AN9768 [LITTELFUSE]

Transient Suppression Devices and Principles; 瞬态抑制器件和原理


型号：	AN9768
厂家：	LITTELFUSE
描述：	Transient Suppression Devices and Principles 瞬态抑制器件和原理瞬态抑制器
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Transient Suppression Devices and Principles

Application Note

January 1998

AN9768

Transient Suppression Devices

Z = 1Ω

1000

100

There are two major categories of transient suppressors: a)

those that attenuate transients, thus preventing their

propagation into the sensitive circuit; and b) those that divert

transients away from sensitive loads and so limit the residual

voltages.

NONLINEAR Z (α = 5)

Z = 10Ω

[ /Title

(AN97

68)

/Sub-

ject

(Tran-

sient

Sup-

Attenuating a transient, that is, keeping it from propagating

away from its source or keeping it from impinging on a

sensitive load is accomplished with ﬁlters inserted in series

within a circuit. The ﬁlter, generally of the low-pass type,

attenuates the transient (high frequency) and allows the signal

or power ﬂow (low-frequency) to continue undisturbed.

0.01

0.1

100

1000

CURRENT (A)

Diverting a transient can be accomplished with a

voltage-clamping type device or with a “crowbar” type

device. The designs of these two types, as well as their

operation and application, are different enough to warrant a

brief discussion of each in general terms. A more detailed

description will follow later in this section.

LINEAR IMPEDANCE: I =

pres-

sion

NONLINEAR IMPEDANCE (POWER LAW): I = KV

FIGURE 1. VOLTAGE/CURRENT CHARACTERISTIC FOR A

Device

s and

Princi-

ples)

/Autho

r ()

/Key-

words

(TVS,

Tran-

sient

Sup-

pres-

sion,

Protec-

tion,

High-

reli-

LINEAR 1Ω RESISTOR AND NONLINEAR

A voltage-clamping device is a component having a variable

impedance depending on the current ﬂowing through the

device or on the voltage across its terminal. These devices

exhibit a nonlinear impedance characteristic that is, Ohm’s

law is applicable but the equation has a variable R. The

variation of the impedance is monotonic; in other words, it

does not contain discontinuities in contrast to the crowbar

device, which exhibits a turn-on action. The volt-ampere

characteristic of these clamping devices is somewhat

time-dependent, but they do not involve a time delay as do

the sparkover of a gap or the triggering of a thyristor.













---------------------

+ Z

FIGURE 2A. VOLTAGE CLAMPING DEVICE

With a voltage-clamping device, the circuit is essentially

unaffected by the presence of the device before and after the

transient for any steady-state voltage below the clamping

level.The voltage clamping action results from the increased

current drawn through the device as the voltage tends to

rise. If this current increase is greater than the voltage rise,

the impedance of the device is nonlinear (Figure 1). The

apparent “clamping” of the voltage results from the

increased voltage drop (IR) in the source impedance due to

the increased current. It should be clearly understood that

the device depends on the source impedance to produce the

clamping. One is seeing a voltage divider action at work,

where the ratio of the divider is not constant but changes.

However, if the source impedance is very low, then the ratio

is low. The suppressor cannot be effective with zero source

impedance (Figure 2) and works best when the voltage

divider action can be implemented.

SCR

ability,

High

Reli-

ability,

Mil,

FIGURE 2B. CROWBAR DEVICE

FIGURE 2. DIVISION OFVOLTAGEWITHVARIABLE

IMPEDANCE SUPPRESSOR

10-102

Application Note 9768

Crowbar-type devices involve a switching action, either the

A Simpliﬁed Comparison Between

Protection with Linear and Nonlinear

Suppressor Devices

breakdown of a gas between electrodes or the turn-on of a

thyristor, for example. After switching on, they offer a very

low impedance path which diverts the transient away from

the parallel-connected load.

Assume an open-circuit voltage of 3000V (see Figure 2):

1. If the source impedance is Z = 50Ω

These types of crowbar devices can have two limitations.

One is delay time, which could leave the load unprotected

during the initial transient rise. The second is that a power

current from the voltage source will follow the surge

discharge (called “follow-current” or “power-follow”). In AC

circuits, this power-follow current may not be cleared at a

natural current zero unless the device is designed to do so;

in DC circuits the clearing is even more uncertain. In some

cases, additional means must be provided to “open” the

crowbar.

With a suppressor impedance of Z = 8Ω

The expected current is:

3000

50 + 8

---------------

1 =

= 51.7A and V = 8 × 51.7 = 414V

The maximum voltage appearing across the terminals of a

typical nonlinear V130LA20A varistor at 51.7A is 330V.

Note that:

× I = 50 × 51.7 = 2586V

× I = 8 × 51.7 = 414V

= 3000V

Filters

The frequency components of a transient are several orders

of magnitude above the power frequency of an AC circuit

and, of course, a DC circuit.Therefore, an obvious solution is

to install a low-pass ﬁlter between the source of transients

and the sensitive load.

2. If the source impedance is only 5Ω (a 10:1 error in the as-

sumption), the voltage across the same linear 8Ω sup-

pressor is:

5 + 8

= 3000 ------------ = 1850V

The simplest form of ﬁlter is a capacitor placed across the

line. The impedance of the capacitor forms a voltage divider

with the source impedance, resulting in attenuation of the

transient at high frequencies. This simple approach may

have undesirable side effects, such as a) unwanted

resonances with inductive components located elsewhere in

the circuit leading to high peak voltages; b) high inrush

currents during switching, or, c) excessive reactive load on

the power system voltage. These undesirable effects can be

reduced by adding a series resistor hence, the very popular

use of RC snubbers and suppression networks. However,

the price of the added resistance is less effective clamping.

However, the nonlinear varistor has a much lower

impedance; again, by iteration from the characteristic curve,

try 400V at 500A, which is correct for the V130LA20A; to

prove the correctness of our “educated guess” we

calculate I,

x I = 5 x 520 = 2600V

3000-400V

I =

= 520A

400V

= 3000V

which justiﬁes the “educated guess” of 500A in the circuit.

Summary

Beyond the simple RC network, conventional ﬁlters

comprising inductances and capacitors are widely used for

interference protection. As a bonus, they also offer an

effective transient protection, provided that the ﬁlter's front-

end components can withstand the high voltage associated

with the transient.

TABLE 1. 3000V “OPEN-CIRCUIT”TRANSIENT VOLTAGE

ASSUMED SOURCE IMPEDANCE

50Ω

5Ω

PROTECTIVE DEVICE

Linear 8Ω

PROTECTIVE LEVEL ACHIEVED

414V

330V

1850V

400V

There is a fundamental limitation in the use of capacitors

and ﬁlters for transient protection when the source of

transients in unknown. The capacitor response is indeed

nonlinear with frequency, but it is still a linear function

of current.

Nonlinear Varistor

Similar calculations can be made, with similar conclusions,

for an assumed error in open-circuit voltage at a ﬁxed source

impedance. In that case, the linear device is even more

sensitive to an error in the assumption. The calculations are

left for the interested reader to work out.

To design a protection scheme against random transients,

it is often necessary to make an assumption about the

characteristics of the impinging transient. If an error in the

source impedance or in the open-circuit voltage is made in

that assumption, the consequences for a linear suppressor

and a nonlinear suppressor are dramatically different as

demonstrated by the following comparison.

The example calculated in the simpliﬁed comparison

between protection with linear and nonlinear suppression

devices shows that a source impedance change from an

assumed 50Ω to 5Ω can produce a change of about 414V to

1850V for the protective voltage of a typical linear

suppressor. With a typical nonlinear suppressor, the

10-103

Application Note 9768

corresponding change is only 330V to 400V. In other words,

Zener Diodes - Silicon rectiﬁer technology, designed for

transient suppression, has improved the performance of

regulator-type Zener diodes. The major advantage of these

diodes is their very effective clamping, which comes closest

to an ideal constant voltage clamp.

a variation of only 21% in the protective level achieved with a

nonlinear suppressor occurs for a 10 to 1 error in the

assumption made on the transient parameters, in contrast to

a 447% variation in the protective level with a linear

suppressor for the same error in assumption. Nonlinear

voltage-clamping devices give the lowest clamping voltage,

resulting in the best protection against transients.

Since the diode maintains the avalanche voltage across a

thin junction area during surge discharge, substantial heat is

generated in a small volume.The major limitation of this type

of device is its energy dissipation capability.

Crowbar Devices

This category of suppressors, primarily gas tubes or carbon-

block protectors, is widely used in the communication ﬁeld

where power-follow current is less of a problem than in

power circuits. Another form of these suppressors is the

hybrid circuit which uses solid-state or MOV devices.

Silicon Carbide Varistors - Until the introduction of metal-

oxide varistors, the most common type of “varistor” was

made from specially processed silicon carbide.This material

was very successfully applied in high-power, high-voltage

surge arresters. However, the relatively low a values of this

material produce one of two results. Either the protective

level is too high for a device capable of withstanding line

voltage or, for a device producing an acceptable protective

level, excessive standby current would be drawn at normal

voltage if directly connected across the line. Therefore, a

series gap is required to block the normal voltage.

In effect, a crowbar device short-circuits a high voltage to

ground. This short will continue until the current is brought

to a low level. Because the voltage (arc or forward-drop)

during the discharge is held very low, substantial currents

can be carried by the suppressor without dissipating a

considerable amount of energy within it. This capability is a

major advantage.

In lower voltage electronic circuits, silicon carbide varistors

have not been widely used because of the need for using a

series gap, which increases the total cost and reproduces

some of the characteristics of gaps described earlier.

However, this varistor has been used as a current-limiting

resistor to assist some gaps in clearing power-follow current.

Volt-Time Response - When the voltage rises across a spark

gap, no signiﬁcant conduction can take place until transition

to the arc mode has occurred by avalanche breakdown of

the gas between the electrodes.

Power-Follow - The second characteristic is that a power

current from the steady-state voltage source will follow the

surge discharge (called “follow-current” or “power-follow”).

Metal-Oxide Varistors - A varistor functions as a nonlinear

variable impedance. The relationship between the current in

the device, I, and the voltage across the terminals, V is

Voltage-Clamping Devices

typically described by a power law: I = kV . While more

To perform the voltage limiting function, voltage-clamping

devices at the beginning of the section depend on their

nonlinear impedance in conjunction with the transient source

impedance. Three types of devices have been used: reverse

selenium rectiﬁers, avalanche (Zener) diodes and varistors

made of different materials, i.e., silicon carbide, zinc oxide,

etc. [1].

accurate and more complete equations can be derived to

reﬂect the physics of the device, [2, 3] this deﬁnition will

sufﬁce here. A more detailed discussion will be found in

Application Note AN9767, “Littelfuse Varistors - Basic

Properties, Terminology and Theory”.

The term α (alpha) in the equation represents the degree of

nonlinearity of the conduction. A linear resistance has an

α = 1. The higher the value of a, the better the clamp, which

explains why α is sometimes used as a ﬁgure of merit. Quite

naturally, varistor manufacturers are constantly striving for

higher alphas.

Selenium Cells - Selenium transient suppressors apply

the technology of selenium rectiﬁers in conjunction with a

special process allowing reverse breakdown current at high-

energy levels without damage to the polycrystalline

structure. These cells are built by developing the rectiﬁer

elements on the surface of a metal plate substrate which

gives them good thermal mass and energy dissipation

performance. Some of these have self-healing

characteristics which allows the device to survive energy

discharges in excess of the rated values for a limited number

of operations characteristics that are useful, if not “legal” in

the unsure world of voltage transients.

This family of transient voltage suppressors are made of

sintered metal oxides, primarily zinc oxide with suitable

additives. These varistors have α values considerably

greater than those of silicon carbide varistors, typically in the

range of an effective value of 15 to 30 measured over

several decades of surge current.

The high exponent values (α) of the metal-oxide varistors

have opened completely new ﬁelds of applications by

providing a sufﬁciently low protective level and a low standby

current. The opportunities for applications extend from low-

power electronics to the largest utility-type surge arresters.

The selenium cells, however, do not have the clamping

ability of the more modern metal-oxide varistors or

avalanche diodes. Consequently, their ﬁeld of application

has been considerably diminished.

10-104

Application Note 9768

dramatic increase in the standby power. Figure 4 shows that

Transient Suppressors Compared

for a zener-diode suppressor, a 10% increase above rated

voltage increases the standby power dissipation above its

rating by a factor of 30. But for a low-alpha device, such as

silicon carbide, the standby power increases by only 1.5

times.

Because of diversity of characteristics and nonstandardized

manufacturer speciﬁcations, transient suppressors are not

easy to compare. A graph (Figure 3) shows the relative volt-

ampere characteristics of the four common devices that are

used in 120V AC circuits. A curve for a simple ohmic resistor

is included for comparison. It can be seen that as the alpha

factor increases, the curve's voltage-current slope becomes

less steep and approaches an almost constant voltage. High

alphas are desirable for clamping applications that require

operation over a wide range of currents.

α = 35

It also is necessary to know the device energy-absorption

and peak-current capabilities when comparisons are made.

Table 2 includes other important parameters of commonly

used suppressors.

SILICON CARBIDE

0.5

Standby Power - The power consumed by the suppressor

unit at normal line voltage is an important selection criterion.

Peak standby current is one factor that determines the

standby power of a suppressor. The standby power

dissipation depends also on the alpha characteristic of the

device.

α = 25

α = 8

α = 4

0.2

0.1

100

102

104

106

108

110

PERCENT OF RATED VOLTAGE

FIGURE 4. CHANGES IN STANDBY POWER ARE

CONSIDERABLY GREATER WHEN THE

SUPPRESSOR'S ALPHA IS HIGH

As an example, a selenium suppressor in Table 2 can have a

12mA peak standby current and an alpha of 8 (Figure 3).

Therefore, it has a standby power dissipation of about 0.5W

Typical volt-time curves of a gas discharge device are shown

in Figure 5 indicating an initial high clamping voltage. The

gas-discharge suppressor turns on when the transient pulse

exceeds the impulse sparkover voltage. Two representative

surge rates 1kV/µs and 20kV/µs are shown in Figure 5.

When a surge voltage is applied, the device turns on at

some point within the indicated limits. At 20kV/µs, the

discharge unit will sparkover between 600V and 2500V. At

1kV/µs, it will sparkover between 390V and 1500V.

on a 120V

line (170V peak). A zener-diode suppressor

RMS

has standby power dissipation of less than a milliwatt. And a

silicon-carbide varistor, in a 0.75” diameter disc, has standby

power in the 200mW range. High standby power in the lower

alpha devices is necessary to achieve a reasonable

clamping voltage at higher currents.

SILICON CARBIDE VARISTOR

6000

5000

4000

(α ≈ 5)

1000

800

3000

SELENIUM 2.54cm

(1") SQ (α ≅ 8)

2000

500

400

1000

800

300

600

500

400

200

100

LITTELFUSE VARISTOR

(20mm DIA.)

SILICON POWER

TRANSIENT SUPPRESSOR

(ZENER) (α ≅ 35)

(α > 25)

300

20 30 40 50

80 100

300

9.5mm OD, 230V

GAS-DISCHARGE SUPPRESSOR

INSTANTANEOUS CURRENT (A)

200

-9

-8

-7

-6

-5

-4

-3

FIGURE 3. V-I CHARACTERISTIC OF FOURTRANSIENT

SUPPRESSOR DEVICE

SHORT-TIME SURGE RESPONSE (S)

FIGURE 5. IMPULSE BREAKOVER OF A GAS-DISCHARGE

DEVICE DEPENDS UPON THE RATE OF

VOLTAGE RISE AS WELL AS THE ABSOLUTE

VOLTAGE LEVEL

The amount of standby power that a circuit can tolerate may

be the deciding factor in the choice of a suppressor. Though

high-alpha devices have low standby power at the nominal

design voltage, a small line-voltage rise would cause a

10-105

Application Note 9768

TABLE 2. CHARACTERISTICS AND FEATURES OF TRANSIENT VOLTAGE SUPPRESSOR TECHNOLOGY

ENERGY

RE-

DEVICE

TYPE

LEAK- FOLLOW CLAMPING CAPABIL- CAPACI- SPONSE

V-I CHARACTERISTICS

AGE

ON I

VOLTAGE

ITY

TANCE

TIME

COST

Ideal Device

Zero

Low

High

Low

Fast

Low

High

CLAMPING VOLTAGE

WORKING VOLTAGE

TRANSIENT CURRENT

Zinc Oxide

Varistor

Low

ModerateTo

Low

High

Low

Moderate

High

Fast

Low

WORKING

VOLTAGE

Zener

Low

High

MAX I LIMIT

WORKING

VOLTAGE

Crowbar

(Zener - SCR

Combination)

Low

Yes

(Latching

Holding I)

Medium

Fast

Moderate

PEAK VOLTAGE

(IGNITION)

WORKING

VOLTAGE

Spark

Gap

Zero

Yes

High

Ignition

Voltage

High

Low

Slow

Low

High

PEAK VOLTAGE

(IGNITION)

WORKING

VOLTAGE

Low Clamp

Triggered

Spark

Gap

Zero

Yes

Lower

Ignition

Voltage

High

Low

Moderate Moderate

PEAK VOLTAGE

(IGNITION)

WORKING

VOLTAGE

Low

Clamp

Selenium

Very

High

Moderate

High

Moderate

High

Fast

High

WORKING

VOLTAGE

Silicon

Carbide

Varistor

High

Low

WORKING

VOLTAGE

10-106

Application Note 9768

The gas discharge device may experience follow-current. As

At 1ms, the two devices are almost the same. At 2µs the

varistor is almost 10 times greater, 7kW for the P6KE 6.8

Zener vs 60kW for the varistor V8ZA2.

the AC voltage passes through zero at the end of every half

cycle the arc will extinguish, but if the electrodes are hot and

the gas is ionized, it may reignite on the next cycle.

Depending on the power source, this current may be

sufﬁcient to cause damage to the electrodes. The follow

current can be reduced by placing a limiting resistor in series

with the device, or, selecting a GDT speciﬁcally designed for

this application with a high follow-current threshold.

Clamping Voltage

Clamping voltage is an important feature of a transient

suppressor. Zener diode type devices have lower clamping

voltages than varistors. Because these protective devices

are connected in parallel with the device or system to be

protected, a lower clamping voltage can be advantageous in

certain applications.

The gas discharge device is useful for high current surges

and it is often advantageous to provide another suppression

device in a combination that allows the added suppressor to

protect against the high initial impulse. Several hybrid

combinations with a varistor or avalanche diode are

possible.

VARISTOR

ZENER

Comparison of Zener Diode and Littelfuse

Varistor Transient Suppressors

CURRENT

Peak Pulse Power

Transient suppressors have to be optimized to absorb large

amounts of power or energy in a short time duration:

nanoseconds, microseconds, or milliseconds in some

instances.

FIGURE 7. CHARACTERISTICS OF ZENER AND VARISTOR

Speed of Response

Response times of less than 1ps are sometimes claimed for

zener diodes, but these claims are not supported by data in

practical applications. For the varistor, measurements were

made down to 500ps with a voltage rise time (dv/dt) of 1

million volts per microsecond. These measurements are

described in Application Note AN9767. Another

consideration is the lead effect. Detailed information on the

lead effect can be found further in this section and in

Application Note AN9773. In summary, both devices are fast

enough to respond to real world transient events.

Electrical energy is transformed into heat and has to be

distributed instantaneously throughout the device. Transient

thermal impedance is much more important than steady

state thermal impedance, as it keeps peak junction

temperature to a minimum. In other words, heat should be

instantly and evenly distributed throughout the device.

The varistor meets these requirements: an extremely reliable

device with large overload capability. Zener diodes dissipate

electrical energy into heat in the depletion region of the die,

resulting in high peak temperature.

Leakage Current

Leakage current can be an area of misconception when

comparing a varistor and zener diode, for example. Figure 8

shows a P6KE 6.8 and a V8ZA2, both recommended by their

manufacturers for protection of integrated circuits having 5V

supply voltages.

Figure 6 shows Peak Pulse Power vs Pulse width for the

V8ZA2 and the P6KE 6.8, the same devices compared for

leakage current.

200

60kW

100

10kW

7kW

3.5kW

0.5

0.2

0.1

100ns 200

1µs

10µs 20

PULSE TIME

100µs

1000µs

FIGURE 6. PEAK PULSE POWER vs PULSE TIME

10-107

Application Note 9768

The zener diode leakage is about 100 times higher at 5V

Failure Mode

than the varistor, 200µA vs less than 2µA, in this example.

Varistors subjected to energy levels beyond speciﬁed ratings

may be damaged. Varistors fail in the short circuit mode.

Subjected to high enough energy, however, they may

physically rupture or explode, resulting in an open circuit

condition. These types of failures are quite rare for properly

selected devices because of the large peak pulse

capabilities inherent in varistors.

100µA PER VERTICAL DIV.

1V PER HORIZONTAL DIV.

Zeners can fail either short or open. If the die is connected

by a wire, it can act as a fuse, disconnecting the device and

resulting in an Open circuit. Designers must analyze which

failure mode, open or short, is preferred for their circuits.

P6KE 6.8

V8ZA2

When a device fails during a transient, a short is preferred,

as it will provide a current path bypassing and will continue

to protect the sensitive components. On the other hand, if a

device fails open during a transient, the remaining energy

ends up in the sensitive components that were supposed to

be protected.

FIGURE 8. CHARACTERISTIC OF ZENER P6KE 6.8 vs

LITTELFUSE VARISTOR V8ZA2

Another consideration is a hybrid approach, making use of

the best features of both types of transient suppressors (See

Figure 10).

The leakage current of a zener can be reduced by specifying

a higher voltage device.

“Aging”

It has been stated that a varistor's V-I characteristic changes

every time high surge current or energy is subjected to it.

That is not the case.

INPUT

ZENER

VARISTOR

As illustrated in Figure 9, the V-I characteristic initially

changed on some of the devices, but returned to within a few

percent of its original value after applying a second or third

pulse. To be conservative, peak pulse limits have been

established on data sheets. In many cases, these limits have

been exceeded many fold without harm to the device. This

does not mean that established limits should be exceeded,

but rather, viewed in perspective of the deﬁnition of a failed

device. A “failed” varistor device shows a 10% change of

the V-I characteristic at the 1mA point.

INPUT

ZENER

VARISTOR

FIGURE 10. HYBRID PROTECTION USINGVARISTORS,

ZENERS, R AND L

8 x 20µs WAVE V31CP20

Capacitance

Depending on the application, transient suppressor

capacitance can be a very desirable or undesirable feature.

Varistors in comparison to zener diodes have a higher

capacitance. In DC circuits capacitance is desirable, the

larger the better. Decoupling capacitors are used on IC

supply voltage pins and can in many cases be replaced by

varistors, providing both the decoupling and transient

voltage clamping functions.

The same is true for ﬁlter connectors where the varistor can

perform the dual functions of providing both ﬁltering and

transient suppression.

NUMBER OF PULSES

There are circuits however, where capacitance is less

desirable, such as high frequency digital or some analog

circuits.

FIGURE 9. 250A PULSE WITHSTAND CAPABILITIES

10-108

Application Note 9768

As a rule the source impedance of the signal and the

frequency as well as the capacitance of the transient

suppressor should be considered.

References

For Littelfuse documents available on the web, see

http://www.littelfuse.com/

The current through C is a function of dv/dt and the

distortion is a function of the signal's source impedance.

Each case must be evaluated individually to determine the

maximum allowable capacitance.

[1] Sakshaug, E.C., J.S. Kresge and S.A. Miske, “A New

Concept in Station Arrester Design,” IEEE Trans.

PAS-96, No. 2, March-April 1977, pp. 647-656.

The structural characteristics of metal-oxide varistors

unavoidably result in an appreciable capacitance between

the device terminals, depending on area, thickness and

material processing. For the majority of power applications,

this capacitance can be of beneﬁt. In high-frequency

applications, however, the effect must be taken into

consideration in the overall system design.

[2] Philipp, H.R. and L.M. Levinson, “Low Temperature

Electrical Studies in Metal Oxide Varistors - A Clue to

Conduction Mechanisms,” Journal of Applied Physics,

Vol. 48, April 1977, pp. 1621-1627.

[3] Philipp, H.R. and L.M. Levinson, “Zinc Oxide for

Transient Suppression,” IEEE Trans. PHP, December

1977.

[4] “Surge Arresters for Alternating Current Power Circuits,”

ANSI Standard C62.1, IEEE Standard 28.

[5] “Lightning Arresters. Part I: Nonlinear Resistor Type

Arresters for AC Systems,” IEC Recommendation

99-1,1970.

[6] Matsuoka, M., T. Masuyama andY. Iida, “Supplementary

Journal of Japanese Society of Applied Physics,” Vol.

39, 1970, pp. 94-101.

[7] Harnden, J.D., F.D. Martzloff, W.G. Morris and F.B.

Golden, “Metal-Oxide Varistor: A New Way to Suppress

Transients,” Electronics, October 2, 1972.

[8] Martzloff, F.D., “The Development of a Guide on Surge

Voltages in Low - Voltage AC Power Circuits,” Report

81CRD047, General Electric, Schenectady, New York,

1981.

[9] Martzloff, F.D., “Varistor versus Environment:Winning

the Rematch,” Report 85CRD037, General Electric,

Schenectady, New York, May 1985.

10-109

AN9768 [LITTELFUSE]

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