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Metal-oxide varistor 

halts component 

burnout 
Metal-oxide varistor: 

a new way to suppress transients 

Able to withstand peak currents of hundreds of amperes, the small 

disk-shaped metal-oxide type of varistor dissipates negligible standby 

power while guarding against power-line surges and turn-on transients 

by J.D. Harnden Jr.' and F.D. Martzloff,' corp.rate Research and ~evs~opment. 

and W.G. Morris* and F.B. Golden," semiconductor ~~d~~& ~~~~~f~~~t. G~~~~~I ~i~~t~i~ co. 

0 Although it belongs to the established family of re- the liquid phase fom a rigid amorphous coating 

. liable varistor protection devices, the recently intro- around each zinc oxide grain, yielding a microstructure 

duced metal-oxide type of varistor is adding a new di- of zinc oxide grains that are isolated from each other by 

mension to the technology of protecting circuits and a thin continuous intergranular phase. It is this complex 

components. Trademarked as the GE-MOV and Mov two-phase microstruchlre that is responsible for the 

varistor by General Electric Co., the metal-oxide-type nonlinear characteristic 

varistor offers the advantages of nanosecond switching The voltage across a metal-oxide-type varistor and 

speeds and small size, while being able to handle cur- the current through it are related by the power law, 

rent surges on the order of hundreds of am~eres. I = kVn, where k is a constant. Exponent n, which is 

Like lother varistor transient suppressbrs, the new 

varistor has a nonlinear voltage-current characteristic 

that makes it useful in voltageregulation applications. 

And because its nonlinear V-I curve is very steepsteeper than that of most other varistors-it can pass 

widely varying currents over a narrow voltage range. In 

some applications, using the device allows a circuit to be 

redesigned with fewer components. 

At low applied voltages the metal-oxide-type varistor 

looks like an open circuit because its unique two-phase 

material assumes the properties of an insulator. When 

applied voltage exceeds rated clamping voltage, the device effectively becomes a short circuit, protecting the 

component that it shunts. 

The unit, moreover, requires very little standby 

power, making it useful for guarding semiconductors. 

Steady-state power dissipation is typically a fraction of 

a milliwatt, as compared to the hundreds of watts dissipated by some other varistor dences 

At present, operating voltage ratings for MOV-brand 

series VP varistors range from 140 to 1,400 volts peak, 

watt-second ratings from 10 to 160 joules, and continuous power ratings range from 0.5 watt to 1.3 w. Units 

are priced from less than $1 to $14 in 1,000-unit lots. 

An inside look 

The metal-oxide-type varistor (Fig. 1) has an encapsulated polycrystalline ceramic body, with metal 

contacts and wire leads. Zinc oxide and bismuth oxide, 

,- ELECTRODE fly LEAD 

ZINCOXIDE 

GRAINS 

ELECTRODE 

WIRE LEAD 

- 

2 1000 .a 

0 

-a too 

5 

0 10 > 10" to-" to 1 

CURRENT IAI the essential ingredients, are mixed with proprietary 

qowdered additives, and then pressed into disks and 

slntered at a temperature greater than 1,200"C. 

L 

1. Pr ope*. Two-phase material in body of metal-oxide-type varBecause the bismuth oxide is molten above 825'C, it istor acts as insulator for low applied Mlt~gff and as conductor for 

assists in the formation of a dense polycrystalline ce- transients mat exceed device's clamping wltage. Bistable interramic through liquid-phase sintering. During cooling, granular phase containing bismuth oxae surrounds each conductive 

zinc oxide grain. Idealized V-1 curve illustrates unil's nonlinear be- 

'Schenecfady. N I. 

~~blm. N.Y. havior in varistor voltage range. 
also referred to as alpha (a), typically has a value between 25 and 50 or more, leading to the idealized V-I 

curve of Fig. 1. Over a wide current range, the voltage 

remains within a narrow band that is commonly called 

the varistor voltage. 

Many properties of the varistor can be directly related to the microstructure and the properties of the two 

phases. As shown in Fig. 1, the unit's idealized V-I 

curve consists of two linear segments for extremely low 

and extremely high currents, and a nonlinear segment 

in the varistor voltage range. 

At low applied voltages, the device's linear characteristic can be attributed primarily to leakage cur~ent 

through the intergranular phase. Device behavior is 

roughly that of an insulator, indicating that low applied 

voltages cause a high electric field across the intergranular phase and a low field within the zinc oxide 

grains. 

. In the varistor voltage range, the intergranular phase 

becomes nonlinear, and wrent through it inaeases 

rapidly as the voltage is raised slowly. The conduction 

mechanism that &ects the nonlinear characteristic is 

now under investieation and is thoueht to be soacecharge-limiting or t&nel'ig phenome&. For ex&mely 

high currents, the resistance of the intergranular layer 

becomes less than that of the zinc oxide grains, causing 

the V-I curve to tend towards linearity again. 

shown. The higher a device's alpha is, the better is its 

voltage suppression capability. 

The parameters of some of the most widely used transient voltage suppressors are summarized in the table of 

Fig. 2. (The commercial voltage ratings reflect peak values; they must be divided by 1.41 to obtain root-meansquare values.) It should be noted that selenium sup 

pressors are normally supplied as a single package of 

several seriesconnected plates, so that the selenium 

voltage rating is not that of a single plate. The other 

varistor data, however, reflects single-unit ratings. 

Metal-oxide-type varistors can also be connected in 

series to increase clamping-voltage rating. Moreover, 

they can serve in a structural mode as well as an electrical one, for instance, by replacing the spacer between 

switch contacts to allow approaching arcless switching 

commutation, especially in dc applications. 

One of the most important considerations when 

choosing a protection device is its steady-state power 

dissipation. In the case of varistors, the higher the alpha 

or exponent, the lower will be the standby loss. For example, if a varistor is shunted across a load, protecting 

it at a 300-v level in the presence of a 2,000-v transient 

with a surge impedance of 10 ohms, a steady-state 

power dissipation is required of the varistor, since it is 

connected continuously to the 117-V rms line. For the 

silicon-carbide varistor, which has a typical alpha of 3, 

Comparing lranslent wppcaosors 

A comparison of the volt-ampere characteristic of the 

metal-oxide-type varistor to that of other voltage suppressors yields the graph shown in Fig. 2. A number of 

varistor devices are represented, including siliam, selenium, silicon-carbide, and metal-oxide types. A pointof-reference curve for a linear ohmic resistor is also 

2. -.or -r(.on. V-l curves d various varistor mnsient 

suppressors and linear ohmic resistw show that characterisbic for 

metal-mae-type varistor is nwiy horizontal because of its high a!- 

pha (-) due. (All varistors - power law. I = kW.) Taw corn 

pares sane key specifications fw several poprlar surge protectors. 
the necessary steady-state dissipation will be 6154 w. On 

the other hand, a metal-oxide-type varistor with an exponent of 30 will dissipate only 0.1 milliwatt. 

Exarnlning device behavior 

Fig. 3a shows the simplified equivalent circuit for the 

metal-oxide-type varistor, as well as its schematic symbol; representative capacitance and resistance values 

are used. The low-current-level resistance, &,, is due 

primarily to the resistivity of the intergranular phase. 

Capacitance C can be attributed to the very thin dielectric of the intergranular phase; it becomes a fairly 

significant factor for the varistor's dynamic characteristic. The highcnrrent-level resistance, &, is due to the 

intrinsic resistivity of the zinc oxide grains; it represents 

the unit's limiting resistance. The component values in 

the figure are for a typical MOV-brand series VP device. 

As indicated in Fig. 3b, the device's V-I curve ordinarily exhibits a slightly negative voltage temperatwe 

coefficient, in the order of O.Ol%/"C to 0.05%/'C for 

the series VP varistors. At very low current levels, this 

coefficient is substantially larger but does not affect the 

normal operating range. Maximum steady-state power 

dissipation for a disk with 3 square centimeters of total 

surface area is usually 1 w in a 70°C free-convection 

ambient environment. 

Typical capacitance curves are depicted in Fig. 3c for 

units with differing diameters. Device capacitance influences high-frequency impedance properties, as 

shown by the plots of Fig. 3d for two production units3. El.S1I1C.I bdu~vior. Eouivalem circua (a) . . . of meat-oxae-Woe . varistor contains hioh-resistance - element to model device's irsulatw sfate 

and low-resisrence elemem for wnductor stale. Temperature rise from 25% to 100% produces only small negative voltage shin in V-l cuNe 

.(b). ~. . Device capacitance (c), which is due to dielectriclwnductor intergranular phase. depends on varistor size and WMltage rating. Abwe Ire- . . 

quencies of about 10 megahertz, varistor impedance (d) becomes nonlinear because device capacitance increws 

ILEAD 

INDUCTANCE) 

METAL-OXIDE-TYPE 

VARISTOR 

CURRENT (A) 

CAPACITANCE (PICOFARADS) 

1 10 100 

FREQUENCY (MHz) 
one with a 130-v rms rating, and the other with a 1,000- sion but is dramatically reduced with varistor suppresv rms rating. A point-of-reference curve for a fixed sion. Because of this significant transient reduction, the 

0.005-microfsad ipacitor is also included. circuit can be ralesig& with fewer parts. (The third 

When a fast-rising surge IS applied to a metal-oxide- scope trace displays varistor current.) - - type varistor, its cGacit&ce immediately makes it appear as a low impedance. Atter the capacitance be- Choosing the right varistor 

comes fully charged, the unit simply operates at the Selecting the correct metalnxide-type varistor is a 

point predicted by its V-I characteristic. simple, logical procedure. First, find the device with a 

peak operating voltage rating that is close to, yet higher Tracing the origin d transients than, the normal peak ac-line voltage. Next, determine 

Transient surges originate from a variety of sources. or estimate the energy level of the transient to be sup 

Regardless of whether electrical circuits operate from ac pressed. This energy level is usually determined by the 

or dc sources, they are oRen plagued by voltage tran- energy term, L12/2, where I is the peak magnetizing 

sients that are either generated within the circuit itself current flowing in inductance L, which stores the tranor transmitted into the circuit from external sources. sient energy in its field. In the case of transformers, I 

One of the most common sources of transients in may be considered the peak exciting current. 

power distribution systems is the L(di/dt) voltage The expected transient current level must be found 

caused by transformer magnetizing currents as the next (Peak feeder transformer magnetizing current, retransformers are switched within either a feeder utility flected to the secondary, is offen used to estimate the 

system or an industrial plant's own distribution system peak transient current.) Then, the varistor unit can be 

Residual lightning surges are another source of concern. selected that has the proper ratings for recurrent voltThese surges can be thought of as an overtlow on the age, clampingvoltage, and energy level. 

main lightning arrestors with which most consumer, In the case of dc applications, varistor steady-state 

commercial, and industrial distribution systems are protected at the interface between the utility and the user's 

distribution svstems. Still other sources can be found in 

homes themklves, resulting from equipment that is 

comected to the utility system. 

Even a small synchronous line clock in the home can 

be subjected to a number of surges within just a 24-hour 

period. In fact, with the introduction of more automatic 

switching functions and complex electrical equipment 

into the home, line surges are becoming more frequent 

And oddly, the better and newer the wiring and installation, the less inherent protection is provided by spillover occurring in poorly wired outlets, switches, and fixtures, and the greater the resulting impressed voltage 

level on comected equipment. 

Using metaloxide-type varistors 

When a steodown transformer is switched on. it can 

impress severe' transients on any components connected 

to its secondan, winding due to its interwindie ca~acitance. lnstallidg a me&-oxide-type varistor, sliown 

in Fig. 4a, can eliminate this stamp transient 

Another source of component failure, which is sometimes overlooked, can occur in the conventional transistor series-pass voltage regulator (Fig. 4b). When the circuit is turned on, the capacitor appears to be a short 

circuit, and the transistor is exposed to the full unregulated bus voltage. Placing a metal-oxide-type varistor 

across the transistor allows a soft current rise to pass 

through the regulator without the usual voltage surge. 

Semiconductors can be protected with the new varistor, resulting in a design with fewer components, because the device can improve the electrical properties of 

the circuit in which it is installed. As an example, wnsider the out~ut staee of small line-onerated radio Wig. 

FREE 

SAMPLE 

KIT 

4c) that req&es hi&-voltage transisiors and an akY- 1- 

Contact General your EIe~tr~~i$$~ctor lo a1 aut rized 

distributor, ECS gales Ice 

or write to: 

General Electric c?;ri& Semiconductor Pro cts D t. 

Building 7, Electroni 

Syracuse, N.Y., U.S.A. 13201 

ated RC nitwork tiwithstaid the voltage spikes generated by distortion during overload. 

The oscilloscwe traces of transistor stress show the TM VARlsTon 
power dissipation should he checked. Be sure to derate 

the device's energy rating if power dissipation demands 

will elevate the varistor case temperature above 85°C. 

For applications where repetitive transients may be encountered, calculate the expected watt-seconds per 

pulse and multiply this figure by the pulse repetition 

rate to determine the additional steady-state power dissipation required. And finally, make certain that the 

unit can comply with such ambient environmental factors as operating and storage temperatures. 

Looklng ahead 

The next major developments for MOV-brand varistors will revolve around package changes to enhance 

applications versatility and improve heat transfer. Future packages will be available in a variety of mounting 

schemes-for example, a bolt-down version with or one without heat-transfer capability that could be directly 

fastened to chassis, brackets, bus bars, and heat ex- change~. LOW-inductance pill-shaped that are 

compatible with pressure-mounted thyristors are also 

beine considered.Another oossibilitv <s a finned nackage 61 direct air convection'and for&d cooling. 

Since metal-oxide-type varistors can be made rather 

thick, they should be available in the near future with 

kilovolt ratin~s. MOV-brand series VP units are curINTERWINDING 

VOLTAGE SWITCH 

CLOSED 

INTERWINDING 

CAPACITANCE 

VARISTOR 

/- 

TRANSIENT cKz$RY 

(.I 

6 VARISTOR 

- 

4. AwUuUons. Switching transient on stepdown transformer (a) can 

L 

be prevented from damaging components by placing varistor across 

secondary winding. In (b), varistor protects transistor in seriespass regulator circua from possible turns transient. Guarding transistor in 

radio outpuf stage (c) wah MristOr permls circuit on left to be redesigned wim fewer components, as shown on right. Scope traces depict 

vdtage across unprotected transistor (I&) and protected transistor (right), as weU as MriSmr current (right). 

Copyright 1972 by McGras-Hill Inc. 1221 Avenue ol the Arnerlcar. New Yon. N.I. ,0320