide different from ordinary resistor, which is based on nonlinear characteristics of semiconductor material made of
Ohm's law to comply with ordinary resistor, and varistor voltage and current is nonlinear relationship was special. When applied across the varistor voltage is lower than the nominal rated voltage, resistance value varistors approaches infinity, almost no current flows through the interior. When the voltage across the TGE Metal Oxide Varistors(MOV) slightly higher than the nominal rated voltage, the varistor will quickly breakdown conduction by a High Performance Varistor state to a low resistance state, current also increases dramatically.

metal oxide varister

Innovative elements: Zinc Oxide blocks for high performance requirements, system studies functional powder type, size, density, surface condition, surface treatment, surface area and powder activity of the influence of the resistance properties, the successful development of sensitive resistance high-performance composite particles. The product is uniformly mixed with zinc oxide, after pressing and sintering can effectively improve the performance of varistors, reached the advanced level of developed countries today.
Benefit Analysis: Zinc Oxide discs and surge arresters have important applications in the field of transmission and distribution engineering and construction of power grids, subway rail transportation, military and civilian communications facilities, the development of high performance metal oxide varister and surge arresters is significant .

High Performance Varistor

High Performance Varistor and surge arrester varistor high performance requirements, system studies functional powder type, size, density, surface condition, surface treatment, surface area and powder activity of the influence of the resistance performance, success develop efficient varistor composite particles.
Second, the specific description:
1. Application: metal oxide varistors and surge arresters manufacturing enterprises.
2, the technical indicators: application to high-performance anti-corona resistance, surge voltage insulation coating material, its performance over similar foreign products; applied to large current insulating coating material, such as zinc oxide insulating layer side valve D3 with ordinary Composite arrester valve side insulation coating compared to the 4 / 10μs standard indicators of the impact of high current 65 ~ 70kA, more than GB11032-2000 standard (45kA), and reached the international IEC-60099-4 standard (65kA).

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