OEM Fast Response 5D 511k Metal Oxide Varistor High Peak Current Withstand Capability​

The most common modern type of varistor is the metal oxide varistor (MOV). Ceramic blocks of this type containing particles of zinc oxide, in a matrix of other metal oxides, such as small amounts of bismuth, cobalt, manganese oxides, are sandwiched between two metal plates to form the electrodes of the device. The boundaries between each die and adjacent elements form diode junctions, which allow 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 in parallel with many other pairs.

When a small voltage is applied across the electrodes, only the reverse leakage through the diode junction causes a tiny current to flow. When a large voltage is applied, the diode junction breaks down due to a combination of thermionic emission and electron tunneling, causing a large current to flow. The result of this behavior is a nonlinear current-voltage characteristic, where MOVs have high resistance at low voltages and low resistance at high voltages.

Electrical characteristics

Varistors remain non-conductive during normal operation. As a shunt mode device, varistors are often used to suppress line voltage surges when the voltage across them remains well below their "clamping voltage". Varistors can fail for two reasons.

Catastrophic failures have occurred due to unsuccessful attempts to limit very large surges caused by events such as lightning strikes, where the energy involved is many orders of magnitude higher than the varistor can handle. Subsequent current flow from the impact could melt, burn or even vaporize the varistor. This thermal runaway is due to the lack of uniformity at the junctions of individual grain boundaries, leading to the failure of the dominant current path under thermal stress when the energy in the transient pulse (usually measured in Joules) is too high (i.e. significantly exceeds the fabrication quotient of the "Absolute Maximum Ratings"). The potential for catastrophic failure can be reduced by overrating or using specially selected MOVs in parallel.

Cumulative degradation occurs as more surges occur. For historical reasons, many MOVs have been misspecified, allowing frequent inflation that also reduces capacity. In this case, the varistor is not visibly damaged and outwardly appears to be functioning (no catastrophic failure), but it no longer provides protection. Eventually, it goes into a short circuit as the energy discharge creates a conductive path through the oxide.

The main parameter affecting the lifetime of a varistor is its energy (joule) rating. Increasing the energy level increases the number (of a defined maximum size) of transient pulses it can accommodate exponentially as well as the cumulative energy sum from clamped smaller pulses. When these pulses occur, the "clamping voltage" it provides during each event decreases, and a varistor is generally considered functionally degraded when its "clamping voltage" changes by 10%. Manufacturer's life expectancy charts involve current, severity, and number of transients for failure prediction based on the total energy dissipated over the life of the part.

In consumer electronics, especially surge protectors, the MOV varistors used are small enough that they will eventually fail. Other applications, such as power transmission, use VDRs of different structures in multiple configurations, designed for long life.

Characteristics of varistors

(1) Advantages: the voltage range is very wide, from several volts to several thousand volts, the surge current can be from tens to several thousand amperes, the response speed is fast, there is no polarity, no freewheeling, and the peak current withstand capacity is large. low price.
(2) Disadvantages: The clamping voltage is relatively high, which can generally reach 2 to 3 times the working voltage. In addition, pressure
The leakage current of the varistor increases with the increase of the number of surge impacts. The response time is longer and the parasitic capacitance is larger.
(3) Applicable occasions: DC power lines, low-frequency signal lines, or used in series with gas discharge tubes for AC power lines.

Specifications: