Varistor

Designation on the diagram

Current-voltage characteristics of varistors: blue - based on ZnO, red - based on SiC.

Different varistors

Varistors

A varistor (lat. Vari (able) - variable (resi) stor - resistor) is a semiconductor resistor whose electrical resistance ( conductivity ) is non-linearly dependent on the applied voltage , that is, it has a non-linear symmetric current-voltage characteristic and has two terminals. It has the property to sharply reduce its resistance from billions to tens of ohms with an increase in the voltage applied to it above the threshold value ^[1] . With a further increase in voltage, the resistance decreases even more. Due to the absence of accompanying currents in case of an abrupt change in the applied voltage, varistors are the main element for the production of surge protective devices (SPDs).

Content

Fabrication

Varistors are made by sintering a semiconductor, preferably powdered silicon carbide (SiC) or zinc oxide (ZnO), and a binder (for example, clay , water glass , varnishes , resins ) at about 1700 ° C. Next, the two surfaces of the obtained element are metallized (usually the electrodes are in the form of discs) and metal wire leads are soldered to them.

Structurally, varistors are usually made in the form of disks, tablets, rods; There are bead and film varistors. Widespread rod trimming varistors with a moving contact.

Properties

The nonlinearity of the characteristics of varistors is due to the local heating of the contacting faces of numerous silicon carbide crystals (or other semiconductor). With a local increase in temperature at the crystal boundaries, the resistance of the latter is significantly reduced, which leads to a decrease in the total resistance of varistors.

One of the main parameters of the varistor - the nonlinearity coefficient λ - is determined by the ratio of its static resistance R to dynamic resistance R _d :

\lambda ={\frac {R}{R_{d}}}={\frac {U}{I}}:{\frac {dU}{dI}}\approx const

{\ displaystyle \ lambda = {\ frac {R} {R_ {d}}} = {\ frac {U} {I}}: {\ frac {dU} {dI}} \ approx const}

,

where U is the voltage, I is the varistor current

The nonlinearity coefficient is in the range of 2-10 for SiC-based varistors and 20-100 for ZnO-based varistors.

The temperature coefficient of resistance (TKS) of the varistor is negative.

Application

Low-voltage varistors are manufactured for operating voltages from 3 to 200 V and currents from 0.0001 to 1 A ; high voltage varistors - for operating voltage up to 20 kV .

Varistors are used to stabilize and control low-frequency currents and voltages; in analog computers, they are used for exponentiation, extraction of roots and other mathematical operations, in overvoltage protection circuits (for example, high-voltage power lines , communication lines, electrical devices), etc.

High voltage varistors are used for the manufacture of surge arresters .

As electronic components, varistors are cheap and reliable, capable of withstanding significant electrical overloads, can operate at high frequencies (up to 500 kHz ). Among the drawbacks - significant low-frequency noise and aging - changes in parameters with time and with temperature fluctuations.

Varistor Materials

Tirite , vilit , latin , silit - semiconductor materials based on silicon carbide with different ligaments. Zinc oxide is a new material for varistors.

Parameters

When describing the characteristics of varistors, the following parameters are mainly used ^[1] :

Classification voltage U _n - voltage at a certain current (usually 1 mA), conditional parameter for marking products;
The maximum allowable voltage U _m for direct current and for alternating current (rms or rms value), the range is from several V to several tens of kV; can be exceeded only with overvoltages;
The nominal average power dissipation P is the power in watts (W), which the varistor can dissipate during the entire service life while maintaining the parameters within the specified limits;
The maximum pulse current I _pp ( Peak Surge Current ) in amperes (A), for which the rise time and the pulse duration are normalized;
Maximum allowable absorbed energy W ( Absorption energy ) in joules (J), when exposed to a single pulse;
Capacity C _o , measured in the closed state at a given frequency; depends on the applied voltage - when the varistor passes a large current through itself, it drops to zero.

The varistor operating voltage is selected based on the allowable dissipation energy and maximum voltage amplitude. It is recommended that at alternating voltage it does not exceed 0.6 U _n , and at constant voltage - 0.85 U _n . For example, in a network with a current voltage of 220 V (50 Hz), varistors are usually installed with a classification voltage not lower than 380 ... 430 V.

Notes

↑ ¹ ² Shelestov, 2002 .

Literature

V. G. Gerasimov, O. M. Knyazkov, A. E. Krasnopolsky, V. V. Sukhorukov. Fundamentals of industrial electronics: A textbook for universities / ed. V. G. Gerasimova. - 2nd ed., Pererab. and add. - M .: High School, 1978.
Electronics: Encyclopedic dictionary / VG Kolesnikov (editor-in-chief). - 1st ed. - M .: Owls. encyclopedia, 1991. - p. 54. - ISBN 5-85270-062-2 .
I.P. Shelestov. Useful schemes. Book 5 . - M .: SOLON-R, 2002. - 240 p. - (Amateur radio). - 7000 copies - ISBN 5-93455-167-1 .

[_421e5c8f5e4b2339-1] ¹ ² Shelestov, 2002 .