Metal-oxide varistor:

a new way to suppress transients

by J. D. Harnden Jr. and F.D. Martzloff Corporate Research and Development, General Electric Co

and W . G. Morris and F. B. Golden Semiconductor Products Department, General Electric Co.

Reprint of reprint from Electronics, October 9, 1972 (distributed by GE)

Significance

Part 6: Textbooks, tutorials and reviews

Part 7: Mitigation techniques

Trade press announcement of the introduction to the U.S. market of Metal-Oxide Varistors intended fo

incorporation in original equipment manufacturers (OEM) equipment, with discussion of the principle of

operation, background information on the origin of transients, and application information.

It is noteworthy that this announcement was released as early a October 1972, aimed at electronic OE

equipment but the corresponding announcement (with research results) of the availability of MOV-base

surge arresters for the utility market was published much later, via the IEEE Transactions paper of Mar

1977 by Sakshaug et al., “A New Concept in Station Arrester Design” – see the file “New concept” in th

annex of Part.7 of this Anthology.
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,* Corporate Research and Developmsnt, 

and W.G. Morris* and F.B. Golden,** Semiconductor Products Departmenf, General Electrlc co 

Although it belongs to the established family of re- the liquid phase forms 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 microstructure 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 amveres. I = kVn, where k is a constant. Exponent n, which is " 

Like other varistor transient suppress& the new 

varistor has a nonlinear voltage-current characteristic 

that makes it useful in voltage-regulation 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 devices. 

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, 

the essential ingredients, are mixed with proprietary 

~owdered additives, and then pressed into disks and 

CURRENT FLr 

INTERGRANULAR 

\I 

Wl RE LEAD 

/ 

ZINC OXIDE 

GRAINS 

ELECTRODE 

EPOXY 

ENCAPSULANT WIRE LEAD 

c h , VARISTOR VOLTAGE RANGE ,, 

CURRENT (AI 

iintered at a temperature geaterthan 1 ,200"C. 1. Properties. 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 voltages and as conductor for 

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

* Schenectady, N Y. zinc oxide grain. Idealized V-l curve illustrates unit's nonlinear be- 

-. Auburn, 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 current 

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 current through it increases 

rapidly as the voltage is raised slowly. The conduction 

mechanism that affects the nonlinear characteristic is now under investigation and is thought to be spacecharge-limiting or tunneling phenomena. For extremely 

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. 

Comparing transient suppressors 

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 silicon, selenium, silicon-carbide, and metal-oxide types. A pointof-reference curve for a linear ohmic resistor is also 

2. Suppressor comparison. V-l curves of various varistor transient 

suppressors and linear ohmic resistor show that characteristic for 

metal-oxide-type varistor is nearly horizontal because of ~ts high alpha (=) value. (All varistors obey power law. I = kVa.) Table compares some key specifications for several popular surge protectors. 

Shortly, clamping-voltage range will be broader for 9MOV varistors. 

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-meausquare values.) It should be noted that selenium suppressors are normally supplied as a single package of 

several series-connected 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, 

I INSTANTANEOUS CURRENT [A) I 

TYPICAL SURGE SUPPRESSOR PARAMETERS 

TRANS1 ENT SUPPRESSOR 

MOV-brand varistors 

(26.21 mm OD) 

Selenium 

(25.4 mm sq) 

Zener, 6-cell cluster 

(38 mm sq) 

Zener, single 

(DO-13 case) 

Spark gap 

(8 mm OD) 

PEAK IDLE 

CURRENT 

(mA1 

1 

12 

0.05 

0.005 

- 

MAX 

CURRENT 

- 1 ms - (A) 

65 

30 

20 

5.7 

<lo0 

PEAK 

POWER 

- 1 ms - (kW) 

18 

9 

7.7 

1.65 

50 

PEAK 

ENERGY 

- 1 ms - (joules) 

18 

9 

7.7 

1.65 

50 

EFFECTIVE 

CLAMPING 

RATIO AT 

10 A 

2.0 

2.3 

1.50 

1.65 

2.55 

(1 00 V/!4 

WEIGHT 

(grams) 

5 

35 

30 

1.5 

1.5 

VOLUME 

(cm3) 

4.4 

20 

24.5 

0.5 

0.6 

COMMERCIAL 

PER-DEVICE 

VOLTAGE RANGE 

(V) 

140- 1,400 

35-700 

14-165 

1.8-300 

150 MIN 
the necessary steady-state dissipation will be 660 W. On 

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

Examlnlng devlce 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, R,ff, 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 high-current-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 temperature 

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. Electrical behavlor. Equivalent circuit (a) of metal-oxide-type varistor contains high-resistance element to model device's insulator state 

and low-resistance element for conductor state. Temperature rise from 25'C to 100°C produces only small negative voltage shifl in V-l curve 

(b). Device capacitance (c), which is due to dieiectric/conductor intergranular phase, depends on varistor size and voltage rating. Above frequencies of about 10 megahertz, varistor impedance (d) becomes nonlinear because device capacitance increases 

(a) 

(LEAD 

INDUCTANCE) 

METAL.OXIDE-TYPE 

VARISTOR 

CURRENT (A1 

CAPACITANCE (PICOFARADS) 
one with a 130-v rms rating, and the other with a 1,000- 

v rms rating. A point-of-reference curve for a fixed 

0.005-microfarad capacitor is also included. 

When a fast-rising surge is applied to a metal-oxidetype varistor, its capacitance immediately makes it appear as a low impedance. After the capacitance becomes fully charged, the unit simply operates at the 

point predicted by its V-I characteristic. 

Tracing the origin of transients 

Transient surges originate from a variety of sources. 

Regardless of whether electrical circuits operate from ac 

or dc sources, they are often plagued by voltage transients that are either generated within the circuit itself or transmitted into the circuit from external sources. 

One of the most common sources of transients in 

power distribution systems is the L(di/dt) voltage 

caused by transformer magnetizing currents as the 

transformers are switched within either a feeder utility 

system or an industrial plant's own distribution system. 

Residual lightning surges are another source of concern. 

These surges can be thought of as an overflow on the 

main lightning arrestors with which most consumer, 

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

distribution systems. Still other sources can be found in 

homes themselves, resulting from equipment that is 

wnnected 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 connected equipment. 

Using metal-oxide-type varistors 

When a stepdown transformer is switched on, it can 

impress severe transients on any components connected 

to its secondary winding due to its interwinding capacitance. Installing a metal-oxide-type varistor, as shown 

in Fig. 4a, can eliminate this startup 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, be- cause the device can improve the electrical properties of 

the circuit in which it is installed. As an example, consider the output stage of small line-operated radio (Fig. 

4c) that requires high-voltage transistors and an associated RC network to withstand the voltage spikes generated by distortion during overload. 

The oscilloswpe traces of transistor stress show the 

voltage transient that occurs with normal RC suppression but is dramatically reduced with varistor suppression. Because of this significant transient reduction, the 

circuit can be redesigned with fewer parts. (The third 

scope trace displays varistor current.) 

Choosing the right varistor 

Selecting the correct metal-oxide-type varistor is a 

simple, logical procedure. First, find the device with a 

peak operating voltage rating that is close to, yet higher 

than, the normal peak ac-line voltage. Next, determine 

or estimate the energy level of the transient to be suppressed. This energy level is usually determined by the 

energy term, LI2/2, where I is the peak magnetizing 

current flowing in inductance L, which stores the transient energy in its field. In the case of transformers, I 

may be considered the peak exciting current. 

The expected transient current level must be found 

next. (Peak feeder transformer magnetizing current, reflected to the secondary, is often used to estimate the 

peak transient current.) Then, the varistor unit can be 

selected that has the proper ratings for recurrent voltage, clamping voltage, and energy level. 

In the case of dc applications, varistor steady-state 

sample, s&cific$$ions, 

Contact your I 

General Elect 

distributor, ECS 

or write to: 

General Electric 

Semiconductor Pr 

Building 7, Electro 

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

- - - ASSURE YOUR EQUIPMENT WILL ENDURE 
power dissipation should be 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. 

Looking 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 exchangers. Low-inductance pill-shaped packages that are 

compatible with pressure-mounted thyristors are also 

being considered. Another possibility is a finned package for direct air convection and forced cooling. 

Since metal-oxide-type varistors can be made rather 

thick, they should be available in the near future with 

kdovolt ratings. MOV-brand series VP units are currently being developed to cover the clamping voltage 

range of 30 V to 10 kv. 0 BIBLIOGRAPHY 

R A. Oeianey and H.0. Kalser. Journal of Electrochemical Society, Solid State Sciences, 

Vol. 114, pp. 883-842. 1967. 

M Matsuoka. T Masyana, and Y. Ilda. Supplementary Journal of Japanese Society of Applied Phys~cs, Vol. 39, 94-101, 1970. 

M MatSUOka. Japanese Journal of Applled Ph~lcs, Vol. lo. 736.746, 1971 

W G Morrls, The Electrical Propertin of Z~O-BI~OS Ceramics." General Electric Co , Corporate Research and Development Repon 72CRO069. 

F B. Golden, R.W. Fox, GE-MOV Vanstor Voltage Translent Suppressors," General Electric Co.. Semiconductor Products Dept . Application Note 200 60 

F B Golden, A New Voltage Transeni Suppressor," IEEE International Smiconductor 

Power Converter Conference Record, Publication 72CHO 602-3-1 A, pp. 2-6-1 to 2-6-9. 

I-ji VOLTAGE 

NDlNG 

ANCE 

1: 

It 

- 

Switching transient on step-down transformer (a) can 

y VARISTOR r-l 

be prevented from damaging components by placing varistor across 

secondary winding. In (b), varistor protects transistor in series-pass regulator circuit from posstble turn-on transient. Guarding transistor in 

radio output stage (c) with varistor permits circuit on left to be redesigned with fewer components, as shown on right. Scope traces depict 

voltage across unprotected transistor (left) and protected transistor (right), as well as varistor current (right). 

Reprinted from Electronics, October 9, 1972 Copyright 1972 by McGraw-Hill Inc. 1221 Avenue of the Americas, New York, N.Y 10020