Varistor

A varistor is an electronic component. This is a voltage-dependent resistor. Above a certain threshold voltage, which is typical for the respective varistor, the differential resistance is abruptly reduced. The characteristic curve is symmetrical with respect to voltage, the polarity is not important. Varistor is from the English terms "variable resistor" composite portmanteau word. Varistors are also referred to as a VDR. VDR Voltage Dependent Resistor is thus voltage- dependent resistor. For metal oxide varistors the abbreviation MOV is common.

Construction

Varistors are today mostly on the basis of zinc oxide (ZnO ) were prepared. Along with other metal oxides, such as bismuth oxide, chromium oxide or manganese oxide, the powder is pressed and sintered in the form of tablets. On two sides of the blank is contacted with silver or aluminum and provided with connections. Previously Varistors silicon carbide (SiC ) were prepared. These varistors are now considered obsolete, because their current-voltage characteristic is higher than the threshold voltage significantly flatter than that of ZnO varistors.

Operation

The zinc oxide varistor is composed of many small zinc oxide grains with different conductivity. Between the zinc oxide grains are produced at the points of contact barrier layers. By an applied voltage, an electric field, which degrades the blocking layers partially. The larger the applied voltage is, the more barrier layers are degraded and hence the resistance decreases. The thickness of the varistor threshold voltage can be varied: The thicker the varistor disk is, the more zinc oxide grains are connected in series and the higher the threshold voltage.

Current-voltage characteristic

The current-voltage characteristics of a varistor can be approximated using the following formula:

Herein, the voltage at which a current of one ampere flows through the varistor ( not to be confused with the threshold voltage ). A is the unit symbol for ampere. The exponent n determines the slope of the characteristic. It must be n is an odd integer so that the current at negative voltage U is also negative. For zinc oxide varistors n is typically greater than 20, for SiC varistors n is about 5

Designs

The following types are most commonly used are:

  • In disc form with Leads
  • As SMD component
  • Clipped onto contactors

Areas of application

Varistors are suitable for protecting against surges. In normal operation, its resistance is very large, while in case of overvoltage, the resistance almost no delay is very small and the charge is derived. They are used to protect sensitive electronic circuits as well as in power engineering.

Varistors response limit of less than one nanosecond and can be very transient overvoltages without being destroyed. With longer duration of a varistor is overheated because the absorbed energy depends on the total mass. A disadvantage of ZnO varistors is that they "age" with several smaller surges, ie their threshold voltage becomes lower with time, and their leakage current increases. SiC varistors did not show this type of aging.

As an alternative to Varistors Suppressors also be used in protection circuits. Suppressors are, however, already destroyed by overvoltage due to low energies, so that they are mainly used for smaller voltages, such as occur for example in signal lines. On the other hand, do not age Suppressors unlike varistors. For applications in which high frequency signals are to be transmitted, the opposite suppressor significantly higher capacity of the varistors may cause an unacceptable loss of the desired signal.

In addition, there are also gas arrester as coarse protection elements. They can absorb very large amounts of energy in the event of voltages, but compared to varistors have a longer response time of up to several microseconds. Therefore, they are also often used in combination with varistors.

Varistors are used as ( demand) current bridge for the series of bulbs in lights.

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