Abstract: Zinc oxide (ZnO) varistors exhibit a voltage upturn at higher current densities which reduces their effectiveness as over‐voltage protection devices. This effect has been attributed to a limiting series impedance offered by the ZnO grains. Based on literature data, the varistors were made for this investigation wherein the grain impedance has been reduced by doping with Al2O3 or Ga2O3. The high current surge and high frequency impedance data combined with microstructural investigation indicated an improved varistor nonlinearity via donor doping. A simple defect model developed for a pure ZnO crystal has been used to explain the doping behavior of the varistor grain.

Abstract: Compositional changes adjacent to grain boundaries in heavily degraded samples of a ZnO varistor containing the additives Bi and Co have been observed with the scanning transmission electron microscope. A marked asymmetry in the distribution of Bi and Co with respect to the physical location of the boundary is found, consistent with the accumulation of mobile cations on one side of the boundary under the influence of a dc electric field.

Abstract: The major voltage drop in ZnO varistors occurs across the grain boundaries, which behave generally as Schottky barriers supporting a barrier voltage Vg. This voltage is proportional to the device voltage. Experimental evidence shows that there is a time‐dependent reduction in the barrier voltage combined with an increase in resistive current iR when the varistor is subjected to a continuous ac voltage stress. The phenomena is reversed when the applied voltage is removed, showing nearly complete recovery. Instabilities of the resistive current and of the barrier voltage are shown to be manifestations of the same phenomenon and are attributed to a metastable component in the Schottky barriers. It is proposed that this metastable component is due to interstitial zinc ions that are capable of migration under thermal and electrical driving forces. When these ions are removed or stabilized by a suitable heat treatment, the instability of the device is reduced. This paper presents experimental data and analysis to support this hypothesis.

Abstract: A protective means for a series capacitor in a high-voltage network comprises a voltage-dependent resistor (varistor) arranged in parallel with the series capacitor, a spark gap arranged in parallel with the varistor for shunting the latter when it becomes overloaded, a current-sensing member, for example a current transformer, for sensing the current through the varistor, an energy absorption detector for sensing the temperature of the varistor, and a pulse transformer electrically or magnetically connected to the current-sensing member, for triggering the spark gap. During normal operation of the network and the series capacitor the current-sensing member is short-circuited by a normally closed switching device. This switching device is controlled by the energy absorption detector in such a way that the switching device is opened if the temperature of the varistor exceeds a predetermined level, whereby a trigger pulse, generated directly by the varistor current via the current-sensing member and the pulse transformer, may pass to the spark gap.

Abstract: An elongated, insulative tube of high mechanical strength is equipped with fittings at each end for electrical connection with a high voltage transmission line and a grounded tower. An external spark gap is provided between a first arcing ring mounted in electrical connection with one end fitting and a second arcing ring mounted to the tube intermediate its ends and electrically connected with a contact member disposed within the tube. The portion of the tube interior between the contact member and the other end fitting is packed with a series array of varistor discs. The tube maintains the line in spaced relation with the tower, while the varistor array and spark gap absorb voltage surge differentials between the line and tower.

Abstract: The invention is based on an overvoltage protection apparatus which consists of at least one varistor and a spark gap which is connected electrically in parallel with said varistor and is preferably capable of carrying lightning current. This may be an air sliding-flashover spark gap whose air flashover point is located outside its electrodes. A monitoring device (5) for the maintenance of the current/voltage characteristic (12) of the varistor and for disconnecting the varistor in the corresponding fault key is provided in the branch of the parallel circuit containing the varistor (4). Thus, even if a leakage current occurs in the varistor, the operation of the spark gap is not adversely affected, as a result of which the operation of the overvoltage protection apparatus is maintained despite this defect.

Abstract: Electrical equipment protection apparatus is provided comprising a plurality of varistor devices connected mutually in parallel, each of which exhibits a non-linear characteristic with an inherent differential in such characteristics, in combination with current sharing elements comprising a plurality of positive temperature coefficient resistors respectively associated with each of the non-linear resistors to ensure substantially equal current sharing among the plurality of varistors.

Abstract: Ceramic materials suitable for use in varistors, consisting essentially of a major proportion of SrTiO3, the balance being (1) at least one of Nb2O5, Ta2O5, WO3, La2O3 CeO2, Nd2O3, Y2O3, Sm2O3 Pr6O11, Eu2O3, and Dy2O3, for making the materials semiconductive, and (2) Na2O, for making surge-proof the varistors made from the ceramic materials. As desired, the compositions may further include one or more of Ag2O, CuO, MnO2, and SiO2 for a higher nonlinearity coefficient. Containing Na2O in proportions ranging from approximately 0.02 to 2.50 mole parts, the ceramic compositions make possible the provision of varistors suffering little from current or voltage surges in use.

Abstract: This vital solid state relay incorporates only passive elements and is intended for use as a replacement for a two element AC track relay in AC track circuits The local input signal is applied to a transformer whose secondary output, across a Zener diode, is a pulsed, square wave DC with regulated peak The track input is applied through a noise rejection filter, tuned to track circuit frequency, to a coupling transformer A series loop registry circuit includes the secondary of each transformer, a DC relay, and a varistor element The sum of the transformer output signals, due to the Zener diode rectification, has a DC component When local and track signals are both present and substantially in phase, the varistor switches to its high current condition to pick up the DC relay to indicate an unoccupied track section Absence of the track input, out of phase conditions, or unmatched frequencies retains the varistor in its low current state and the DC relay releases to register an occupied section

Abstract: A three-phase power supply has a current transformer in each power line. The transformer secondary winding is connected to a load. Generally, the load is a control means responsive to the current being sensed. A triac in parallel with the secondary winding and load has its gate connected by a voltage sensitive circuit to sense the corresponding secondary winding voltage and to conduct and protect the transformer in the event of load loss. The gate circuit includes a metal oxide varistor and a linear resistor in series connected in parallel with the secondary winding. A second linear resistor is connected to the common node of the first linear resistor and metal oxide varistor. The second linear resistor is in series with a diac and connects the gate to the common node of the varistor and first resistor. A trigger capacitor is connected between the common node of the second resistor and diac and the opposite end of the first linear resistor. On load loss, the secondary voltage rises and the varistor conducts, rapidly charging the trigger capacitor. The diac then conducts and the trigger capacitor supplies a large but controlled gate current to drive the triac "on." During the opposite half cycle, this cycle repeats until a load draws enough current to keep the secondary voltage within a critical level.

Abstract: A series string (17, 18) of plural zinc oxide varistor elements (17A-17N 18A-18N) is electrically connected across each winding (11, 12) of a power transformer, with interior winding taps (20, 22) being electrically connected to the junctions between varistor elements. Each varistor string, disposed within the transformer casing, protects its associated winding from voltage surges in the same manner as externally mounted lightning arresters, provides highly effective voltage grading, and suppresses harmful transient voltage oscillations between the winding taps.

Abstract: A zinc oxide varistor disc is provided with one or more circumferential recesses in its rim to reduce the current density at the edges of the electrodes on the opposed disc faces, which electrodes are of a lesser diameter than the varistor disc. It has been discovered that optimum results are achieved when the diameter at the bottom of the recess is somewhat less than the diameter of the electrodes.

Abstract: The invention relates to a screen with matrix access which makes use of a varistor as a switching element to control the orientation of the molecules forming a liquid crystal layer. The invention has as its object the elimination of the passive surfaces of the varistor substrate and their replacement by an insulating material having a low dielectric constant.

Abstract: The emergence of the metal-oxide varistor (MOV) has made a significant impact on station-class surge arrester technology. Areas in which MOV arresters will provide superior protection on distribution systems are examined even though the current discharge voltage characteristics of MOV arresters are the same as those of conventional surge arresters. Areas discussed include ?Severe switching areas (capacitor banks); ?High temporary overvoltage areas; ?Underground and overhead systems.

Abstract: To record significant discharges of a surge arrester, the energy dissipated in a separate varistor element equivalent to the varistor elements in the arrester and connected in the arrester discharge path is monitored. When an arrester discharge results in a temperature increase above a predetermined minimum in the separate varistor element, heat sensing means thermally coupled therewith triggers a counter to record the event.

Abstract: ZnO varistors are excellent materials for multiplexing dichroic liquid crystal displays. We describe here the development of varistor materials tailored for this application. The varistors are large (∼25 cm diameter), uniform, and have relatively low dielectric constant.

Abstract: The protection circuit has two components - a zener diode and an element having a positive temperature coefficient of resistance. The zener diode has voltage - current characteristics such that beyond a certain point, the voltage across its terminals remains stable. The two components are connected in series across the variable supply voltage, and are in intimate thermal contact. The load is connected across the zener diode terminals. The zener diode may alternatively be a varistor (VDR). This or the diode is bonded directly on to the PTC resistor or thermistor. Silicone grease may ensure a good thermal contact between the two components.

Abstract: Disclosed is a metal oxide varistor which comprises; a sintered body containing (a) ZnO as a principal component, and (b), as auxiliary components, Bi, Co and Mn in amounts of 0.05˜2 mole %, 0.05˜2 mole % and 0.05˜2 mole %, when calculated in terms of Bi2 O3, Co2 O3 and MnO2, respectively, and at least one selected from Al, In and Ga in amounts of 1×10-4 ˜3×10-2 mole %, when calculated in terms of Al2 O3, In2 O3 and Ga2 O3, respectively; said sintered material having been reheated at a temperature of 650°˜900° C. after sintering; and a non-diffusible electrode provided on said sintered body. The metal oxide varistor has excellent pulse response and volt-ampere non-linearity even with respect to a pulse having a short rise time of less than a microsecond.

Abstract: Metal-oxide varistors have a high degree of non-linearity in their voltage-current characteristic, making arrester application without gaps possible. This is a particularly significant advantage in gas-insulated substation (GIS) arrester applications. The metal-oxide varistor elements have excellent long-term stability and compatibility in an SF6 environment. Using multiple soldered columns of metal-oxide varistors, a gapless arrester assembly has been designed that has a simplified overall construction.

Abstract: A zinc oxide varistor disc is provided with a reduced cross section in the vicinity of the rim to deter the transport of fringing current in the vicinity of the edges of the electrodes which are of a lesser diameter than the varistor disc. One emobodiment comprises a varistor disc having a plurality of grooves formed circumferentially around the disc perimeter.

Abstract: VOLTAGE REDUCTION SAFETY CIRCUIT ABSTRACT A circuit is presented for coupling a storage capacitor to a capacitor shorting switch whereby voltage across the shorting switch is maintained at a reduced potential relative to the storage capacitor so that the voltage and potential current at the switch contacts is maintained at levels wherein it is impossible for the release of sufficient energy into the surrounding atmosphere to cause ignition of a flammable mixture of gas and air. This is accomplished by coupling the storage capacitor to the switching device via a variable voltage divider network controlled by an SCR with a zener diode controlled gate circuit and a varistor.

Abstract: PURPOSE:To effectively prevent the damage of a semiconductor laser due to a surge current by setting the breakdown voltage of a zener diode or ZNR (zinc oxide varistor) to more than sum of the voltages at both end of the semiconductor laser and a series resistor. CONSTITUTION:This circuit is constructed to satisfy the relationship of Vx= VL+RImax, where Imax represents the maximum allowable current, VL represents the voltage between the terminals when the current flows, and Vx represents the rising voltage of a voltage nonlinear element. When a positive impulse voltage is applied from a power source side, the rise of a zener diode 7 is abrupt in the voltage region above the Vx. Accordingly, the current flows to the zener diode 7 side, the current above the maximum allowable current will not flow at the side of a semiconductor laser 5, and the laser 5 will be protected. Since the low voltage is lower than the Vx at the ordinary operation time, the current flows only through the laser.