varistor

varistor [və′ris·tər]

(electronics)

A two-electrode semiconductor device having a voltage-dependent nonlinear resistance; its resistance drops as the applied voltage is increased. Also known as voltage-dependent resistor.

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Varistor

Any two-terminal solid-state device in which the electric current I increases considerably faster than the voltage V. This nonlinear effect may occur over all, or only part, of the current-voltage characteristic. It is generally specified as I ∝ Vⁿ, where n is a number ranging from 3 to 35 depending on the type of varistor. The main use of varistors is to protect electrical and electronic equipment against high-voltage surges by shunting them to ground. See Electric protective devices

One type of varistor comprises a sintered compact of silicon carbide particles with electrical terminals at each end. It has symmetrical characteristics (the same for either polarity of voltage) with n ranging from 3 to 7. These devices are capable of application to very high power levels, for example, lightning arresters. See Lightning and surge protection

Another symmetrical device, the metal-oxide varistor, is made of a ceramiclike material comprising zinc oxide grains and a complex amorphous intergranular material. It has a high resistance (about 10⁹ ohms) at low voltage due to the high resistance of the intergranular phase, which becomes nonlinearly conducting in its control range (100–1000 V) with n > 25.

Semiconductor rectifiers, of either the pn-junction or Schottky barrier (hot carrier) types, are commonly utilized for varistors. A single rectifier has a nonsymmetrical characteristic which makes it useful as a low-voltage varistor when biased in the low-resistance (forward) polarity, and as a high-voltage varistor when biased in the high-resistance (reverse) polarity. Symmetrical rectifier varistors are made by utilizing two rectifiers connected with opposing polarity, in parallel (illus. a) for low-voltage operation and in series (illus. b) for high-voltage use. For the high-voltage semiconductor varistor, n is approximately 35 in its control range, which can be designed to be anywhere from a few volts to several hundred. See Semiconductor rectifier