Varistor

Description

The varistors are made up of Oxyde S metal (oxides of Zinc ZnO for the last generations, the Carbure of silicon (Sic) having been used before). The metallic oxide powders are assembled by Frittage in the ceramic shape of block of , generally in the form of cylinder or of disc.

In HTA and HTB, one assembles these blocks in an insulating envelope (ceramic or Polymère) to form a Parafoudre (see photo). These components replaced the spark-gap S with air (said “to horns”) in spite of a higher cost, the latter producing a radioelectric noise incompatible with the regulations in force, and having a reliability and a level of poor protection.

In low or Very low tension, one finds the presentations following:

metallized discs, provided with connections (wire or, for the highest currents, strips or terminals) then isolated by a fluidized layer of epoxy (even technology that for the Condensing S discoïdaux) or, more rarely, moulded: this form was most frequent by far a few years ago,
Parallélépipède S moulded, outcome with component with assembly on the surface (CMS),
networks of varistors weak energy (application: protection of the pins of a connector against the electrostatic discharges of human origin).

Principle

These blocks of varistor have a characteristic extremely nonlinear tension/current. Beyond a certain threshold of tension the impedance of the varistor falls to allow the evacuation of the running creating overpressure; when the tension returns on its normal level the impedance of the varistor takes again its value with the waking state (thus not of current). For the strong amplitudes of current deviated the terminal voltage of the varistor increases.

The lifespan of these components is limited: according to the requests the properties of the blocks of ceramics are degraded, the leakage current with the waking state increases, and the thermal behavior of the blocks decreases. The lightning protectors HTB with varistor comprise a system of valve to avoid the damage related to the thermal capacity overshooting. It is however recommended periodically to check the state of the lightning protectors of this type by a measurement of the leakage current at rest, which is rather good indicating of the state of the lightning protector. The time of starting of this type of lightning protector is much better than that of the spark-gaps (about 30ns).

This type of component is used to protect the distribution network and also the final equipment in the form of Switchgear modular but not for the equipment of Télécommunication out of high banc because the capacity parasite induces a weakening of the signal in the high frequencies.

Note on the significance of the term “varistor”

Étymologiquement, varistor simply means “variable resistor”, i.e. whose quotient U/I is not constant, which indicates in practice about all the Dipôle S used in electricity. While limiting to those whose characteristic U/I is Monotone (what excludes the devices like the spark-gap S or the Diac S), one can quote:

resistances varying with the ambient Temperature

in a nondesired way: all the real resistances
in a desired way: the Thermister S (divided into CTP and CTN) used as temperature gauges

resistances varying with the Temperature by car-heating

the Fusible S réarmables (CTP)
limitings device of inrush current (CTN)

resistances varying with the tension

the Zener and the Transil
the Surge protector S

Nevertheless the use wants that the varistor term indicates only the latter.

See too

Diode Transil
Diode Zener

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