GE-MOV® Varistor – The Super Alpha Varistor

J.D. Harnden, F.D. Martzloff, and W.G. Morris 

General Electric Company 

Reprinted from unclassified General Electric TIS Report 72CRD260, December 1972 

Significance

Part 7 – Mitigation techniques 

A public-domain (unclassified) General Electric report aimed at the engineering community to provide 

technical information in support of the trade announcement published in Electronics (see MOV Announce

in Part 6. 

Note – 2005 perspective:

With hindsight, this report reflecting the enthusiasm at General Electric for the answer to the quest for an 

effective and economical transient suppressor in consumer products (see Evaluate 1963 SPDs in Part 7), 

is probably at the source of the unfortunate selection of device ratings that made the electronics-oriented 

“GE-MOV®” select a disc thickness for its 130-V rating according to what became a perception of “lower 

is better” for surge protection (200 V at 1 mA DC, see Figure 12 in the report). That position was abetted

by some now-forgotten trade publications emphasizing the need to limit transient overvoltages to less 

than 100 V above peak line voltage, hence the citing of a low level 330 V for the “clamping voltage” of 

130-V TVSSs in the fist edition of UL Std 1449. 
General Electric Company 

Corporate Research and Development 

Schenectady, New York 

TECHNICAL lNFORMATlOlV 

SERIES 

Today there is considerable interest and development activity in 

a new class of metal oxide varistor which exhibits a high degree of nonlinearity as compared with previous devices. Because of the metal 

oxide, the device has been named a metal oxide varistor and has been 

trademarked MOV. * This quest forTsuper" or higher performance 

is seemingly a continuous one -- whichfor example, in the case of 

transistors has seen the emergence of the Darlington and super beta 

transistor. 

The voltage across a varistor and the current through it are related 

by a power law I = k Vn. The exponent n will typically have values 25 

to 50 or more, leading to a characteristic very similar to that of a 

Zener diode. Over a wide current range, the voltage remains within 

a very narrow band for a specific device, and can be referred to as the 

"varistor voltage" for that device. The nonlinear electrical characteristic makes the device useful in voltage regulation applications, and 

in particular for limiting surges and transient voltages that may appear 

on power lines. 

Polycrystalline devices using zinc oxide have been the subject of 

much research over the past dozen or so years. Recent work on the 

zinc oxide-bismuth oxide system has shown many improvements in 

properties compared to other devices based on silicon carbide, 

selenium, etc. 

NO 

72CRD260 

hm 

3ecember 1972 

" 1 

AUTHORHarnden, JD, Jr. , Martzloff, FD 

Morris, WGt, Golden, F B+ 

Varistor 

:*Trademark of General Electric Company. 

tGE Semiconductor Products Dept., Schenectady. N. li 

$GE Semiconductor Products Dept., Auburn, N. Y. 

SUBJECT 

varistor 

- 

NO PAGES 

14 

KEY WORDS varistor, nonlinear varistor, metal oxide 

varistor, MOV, GE-MOV 

""' GE-MOV+ Varistor - - The Super Alpha 

ORlGlUATlNGPh ysics and Electrical CORPORATE 

RESEARCH AND DEVELOPMEN1 

Engineering Laboratory SCHENECTADY, N.Y. 

SUMMARY 

INFORMATION PREPARED FOR 

Additional Hard Copies Available From 

Microfiche Copies Available From 

R D-54 ( 10170) 

Corporate Research 8 Development Distribution 

P.O. Box 43 Bldg 5, Schenectady , N.Y., 12301 

Technicol Information Exchange 

P.O. Box 43 Bldg. 5, Schenectady , N.Y., 12301 
GE - MOV:;: VARISTOR --The Super Alpha Varistor 

J. D. Harnden, Jr., F. D. Martzloff, 

INTRODUCTION 

Varistors are perhaps among the oldest of the 

electrical and electronic devices. Yet today there is 

considerable interest and development activity in a 

new class of metal oxide varistor which exhibits a 

high degree of nonlinearity as compared with previous 

devices. Because of the metal oxide, the device has 

been named a metal oxide varistor and has been trademarked MOV::. This quest for "super" or higher 

performance is seemingly a continuous one -- which, 

for example, in the case of transistors has seen the 

emergence of the Darlington and super beta transistor. 

The voltage across a varistor and the current 

through it are related by a power law I = k Vn. The 

exponent n will typically have values 25 to 50 or more, 

leading to a characteristic very similar to that of a 

Zener diode. Over a wide current range, the voltage 

remains within a very narrow band for a specific 

device, and can be referred to as the "varistor voltage" 

for that device. The nonlinear electrical characteristic 

makes the device useful in voltage regulation applications, and in particular for limiting surges and transient voltages that may appear on power lines. 

Polycrystalline devices using zinc oxide have 

been the subject of much research over the past 

dozen or so years. (1) Recent work on the zinc oxidebismuth oxide system has shown many improvements 

in properties compared to other devices based on silicon carbide, selenium, etc. (2-4) 

STRUCTURE AND PROCESSING 

A preferred type of metal oxide varistor consists 

of a polycrystalline ceramic body, with metal contacts 

and wire leads applied, and suitably encapsulated 

(Fig. 1). Zinc oxide and bismuth oxide, desirable 

ingredients for a high exponent varistor, are mixed 

with other (proprietary) additives in powder form, 

pressed into disks and sintered above 1200°C. The 

bismuth oxide is molten above 825OC, assisting the 

formation of a dense polycrystalline ceramic through 

liquid-phase sintering. During cooling, the liquid 

phase forms a rigid, amorphous coating around each 

ZnO grain, giving a microstructure of ZnO grains 

isolated from one another by a thin, continuous intergranular phase. This complex, two-phase microstructure is responsible for the nonlinear characteristic 

MICROSTRUCTURE AND INTERGRANULAR PHASE 

The nature of the continuous intergranular phase 

can be seen in Fig. 2. This is a scanning electron 

::Trademark of the General Electric Company. 

tGE Semiconductor Products Dept. . Schenectady, N. Y. 

$GE SemiconductorProducts Dept.. Auburn, N. Y. 

W. G. Morris? and F. B. Golden* 

N CURRENT FLOW ELECTRODE 

Bl STABLE INTERGRANULAR 

BISMUTH OXIDE 

(INSUMTOR AT LOW VOLTAGE 

CONDUCTOR AT HIGH VOLTAGE) 

CONDUCTIVE ZINC OXIDE 

111111111 

EPOXY ENCAP SUIANT 

V WIRE LEADS 

Fig. 1 Structure of metal oxide varistor. 

Fig. 2 Scanning electron micrograph (860x1. 

micrograph at 2000X of a sintered ceramic, containing 

zinc oxide and bismuth oxide in a ratio of 50: 1. The 

zinc oxide grains have been dissolved in an ammonium 

chloride solution, revealing the amorphous phase. 

The most prominent feature is the triangular rods 

which appear at the intersection of three grain edges. 

although there are several regions where the very 

thin membrane (less than l000A) which separated 

grain faces remains intact. To understand the 

Manuscript Received 9 / 14 / 72 
electrical properties of the MOV varistor, one must 

consider not only the doping of ZnO crystals by 

bismuth ions, but also the doping of the bismuth 

oxide intergranular phase by zinc ions, and the geometrical effects of the two-phase structure. 

PROPERTIES OF ZINC OXIDE GRAINS 

The rather low resistivity, about 1 ohm-cm at 

25'32, of semiconducting zinc oxide is attributed to 

conduction electrons from ionized zinc interstitial 

atoms. The addition of monovalent ions, such as 

Na+, introduces traps, and the resistivity increases. 

Conversely, trivalent ions such as ~r+~ contribute 

additional electrons with a further reduction in resistivity. Bismuth doping of the zinc oxide grains is ' 

less than 100 ppm as determined by electron microprobe analysis. Trivalent bismuth ions would be expected to decrease the resistivity of zinc oxide, but 

in this concentration they probably have a negligible 

effect. It is sufficiently accurate to describe the zinc 

oxide grains in the MOV varistor as being n-type, 

1 ohm-cm semiconductor. 

PROPERTIES OF THE Bi203 INTERGRANULAR 

PHASE 

Unlike the abundance of material available on 

zinc oxide, very little is known about the electrical 

behavior of Bi,O,. It has a much higher resistivity, 

about 1011 ohm-cm at 25OC. but virtually nothing is 

known about the nature of the carriers and the defect 

structure. The melting point is 825°C with the liquid 

dissolving an appreciable amount of ZnO. According 

to a limited phase diagram, it forms a 6Bi20,- ZnO 

cubic compound below 800°C. Accurate analysis of 

the intergranular phase in the ZnO-Bip, ceramic is 

difficult, but most evidence points towards an amorphous structure, containing up to perhaps 20 percent 

zinc, with a resistivity much greater than the zinc 

oxide grains. 

RELATING PROPERTIES TO MICROSTRUCTURE 

Many properties of the MOV varistor can be 

directly related to the microstructure, and the properties of the two phases. At low applied voltages, 

the resulting electrical characteristics is primarily 

that attributable to the leakage current through the 

intergranular phase. The behavior is approximately 

that of an insulator at the low ranges of applied voltage, 

and the applied voltage thus results in a very high field 

across the intergranular phase and a low field in the 

grains. In the varistor range, the intergranular phase 

becomes nonlinear and current through it increases 

very rapidly as the voltage is slowly increased. The 

conduction mechanism which results in the nonlinear 

characteristic is a source of intense investigation and 

is probably space charge limited currents or tunneling. In the very high current range, the resistance of 

the intergranular layer becomes less than the ZnO 

grains, causing the V-I characteristic to once again 

tend towards linearity. Capacitance-frequency and 

capacitance-voltage curves for the ceramic body 

indicate conduction limited by hopping or tunneling in 

the thin layers, with relatively easy conduction through 

the n-type ZnO grains. 

EVAPORATED THIN-FILM EXPERIMENT 

Experiments on evaporated films of Big, on ZnO 

show that the intergranular layer is capable of nonlinear characteristic alone. Figure 3 illustrates the 

fact that a ZnO single crystal is linear, whereas the 

same crystal with an evaporated Bi,O, film shows 

nonlinearity similar to the metal oxide varistor. 

This, together with the microstructure shown in Fig. 

2, emphasizes the importance of the intergranular 

phase in order to achieve good varistor properties. 

OPERATION 

Fig. 3 V-I characteristic of zinc oxide and ZnOBi ,O, . 

PROPOSED MECHANISM - - HOPPING 

By comparing all the electrical properties measured in this work, it is proposed that the mechanism 

is a combination of electron hopping and tunneling in 

the amorphous thin layers. The specific areas of 

agreement with the hopping model are (1) a very 

slight decrease in polarizability with frequency, (2) 

a decrease in AC resistivity with frequency, ap- proaching l/f2 near 105Hz, and (3) a low activation 

energy value of less than 0. 01 ev is calculated. 

PROPOSED MECHANISM -- HOPPING AND 

TUNNELING 

Linear conduction is one feature of the hopping 

model which does not agree with the measured characteristics. Electron tunneling is a nonlinear conduction 

mechanism usually associated with a small temperature coefficient which may be considered as a series 

step in the conduction process. The estimated thickness of approximately 600A for the intergranular 

phase is, however, rather large for tunneling through 
it to take place; but if the process is assumed to consist of a series of tunneling and hopping steps, the 

highly nonlinear conduction feature of tunneling can 

be introduced. The fields calculated in the intergranular phase are approximately 10' V/cm so that 

one might expect local field concentrations sufficient 

to initiate tunneling. 

COMPARISON OF THIN-FILM PROPERTIES WITH 

SINTERED PROPERTIES 

In the evaporated thin-film experiment, it is required that the ZnO single crystal substrate temperature be above 20OoC, or that a subsequent annealing be 

done, in order to get an insulating layer. Thus, it is 

likely that diffusion of Zn into the film will result in a 

doping level comparable to the intergranular phase. 

The inherent surface roughness even on a polished 

surface results in thin regions which are more susceptible to electric breakdown. The thin-film samples show similar V-I characteristics to the sintered 

ceramics in the low current region, but generally 

fail as high localized current densities develop. 

These types of defects are less likely to occur in the 

liquid-phase sintered bodies. An important feature 

of liquid-phase sintering is the increased solubility 

in the liquid phase of regions of small radius of curvature on the surface of the grains, and the transport of 

this material to regions of negative curvature or preferred low energy faces. Thus the interfaces are 

naturally smooth and flat and more uniform conduction is possible. Liquid-phase sintering is a rather 

unsophisticated technique by modern semiconductor 

processing standards. It tends to automatically 

develop the atomically smooth interfaces and uniform 

intergranular phase required for nonlinear conduction. 

This, together with the redundancy of a large threedimensional, series-parallel network, results in the 

MOV varistor being a device which has outstanding 

nonlinear properties and remarkable stability. 

VOLTAGE RANGE, VOLTS PER MIL CONCEPT, 

AND LOWER VOLTAGE LIMIT 

Devices of various varistor voltages are feasible 

from approximately 30 volts to many kilovolts. For 

a given composition and processing, it is convenient 

to talk about volts per mil; the varistor voltage is 

proportional to the thickness of the material between 

the contacts. Indeed, it is possible to mechanically 

slice the varistor material into small pieces, and 

calculate the voltage from the thickness. There is, 

of course, a minimum divisibility which is practical 

in terms of preservation of the high exponent characteristic. This limitation begins to show itself when 

there are insufficient grain boundaries within the 

statistical electrical path remaining for the thin cross 

section. 

IDEALIZED V-I CHARACTERISTICS 

Fig. 4 Idealized V-I characteristic. 

useful characteristic of the new metal oxide varistors. 

The low current level resistance is due basically to 

the resistivity of the intergranular phase. The capacitance is due to very thin dielectric of the inner granular phase and is a fairly significant factor in the dynamic characteristic of the material. The high current characteristic is due to the basic resistivity of 

the zinc oxide grains and represents the limiting 

resistance of the device. 

ELECTRICAL CHARACTERIZATION 

The following treatment serves as an introduction rather than exhaustive treatment of the electrical properties. For more details see Refs. 5 and 6. 

Figure 5 provides an interesting composite view 

of the voltage-current relationship for silicon carbide 

varistors and MOV varistors and parametrically with 

linear resistances. The presence of linear resistor 

curves helps to highlight the great range of varistor 

operation. As pointed out earlier, the zinc oxide 

material by itself is essentially a linear resistor 

highly conducting. 

sic 

Figure 4 shows the idealized voltage versus cur- Fig. 5 Varistor and resistor log-log comparison. rent characteristic. The wide range of current covering approximately 10 orders of magnitude is a highly 
The important electrical significance of the alpha 

or exponent upon clamping ratio is shown in Fig. 6. 

Thus it will be seen, for example, with a typical alpha 

of 30 that the current can vary over a range of 10,000 

to one with a voltage change in the order of approximately 1. 36. 

I 1.1 1.2 I .3 1.4 

VOLTAGE RATIO - 

Fig. 6 Clamping ratios for different current ratios as 

a function of alpha (a ). 

As in the case with any new electronic device, 

questions regarding effects of temperature aging 

under storage and stress, plus overload conditions, 

and high-frequency properties all need to be examined 

and characterized in detail. 

Figure 7 shows the temperature effect for the 

typical VP series of MOV varistors. The material 

exhibits a slight negative voltage temperature coefficient in the order of 0. 05% to 0. 1% per "C. This 

I I I I I I I 

100 1 10 100 1 10 100 

MICROAMPERES MI?[ IAMPERES AMPERES 

Fig. 7 Typical VP series temperature effect. 

coefficient may change with composition. At very 

low current levels the coefficient is substantially 

larger, but this does not affect the normal operating 

range of the device. Storage at temperatures of 

125°C has shown no degradation (the process involves 

sintering above 1200°C and tempering at several 

hundred degrees). There is a relationship between 

maximum steady state dissipation and ambient temperature. Typical values indicate 1 watt for a disk 

with 3 cm square total surface area and a 70°C free 

convection ambient. 

Long-term stability tests under DC and AC conditions have well exceeded 10,000 hours under various 

stress levels with encouraging results. Several important stability areas being investigated include long 

term with DC bias at moderate power dissipation and 

stability with repetitive short-duration, high-energy 

pulses. The DC stability under moderate power dissipation (1 watt for 14 mm disk) has been established 

with a trend to a polarization-like mechanism whereby 

a small shift in the device voltage occurs with time. 

Under a condition of constant current in the device, 

the voltage across the device increases by a few percent and tends toward the stable asymptote. Under 

a condition of constant voltage applied to the device, 

the current drawn by the device decreases with time. 

Because of the high exponent characteristic the constant voltage test with decreasing current results in 

substantial decrease in the power dissipation in the 

device. Hence, even less current and greater 

stability. 

RATINGS 

Repetitive pulses of high energy (up to 60 joules 

in a 20 rnm disk) have also been applied, producing a 

detectable shift in the V-I characteristic with asymptotic 

trends. Most of the changes take place in the first 1 to 

10 pulses, while 90 additional pulses produce the shift 

to decreasing amplitude. The low power ratings reflect a package thermal transfer limitation rather than 

a basic limitation on the part of the varistor. 

Peak current ratings in the case of the larger 

diameter sizes have been arbitrarily limited due to 

lack of rating equipment capability. The larger 

diameter devices have twice the active area of their 

smaller counterparts. Linearly extrapolating the current rating would yield a figure of 2000 amp in place 

of the 1250 amp value of the data sheet. Although 

localized areas of nonuniformity may prevent the use 

of a linear ratio it appears the 1250 amp value is 

ultraconservative and will be upgraded in the near 

future. Likewise, the 7 ysec pulse base width is an 

arbitrary value limited by the first generation of 

rating test equipment. There are strong indications 

that the device may be capable of sustaining rated 

peak currents for base widths in the 50 to 100 psec 

range. The equivalent circuit for the MOV varistor 

is approximately indicated in Fig. 8. The typical 

value of capacitance is broken out and characterized 

in Fig. 9. In addition, the high-frequency properties 

are of interest and are shown for two production devices in Fig. 10, where resonance with the inductance 

of the leads becomes apparent. As a point of comparison a fixed point 0. 005 pF capacitor is included. 

SWITCHING SPEED 

Tests have been performed with current pulses 

of 0. 5 nsec rise time, 2 to 250 nsec pulse widths up 

to 65 amp on devices with 1.4 cm diameter. To perform this test, a disk of MOV material was placed 

in a low inductance test circuit and a current pulse 

was applied. Current in the device and voltage across 
Fig. 8 MOV varistor elementary equivalent circuit. 

it were recorded with a high-speed oscilloscope. There 

does not appear to be a "switching time" nor a dv/dt 

effect involved. When a fast front voltage surge is 

applied to the device, its capacitance will immediately 

make it appear as low impedance and, when the capacitance is fully charged, the device will merely operate 

at the point corresponding to its V-I characteristic. 

This is to be compared with some other suppression 

devices such as spark gaps which possess considerable 

variation in their protection level depending upon the 

rate at which the voltage is applied to them. 

MOV CAPACITANCE (PICOFARADS) --- 

Fig. 9 Typical capacitance. 

OBJECTIONABLE 

... . . . .: . .. . MARGINAL 

MINIMAL 

Fig. 10 Typical VP series capacitance. 

APPLICATION TO DISTRIBUTION SYSTEM PROBLEMS 

Transient surges originate from a variety of 

sources. Regardless of whether they are operating 

on AC or DC voltages, the electrical circuits are 

often plagued by voltage transients that are either 

generated within the individual circuit or transmitted 

into the circuit through external sources. With increasing sensitivity of electronic equipment and its 

widespread application, it is important to understand 

the nature of such disturbances, their source and effective methods of coping with them in order to achieve 

satisfactory levels of reliability consistent with the 

inherent properties of solid-state components. The 

most common source of transients in a distribution 

system is the L(di/dt) transient that results from 

transformer magnetizing currents as they are 

switched, either within the feeder utility system or 

within an industrial plant's own distribution system. 

Another source of concern is due to residual lightning 

surges. These residual surges can be thought of as 

the overflow on the main lightning arresters that most 

consumer, commercial, and industrial distribution 

systems are protected with at the Interface between 

the utility and the user's distribution systems. Still 

other sources will be found in homes themselves resulting from equipment that is connected to the utility 

system. Field exposure over many years and millions 

of device operating hours produced the following typical data for small synchronous line cord clocks. 

Refer to Fig. 11. With the development of high-speed 

continuously monitoring oscilloscopes, pictures of 

some of this electrical pollution sheds further light 

on the energy content and duration of such surges. 

Figure 12(a) shows the results on a log photo of an 

ELECTRIC CLOCK FIELD HISTORY 

3 YEAR PERIOD 

24 HOUR PER DAY 

- CONTINUOUS 

V) W 

LL 

3 _I I - 

2 I I 

2 KV 6 KV 

INSULATION LEVEL 

Fig. 11 Electric clock (line operated) field history. 
Fig. 12 Transient voltages in homes. 

automatic recording camera in a home installation. 

Notice that there are a number of surges within the 

time span of this recording, i. e.. 24 hours. The 

bright stripe is the 117 volts which are present at 

all times, while the transients are a composite of 

all transients photographed during a 24-hour period. 

In (b) a single transient is isolated in order to give 

a clearer view of the oscillatory nature and length of 

such a disturbance. In (c), reference is made to 

several levels of electrical voltage level stress. The 

dotted line indicates the typicalpractice of insulation 

design in which twice rated voltage is added to a 

thousand. It will be noticed that this is really insufficient in view of the oscillograms and data presented 

previously in Fig. 11 if adequate long-term stability 

is to be achieved. By incorporating a MOV device, 
it might be possible to successfully operate at a suppression or clamping level as exhibited by the dashed 

characteristic. Thus, rather than increasing the 

over-all insulation requirements to a higher level it 

would be possible through a coordinated systems design to achieve an improved product. 

The surges presented in Fig. 12 are not at all 

atypical. Indeed, with more automatic switching 

functions and complex electrical equipment in the 

home they will be found rather frequently. In fact. 

the better and newer the wiring and installation, the 

less protection is inherently provided by spillover 

occurring in poorly wired outlets, switches, etc., 

so that there will be an increase in the resulting 

impressed level on connected equipment. As more 

and more automatic controls begin to make their way 

into home applications with the upgrading of heating 

systems, cooking equipment, air conditioning--not to 

name the sophisticated electronic entertainment equipment to be found in a typical home. This gives rise to 

concern on the part of the manufacturer to adequately 

insure for the user the longevity and satisfaction that 

he is entitled to. 

Thus, the challenge of how to protect and improve 

"environment" certainly must include the electrical 

spectrum. The following Fig. 13 very graphically 

illustrates the importance of the high alpha in order 

to provide varying degrees of protection. Again, as 

a matter of convenience, the dotted and dashed bands are 

superimposed upon the 60 Hz wave with its impressed 

transient. In case 1 the assumption is made that the 

varistor shunted across the load in the presence of 

the 2000-volt transient with a surge impedance of 50 

ohms will be protecting at 400-volt level. This will 

require a steady-state power dissipation in the varistor 

since it is connected contmuously to the 117 volt rms 

line. For the silicon carbide varistor with a typical 

alpha of 3, this would result in 600 watts of steadystate dissipation. In many applications, this, of 

course, would be an unacceptable limitation and 

penalty on the total equipment. Notice that in the 

case of the metal oxide varistor with an exponent of 

-30, the dissipation is many orders of magnitude 

lower and completely satisfactory for such applications. The lower the exponent, the higher will be the 

power dissipation under steady-state conditions; and 

conversely the higher the exponent, the lower will become the standby losses. In case 2, each example 

is shown for steady-state power dissipation of 1 watt 

in the varistor bodies and the resulting clamping peak 

voltages. 

APPLICATION TO EQUIPMENT PROBLEMS 

Arcless Switching 

Arcless or black switching commutation can be 

approached by placing the varistor directly across 

switch contacts. Where explosive environments may 

be encountered such as in the oil refinery industry, 

considerable savings can be achieved by eliminating 

the threat of an arc-induced explosion. Applications 

requiring high reliability and long life will want to 

use the varistor across mechanical contacts for purposes of reducing contact erosion inherent with arcing 

contacts. This problem is especially acute when 

operating on DC. The oscillograms in Fig. 14 show 

SIC VARISTOR 10 = 3 1 

CASE I I CASE 2 I 

Fig. 13 Varistor steady state losses. 

Fig. 14 Snap action contact voltage-time oscillograms 

(a) no varistor (b) with varistor. 

recordings of the voltage generated by a snap-acting 

contact operating from a 90 volt DC supply interrupting 

an inductive load. With a MOV varistor attached externally to the contact structure, the voltage is 

virtually that of the varistor, and restriking is greatly 

reduced, as is of course the magnitude, since it is 

limited in a very effective manner. This is in direct 

contrast to the normal situation in which the energy 

appears as heating of contacts and blades. 
This application will also serve to introduce a 

new concept of integrated design. Unlike other suppressor components, this new varistor can also 

serve in a structural mode. Thus, by removing a 

typical spacer from the switch contacts, it is possible 

to provide both functions as shown in Fig. 15. 

Fig. 15 Integral use of MOV varistor in contact 

assembly. 

Start-up Transients 

Because of interwinding capacitance, step-down 

transformers when switched on can impress severe 

transients on any connected secondary as shown in 

Fig. 16. Another sometimes overlooked source of 

Fig. 16 Voltage transient due to energizing stepdown transformers. 

component failure can occur in the transistor series 

pass regulation as shown in Fig. 17. Upon turn-on. 

the capacitor appears as a short and the transistor 

is exposed to the full unregulated bus voltage. By 

placing the MOV varistor across the transistor, it 

FROM DRIVER AMPLIFIER p+77 "T E OUT E IN 

1 (UNREGULATED) (REGULATED) I1 

Fig. 17 Start up transient voltage surge protection. 

enables the circuit designer to program a soft current 

rise through the series regulator while avoiding the 

accompanying voltage surge that would be present 

during slow charging of Cl wlthout the varistor. 

Another example of semiconductor voltage protection 

shows us the simplification of component count resulting from the improved electrical properties of 

the metal oxide varistor. The small 117 volt lineoperated radio chassis (Fig. 18) requires the use of 

resistors and capacitors and high-voltage transistors 

in the output stage in order to successfully stand the 

voltage spikes which result from distortion under overloading conditions. The oscillogram for the transistor 

stress with normal resistor and capacitor suppression 

should be compared with the simplified version and 

its resulting oscillogram when the MOV alone is used 

for suppression. Note that the resulting voltage stress 

on the transistor is reduced to a significantly low value 

allowing a re-optimization of the transistor and its 

circuit components. Resulting current through the 

metal oxide varistor is also indicated. 

Protection from Color TV Kinescope Interelement 

Arcing 

Metal oxide varistors can be made rather thick 

and thus readily applied to high-voltage circuits. 

Kilovolt (per device) applications should become 

common in the near future. Such a possible application might be in television sets. At the present time, 

sparkgaps together with diode clamp circuits are often 

used for video driver transistor protection. These, 

however, suffer from sensitivity to molsture and 

dust and limited life. It is believed that by replacing 

the gap with a metal oxide varistor improved reliability 

will result. This is shown schematically in Fig. 19. 

Other potential high-voltage applications which 

can be achieved with increased power handling metal 

oxide varistor packages include high-voltage stabilization circuits, television vertical and horizontal 

output limiting and driver protection, microwave oven 

magnetron protection and x-ray generation control, 

and high-voltage AC clamping. 
7 VOLTAGE 

-m VOLTAGE 

CURRENT 

Fig. 18. Application of varistors in A-M receiver. 
COLOR 

KINESCOPE 

FROM 

i 

Y VIDEO DRIVER 

Fig. 19 Color RGB video driver protection from 

arcing kinescope. 

Typical Steps in Selecting a Metal Oxide Varistor 

At the present time, devices are available covering a range of voltage from 140 to 1400 and in continuous power ratings from 0. 5 to 1. 3 watts. The 

following guides will help to illustrate the major considerations in the proper device selection: 

Step #1: Consider the normal high-lineAC peak 

terminal voltage to be applied to the MOV varistor. 

Find the model with the closest higher peak recurrent 

voltage rating. 

Step #2: Determine or estimate the energy level 

of the transient to be suppressed. This energy level 

is usually determined by 112 L12 where switching 

transients are involved. "I" is the peak of the magnetizing current flowing in the inductance "L, " the 

field of which stores the transient energy. In the 

case of transformers, "I" may be considered as the 

peak of the exciting current. 

Step #3: Determine or estimate the current level 

of the transient to be suppressed. From step #2 the 

peak magnetizing current of feeder transformers, 

reflected to the secondary, is often used to estimate 

the peak transient current. From the specification 

sheet, determine the maximum clamping voltage that 

can be obtained. If this value is above the required 

clamping voltage, a special selection will be required. 

Step #4: Select the model from the specification 

sheet that provides the required peak recurrent 

voltage, maximum clamping voltage, and energy 

rating. The larger varistors will be needed for the 

very high energy applications. 

Step #5: Check steady-state power dissipation in 

the case of DC applications. Derate energy rating 

where power dissipation demands elevate the varistor 

case temperature beyond 85OC. 

Step #6: Where repetitive transients may be 

encountered, calculate watt-seconds per pulse and 

multiply by repetition rate to determine added power 

device must dissipate. Add to steady state idling 

power of previous step #5. 

Step #7: Review ambient environment factors 

such as operating and storage temperatures to insure 

compliance within specifications. 

HOW DO METAL OXIDE VARISTORS COMPARE 

WITH OTHER DEVICES? 

In Fig. 20 a comparison is presented for selenium 

and metal oxide varistors. Figure 21 indicates the 

peak current capability for selenium and metal oxide 

0 200 400 600 

RUS VOLTAGE RATING -VOLTS 

Fig. 20 Suppressor volume requirements. 

Fig. 21 Suppressor peak current capability. 
varistors, while Fig. 22 indicates the surge energy 

dissipation capability. Figure 23 compares a number 

of varistor devices including silicon, selenium, silicon carbide, and metal oxide varistor, and as a point 

of reference a linear ohmic resistor. Figure 24 is a 

linear plot of some of the same characteristics. 

While the log plots are to be preferred in most applications, it does represent a somewhat new form of 

presentation for electronic component users who most 

SELENIUM -, 

I 

RMS VOLTAGE RATING - vous 

Fig. 22 Suppressor energy dissipation capability. 

INSTANTANEOUS CURRENT (AMPSI 

Fig. 23 Log-log volt/amperes characteristics of 

common voltage suppressors. 

usually in the past have been accustomed to using 

linear scales. 

Figure 25 attempts to summarize in tabular form 

a variety of characteristics for some of the more 

competitive transient voltage suppressors. The 

voltage rating shown per device is the peak voltage. 

so that for rms operation the values are 1.41 lower. 

This portion of the table is not extirely consistent 

since many plates are used in a series connection for 

selenium and packaged as one to achieve higher than 

single plate voltages, whereas the other data are 

truely for single units. Series operation of MOV 

varistors is very practical. 

INSTANTANEOUS VOLTS - 

Fig. 24 Linear volt/ampere characteristics of common voltage suppressors. 

'8 nn, D.O.1 

*b CrI% ' 30 - lO.000 ," %.*Iepm=", 

Fig. 25 Comparison of transient voltage suppressors. 

FUTURE DEVICE TRENDS 

The flexibility of polycrystalline material should 

allow great versatility in meeting the great needs of 

the electronic and electrical industries. Mechanical 

adjustment of thickness by the manufacturer to provide exact voltage is an example. The ability to slice 

and dice from larger pieces should provide a flexibility in sample and prototype production while 

machining and shaping open up wide possibilities. 

A further development of a number of processes 

beyond the simple and conventional pill press process 

is very encouraging. 

Figure 26 indicates a range of two leaded encapsulated devices covering a range of voltages up 

to 1400 some of which are typical of the VP series 

commercially available. Also included in this 

photograph are samples of higher power and voltage 

devices which can be achieved through simply increasing the volume of the material. 

The next major developments will revolve 

around package changes for enhanced application 

versatility and improved heat transfer purposes. 

Packages will become available that will allow a 

variety of mounting methods for the equipment 
Fig. 26 Lead mounted MOV varistors and bulk 

varistor material. 

designer. Bolt-down capability with and without heat 

transfer capability is envisioned. This will allow 

direct fastening to chassis, brackets, bus bars, and 

heat exchangers. Furthermore, low inductance pillshaped packages that are compatible with pressuremounted thyristor packaging are being investigated. 

Finned design for direct air convection and 

forced cooling is very feasible. Initial prototypes 

a few being shown in Fig. 27 indicate 5 watt dissipation capability at 25OC ambient convection cooled 

using 1 1/2-inch-square plates and up to 50 watts for 

stud-mounted, water-cooled units (as with semiconductors electrical isolation can also be introduced 

into the package). The degree to which the application requires improved transient rating versus steadystate rating will, of course, influence the success of 

an individual approach. 

Fig. 27 Experimental higher power finned devices. 

As mentioned earlier, increasing the heat dissipating capability of the metal oxide varistor substantially improve the effective electrical characteristics 

by changing the effective clamping ratio. This is 

accomplished by increasing the design maximum 

peak idle current. When the idle current is increased 

the clamping ratio is decreased. For a single device 

the exact value of the decrease in clamping ratio for 

a fixed level of peak suppression current can be 

determined by referring to Fig. 6. For example. 

assuming an alpha of 25 and a 1 ma peak idling current, the clamping ratio of 10 amp is l. 45. Changing 

the idling current by a decade to 10 ma results in a 

clamping ratio of 1. 32 at the same peak current of 

10 amp. 

Beyond package improvements, the future holds 

promise of having varistors with alphas exceeding 35. 

Low-voltage processes (under 100 volts) should make 

them attractive for transportation and signal circuitry 

applications. 

CONCLUSIONS 

These developments coupled with the device's 

extraordinary speed should now allow the solution of 

many of the difficult problems of voltage control in 

the nanosecond and kilowatt area. A few applications 

have been cited by way of illustration, but knowingly 

to the exclusion of many others of interest such as 

semiconductor snubber networks, and storage recovery transients, industrial control circuits, instrumentation, and communications. The appended 

bibiography should be of keen interest in this regard. 

It is becoming increasingly clear that the new 

metal oxide varistor is a valuable addition to the 

power conditioning design engineers tool kit. Its 

high-energy absorption capability coupled with tight 

clamping ratios make it a superior transient suppressor. High-voltage stabilization requirements 

may also be met, using the metal oxide varistor, as 

future packaging and higher alphas evolve. Commercially, it is expected that the market for these 

new polycrystalline semiconductors will amount to 

between $20 to $30 million within 5 years. 

ACKNOWLEDGMENTS 

The authors wish to acknowledge the invaluable 

support and encouragement of their co-workers, 

T. E. Anderson, K. L. Benson, D. P. Shattuck, and 

D. M. Tasca, as well as many other colleagues in 

support of these developments. 

REFERENCES 

1. R. A. Delaney and H. D. Kaiser, J. Electrochem. 

Soc., Solid State Sci. 114, 883-842 (1967). 

2. M. Matsuoka, T. Masuyama, and Y. Iida. Suppl. 

J. Japan. Soc. Appl. Phys. 2, 94-101 (1970). 
M. Matsuoka, Japan. J. Appl. Phys. 10, 

736-746 (1971). 

W. G. Morris, "The Electrical Properties of 

ZnO - Bi,O, Ceramics, " General Electric TIS 

Report 72CRD069. 

F. B. Golden and R. W. Fox, "GE-MOV~~ 

Varistor Voltage Transient Suppressors, " 

General Electric Company, Semiconductor 

Products Dept. Application Note 200. 60. 

F. B. Golden. "A New Voltage Transient Suppressor, " Conference Record IEEE International 

Semiconductor Power Converter Conference, 

Publication 72CH0602-3- lA, pp. 2-6- 1 to 2-6-9. 

APPENDED BIBLIOGRAPHY 

Material Considerations 

G. Heiland, E. Mollwo, and F. Stockmann. "Electronic Processes in Zinc Oxide, Solid State Physics 

Series, edited by F. Seitz and D. Turnbull. Academic 

Press (1959). pp. 191-323. 

Zinc Oxide Rediscovered (100 pages), published by 

the New Jersey Zinc Co., New York (1957). 

Surge and Transient Effects on Semiconductors 

P. Chowdhuri, "A Study of Impulse Breakdown 

Phenomena of Silicon Power Rectifiers, " IEEE 

Conf. Record of IGA 197 1 6th Annual Meet ing, 

Publication 71-C1-IGA, pp. 791-798. 

B. Reich, "Protection of Semiconductor Devices, 

Circuits, and Equipment from Voltage Transients, " 

Proceedings of the IEEE. Special Issue on HighPower Semiconductor Devices, Aug. 1967, pp. 1355- 

1361. 

P. Chowdhuri, "Transient Voltage Characteristics 

of Medium Power Silicon Rectifiers," IEEE Conf. 

Record of IGA 1971 6th Annual Meeting Publication 

71-C1-IGA, pp. 781-790. 

Surge and Transient Effects on Circuits and 

Equipments 

A. Greenwood, Electrical Transients in Power 

Systems, John Wiley-Interscience. 

F. D. Martzloff and G. J. Hahn, "SurgaVoltages in 

Residential and Industrial Power Circuits, " IEEE 

Transactions on Power Apparatus and Systems. Vol. 

PAS-89, No. 6, JulyIAug. 1970, pp. 1049-1056. 

"Switching Surges, Part 111: Field and Analyzer 

Results for Transmission Lines -- Past, Present 

and Future Trends, " IEEE Transactions on Power 

Apparatus and Systems, Vol. PAS-89. No. 2. 

February 1970, pp. 173-189, IEEE Committee 

Reports. 

J. Merrett, "The Design of Lower Power Control 

Systems with Particular Reference to Transients and 

to Experience Gained with DC Motor Speed Control, " 

IEEE Proceedings -- Conf. on Power Thyristors and 

Their Applications, London, May 6-8, 1969, pp. 

161-167. 

D. B. Corbyn. "Voltage Surge Control in Thyristor 

Equipment,'' IEEE Conf. Publication, No. 17, Power 

Applications of Controllable Semiconductor Devices, 

Nov. 1965, pp. 89-101. 

Electrical Noise Effects 

A. Wavre, "Application of Integrated Circuits to 

Industrial Control Systems with High Noise Environments, I' IEEE Trans. Industry and General Applications, Vol. IGA-5, pp. 278-281, MaylJune 1969. 

W. C. Schmidt. "Electrical Noise in Control System 

Installations, I' JEEE IGA 3rd Ann. Conf. Record, 

C27-IGA, pp. 229-238, 1968. 

P. N. Budzilovich, "Electrical Noise: Its Nature, 

Causes, SoLutions," Contr. Eng., Vol. 16, pp. 74-78, 

May 1969. 

W. C. Schmidt and J. C. Oliverio, "Electrical Noise 

Reduction in Industrial Control Systems, " IEEE Trans. 

Industry and General Applications, Vol. IGA-5, 

pp. 259-265, May/June 1969. 

"Noise Effects in Control Systems, I' Machine Des.. 

pp. 146-156, Dec. 19. 1968. 

L. 0. Rexrode, "Noiseproof a Digital Electronic 

Control System, " Contr. Eng., Vol. 15, pp. 66-71, 

Nov. 1968. 

F. G. Willard, "Prevention and Treatment of Noise 

in Control Signals, " IEEE Trans. Industry and 

General Applications, Vol. IGA-5, pp. 266-272, 

MayIJune 1969. 

P. Chowdhuri, "Susceptibility of Electrical Control 

Systems to Electromagnetic Disturbances, " IEEE 

Conf. Record of IGA 1971 6th Annual Meeting, 

Publication 71-C1-IGA, pp. 155-161. 

Power Semiconductor Equipment Considerations 

J. D. Harnden, Jr. and F. B. Golden, "Power Semiconductor Applications: Volume 1 -- Basic Considerations." "Volume I1 -- Equipment and Systems,'' IEEE 

Press, 1972, Library of Congress #72-78345. 

H. A. Gauper. Jr.. J. D. Harnden. Jr.. and A. M. 

McQuarrie, IEEE Spectrum, Oct. 1971, pp. 32-43. 

SCR Manual, 5th ed., General Electric Co.. 

Semiconductor Products Dept. . Publication 

ETRC3875B. 
Bibliography. Standards, and Guides 

"Bibliography on Electomagnetic Compatibility, " 

IEEE Publication No. 72 CH0680-9 EMC. 

"Bibliography on Surge Voltages in AC Power Circuits 

Rated 600 Volts and Less, " IEEE Committee Report. 

IEEE Trans. on Power Apparatus and Systems, Vol. 

PAS-89. No. 6, July/August 1970, pp. 1056-1061. 

IEEE Guide for Transformer Impulse Tests, IEEE 

Standard No. 93, 1968. 

H. A. Gauper, Jr.. "Regulatory and Industrial Aspects 

of Power Conversion Conf. Record, IEEE Publication 34C20, pp. 109-111. 

"Surge Arrestors for AC Power Circuits." IEEE 

Standard 28-1972 (ANSlC62.1-1971). 

"Voltage Surges in Relay Control Circuits, " IEEE 

Committee Report, Paper 31 pp. 66-314, IEEE Summer Power Meeting, New Orleans, La., July 1966.