Littelfuse AN9773 Application Note

Varistor Testing

Application Note January 1998

Introduction

This note details the common tests of varistor parameters and
describes suitable test methods using simplied test circuits.

All tests are performed at 25
The test circuits and methods given herein are intended as
a general guide. Since the tests frequently entail high
voltages and currents, the user must exercise appropriate
safety precautions.

Engineering Evaluation

It is important to focus on the key characteristics and
ratings to determine if the component can perform as
expected. Typically, for a varistor, its nominal voltage,
clamping voltage, standby current, insulation resistance,
and capacitance are measured. The surge current, or
energy, and waveshape available in the circuit together with
its frequency of occurrence should be measured or
computed. The characteristics of these expected transients
should then be checked against the pulse ratings and the
power dissipation ratings of the selected varistor type.
Where suitable equipment is available, these ratings may
be veried.

Product Qualication

A product qualication plan often will be used to detail the
electrical and environmental tests to which sample
components may be subjected. The suggested electrical
characteristics tests could include (with appropriate
conditions and limits): nominal varistor voltage, V
maximum clamping voltage,
(optional, especially for AC applications); insulation
resistance; and capacitance. A test to ensure surge current
withstand capability may be included in the qualication
plan. This test must be carefully performed and specied (by
using either 8/20µs or 10/1000µs waveshapes) consistent
with the pulse lifetime rating chart of the varistor selected.
Other qualication tests may be used to ensure mechanical
integrity, humidity resistance, solderability, and terminal/lead
strength.

Incoming Inspection

The equipment maker may wish to verify that shipments
received consist of correct parts at the expected quality
level. For incoming inspection of Littelfuse Varistors, it is
recommended that sample testing include nominal varistor
voltage (V
voltages specied on the purchase drawing/specication.
Other electrical sampling tests frequently performed can
include insulation resistance and capacitance. Tests such as
maximum clamping voltage, V
are usually checked only on a periodic audit basis.

) tested against the minimum and maximum

N

o

C, unless otherwise specied.

;

N

VC; DC standby current, ID

, and DC standby current, ID,

C

AN9773

Field Maintenance

Field maintenance testing is done to verify that the varistor is
still providing the intended protection function.

The nominal varistor voltage should be tested against the
minimum limits for the model using the method described in
the Nominal Varistor Voltage V

section. If the varistor is

N

open, short, or more than 10% outside either limit, it should
be replaced. The DC standby current may also be measured.

Measurement of Varistor Characteristics [1]

Nominal Varistor Voltage V

This is measured at a DC test current, I
models. A simplied circuit for instrumenting this test, shown
in Figure 1, is suitable for varistors up through a rating of
300V
voltage will be needed. Resistor R1 has a dual purpose. In
conjunction with the variable voltage supply, E1, it forms a
quasi-current source providing up to 6mA when switch S1 is
closed. Also, R1 is used as a current sensor to measure
current owing through the varistor-under-test. To use the
circuit, the operator places switch S2 in position I and S3 into
position V
S1 is closed. E1 is then adjusted to obtain a reading of 100V
±5V on the digital voltmeter. Approximately 1mA of current
will be owing in R1. When switch S2 is placed in position V,
the varistor voltage will be indicated on the voltmeter. The
values of R1 and E1 supply voltage can be scaled
appropriately for other voltage-current test points.

If the varistor voltage test is implemented on automatic test
equipment, a “soak” time of 20ms minimum should be
allowed after application of test current before voltage
measurement. This is necessary to allow varistor voltage to
settle toward a steady-state value. Figure 2 illustrates the
time response of a specimen varistor with a constant 1.0mA
current applied. As can be seen, the varistor voltage initially
may rise to a value up to 6% greater than nal. With a 20ms
or greater soak time, the measured value will differ by less
than 2% from the steady-state value.

. Above the 300V

RMS

. A test device is then inserted into the socket and

N

S1

R1

S3

V

0V-600V

E1

FIGURE 1. SIMPLIFIED CIRCUIT FOR VARISTOR VOLTAGE

AND DC STANDBY CURRENT TESTS

N

N

of 1mA for product

N

rating, a higher supply

RMS

R2
I

D

+

I

DVM

S2

-

V

R1 = 100kΩ, 1%, 1W(V
R2 = 1kΩ, 1%, 1/2W(I

TEST)

N

TEST)

D

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1-800-999-9445 or 1-847-824-1188 | Copyright © Littelfuse, Inc. 1998

C

Application Note 9773

V (T)
5V/DIV

250V

240V

230V

FIGURE 2. VOLTAGE-TIME V(T) CHARACTERISTICS OF A

LITTELFUSE VARISTOR (V130LA10A)
OPERATING AT A CONSTANT DC CURRENT OF

0.1ms/DIV

1ms/DIV
10ms/DIV

100ms/DIV
1000ms/DIV

T

For varistor models that are commonly used on 60Hz power
lines, the V

limits may be specified for a 1.0mA peak AC

N

current applied. If an AC test is preferred by the user, a
schematic approach similar to that shown in Figure 1 is used,
except an AC VARIAC™ is substituted for the DC power supply,
and an oscilloscope is substituted for the voltmeter. This circuit
is equivalent to that of a typical curve tracer instrument.

To avoid unnecessary concern over minor measurement
anomalies, three behavioral phenomena of metal-oxide
varistors should be noted. First, it is normal for the peak
varistor voltage measured with AC current to be about 2% to
5% higher than the DC value, as illustrated by Figure 3. This
“AC-DC difference” is to be expected, since the one-quarter
cycle period of a 60Hz wave is much less than the 20ms
minimum settling time required for DC readout.

Second, it is normal for the varistor voltage to increase
slightly when first subjected to electrical current, as shown in
Figure 4. This might be considered a “break-in” stabilization
of the varistor characteristics. During normal measurement

the voltage shift typically is less than 1%. This voltage shift is
of little consequence for most measurement purposes but
might be noticeable when viewing a DVM as in the test
method of Figure 1. The visual DVM observation should be
made shortly after power is applied, with measurement to
not more than three significant figures.

Third, it is normal for the varistor voltage-current
characteristic to become slightly asymmetrical in polarity
under application of DC electrical stress over time. The
varistor voltage will increase in the same direction as the
polarity of stress, while it will be constant or will decrease in
the opposite polarity. This effect will be most noticeable for a
varistor that has been subjected to unipolar pulse stresses
or accelerated DC life tests. Therefore, to obtain consistent
results during unipolar pulse or operating life tests, it is
essential to provide a polarity identification for the test
specimens. However, for initial readout purposes, this effect
usually is insignificant.

Maximum Clamping Voltage, V

Two typical current impulses that may be used to define the
varistor clamping voltage are the 8/20 µ s and the 10/1000 µ s
pulses. Figure 5 shows typical varistor test waveforms for
these two impulses.

The clamping voltage of a given model varistor at a defined
current is related by a factor of the varistor voltage.
Therefore, a test of the nominal varistor voltage against
specifications may be sufficient to provide reasonable
assurance that the maximum clamping voltage specification
is also satisfied. When it is necessary to perform the V
special surge generators are required. For shorter impulses
than 8/20 µ s, precautions must be observed to avoid an
erroneous “overshoot” in the measurement of the clamping
voltage. The Equipment for Varistor Electrical Testing section
gives general information on surge generators; a brief
description of the “overshoot” effect follows.

test,

C

V(T)

5V/DIV

DC

100

VOLTAGE (V)

10

10

FIGURE 3. AC AND DC CHARACTERISTIC CURVES FIGURE 4. V130LA10A) VARISTOR VOLTAGE FOR THE

-6

-7

10

-5

10

I, CURRENT (A)

10-142

AC 60Hz

130V

RMS

PRODUCT
MEDIUM VOLTAGE
MATERIAL

-4

10

10-310-210

RATED

-1

INITIAL CYCLES OF 60Hz OPERATION AT A
PEAK CURRENT OF 1.0mA

VARIAC™ is a trademark of Glen Rad, Inc.

T, 50ms/DIV

Application Note 9773

10A/DIV

100V/DIV

0

FIGURE 5A. 8/20 µ s, WAVE I

OUTPUT LEAD

FROM

TRANSIENT

GENERATOR

AREA ≈

0.5cm

10µs/DIV

= 50A, V

P

= 315V

P

FIGURE 5B. 10/1000 µ s, WAVE I

FIGURE 5. TYPICAL CLAMPING VOLTAGE TEST WAVEFORMS (LITTELFUSE

FIGURE 6. VARISTOR TYPE V130LA10A)

VOLTAGE PROBE

2

COPPER TUBE

SURROUNDING

VOLTAGE PROBE

CURRENT

PATH

AREA

22cm

≈

2

1ms/DIV

= 50A, V

P

= 315V

C

10A/DIV

100V/DIV

0

CURRENT

PAT H

VARISTOR

FIGURE 7A. MINIMAL LOOP AREA

GROUND

VARISTOR

FIGURE 7B. EXCESSIVE LOOP AREA TYPICAL

“OVERSHOOT” OF LEAD-MOUNTED VARISTOR
WITH STEEP CURRENT IMPULSES

FIGURE 7C. CURRENT RISE OF 8 µ s FIGURE 7D. CURRENT RISE OF 0.5 µ s

FIGURE 7. EFFECT OF LEAD LENGTH ON “OVERSHOOT”

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