### Littelfuse Varistor Ordering Information Diagram

Selecting a Littelfuse Varistor
A pplication Note
J u ly 1 9 9 9
AN9771.1
Introduction
The varistor must operate under both a continuous operating
(standby) mode as well as the predicted transient (normal)
mode. The selection process, therefore, requires a knowledge
of the electrical environment. When the environment is not
fully defined, some approximations can be made.
For most applications, the selection is a five-step process:
1. Determine the necessary steady-state voltage rating
(working voltage)
2. Establish the transient energy absorbed by the varistor
3. Calculate the peak transient current through the varistor
4. Determine power dissipation requirements
5. Select a model to provide the required voltage-clamping
characteristic
A final consideration is to choose the appropriate package
style to suit the application.
Consider the maximum continuous voltage that will be
applied to the varistor including any high line conditions
(i.e., 110% or more of nominal voltage). Ratings are given
for continuous sinusoidal AC and DC voltages. If a
nonsinusoidal waveform is applied, the recurrent peak
voltage should be limited to √2x VM(AC).
Specifications for the UltraMOV™ Series varistor, for
example, are shown in Table 1 for 140V AC rated devices to
illustrate the use of the ratings and specifications table.
VM(AC) - These models can be operated continuously with
up to 140VRMS at 50Hz - 60Hz applied. They would be
suitable for 120VAC nominal line operation and would allow
for about a 120% high line condition.
VM(DC) - Operation up to 180VDC applied continuously is
allowed.
Energy
Transient energy ratings are given in the WTM column of the
specifications in joules (watt-second). The rating is the
maximum allowable energy for a single impulse of
10/1000µs current waveform with continuous voltage
applied. Energy ratings are based on a shift of VN of less
than ±10% of initial value.
When the transient is generated from the discharge of an
inductance (i.e., motor, transformer) or a capacitor, the
source energy can be calculated readily but, in most cases
the transient is from a source external to the equipment and
is of unknown magnitude. For this situation an approximation
technique can be used to estimate the energy of the
transient absorbed by the varistor. The method requires
finding the transient current and voltage applied to the
varistor. To determine the energy absorbed the following
equation applies:
E =
τ
∫0 VC ( t )I ( t )∆t
= KV C Iτ
where I is the peak current applied, VC is the clamp voltage
which results, τ is the impulse duration and K is a constant.
K values are given in Figure 1 for a variety of waveshapes
frequently encountered. The K value and pulse width
correspond to the current waveform only, assuming the
varistor voltage waveform is almost constant during the
current impulse. For complex waveforms, this approach also
can be used by dividing the shape into segments that can be
treated separately.
TABLE 1. ULTRAMOV RATINGS AND SPECIFICATIONS EXAMPLE
MAXIMUM RATING (85oC)
CONTINUOUS
DEVICE
MODEL
NUMBER
MODEL
BRANDNUMBER
ING
CHARACTERISTICS (25oC)
TRANSIENT
VARISTOR
VOLTAGE AT 1mA
DC TEST CURRENT
RMS
VOLTS
DC
VOLTS
ENERGY
2ms
PEAK CURRENT
8 x 20µs
VM(AC)
VM(DC)
WTM
ITM
ITM
2 x PULSE 1 x PULSE
(V)
(V)
(J)
(A)
VNOM
MIN
(A)
VNOM
MAX
(V)
MAXIMUM
CLAMPING
VOLTAGE 8 x 20µs
TYPICAL
CAPACITANCE
VC
IPK
f = 1MHz
(V)
(A)
(pF)
V07E140
7V140
140
180
13.5
1200
1750
200
240
360
10
160
V10E140
10V140
140
180
27.5
2500
3500
200
240
360
25
400
V14E140
14V140
140
180
55
4500
6000
200
240
360
50
900
V20E140
20V140
140
180
110
6500
10000
200
240
360
100
1750
10-121
UltraMOV™ is a trademark of Littelfuse, Inc.
Application Note 9771
WAVESHAPE
K†
EQUATION
0.637
IPK
I
Π
PK sin  ----t
τ 
Section (1) E = kVC Iτ = (0.5) (500) (100) (5) (10-6)
t
τ
Peak Current
t
I PK  --
 τ
0.86
IPK
0.5 IPK
I PK sin ( πt )e
t
– t /τ
1.4
IPK
I PK e
-t/1.44τ
0.5 IPK
t
τ
IPK
The peak current rating can be checked against the transient
current measured in the circuit. If the transient is generated
by an inductor, the peak current will not be more than the
inductor current at the time of switching. Another method for
finding the transient current is to use a graphical analysis.
When the transient voltage and source impedance is known,
a Thevenin equivalent circuit can be modeled. Then, a load
line can be drawn on the log - log, V-I characteristic as
shown in Figure 3. The two curves intersect at the peak
current value.
The rated single pulse current, ITM, is the maximum
allowable for a single pulse of 8/20µs exponential waveform
(illustrated in Application Note AN9767, Figure 21). For
longer duration pulses, ITM should be derated to the curves
in the varistor specifications. Figure 4 shows the derating
curves for 7mm size, LA series devices. This curve also
provides a guide for derating current as required with
repetitive pulsing. The designer must consider the total
number of transient pulses expected during the life of the
equipment and select the appropriate curve.
1.0
Where the current waveshape is different from the exponential
waveform of Figure 11 of AN9767, the curves of Figure 4 can
be used by converting the pulse duration on the basis of
equivalent energy. This is easily done using the constants given
in Figure 1. For example, suppose the actual current measured
has a triangular waveform with a peak current of 10A, a peak
voltage of 340V and an impulse duration of 500µs.
IPK
t
τ
3.28J Total
0.5
t
τ
†
= 0.13J
Section (2) E = kVC Iτ = (1.4) (500) (100) (50-5) 10-6) = 3.15J
IPK
τ
The waveform is divided into two parts that are treated
separately using the factors of Figure 1: current waveform
Section (1) 0 to 5µs and (2) 5µs to 50µs. The maximum
voltage across the V130LA1 at 100A is found to be 500V
from the V-I characteristics of the specification sheet.
Based upon alpha of 25 to 40
FIGURE 1. ENERGY FORM FACTOR CONSTANTS
ZS
Consider the condition where the exponential waveform
shown below is applied to a V130LA1 type Littelfuse Varistor.
IV
VOC
VR
100A
FIGURE 3A. EQUIVALENT CIRCUIT
50A
t
0
5µs
50µs
FIGURE 2.
10-122
The pulse rise portion of the waveform can be ignored when
the impulse duration is five times or more longer. The
maximum number of pulses for the above example would
exceed 104 from the pulse derating curves shown in Figure 4.
VR = VOC -IZS
VOC
Varistor Voltage
VC
CLAMP VOLTAGE
VARISTOR V-I
CHARACTERISTIC
IV
-VOC/ZS
LOG VARISTOR CURRENT (A)
FIGURE 3B. GRAPHICAL ANALYSIS TO DETERMINE PEAK I
FIGURE 3. DETERMINING VARISTOR PEAK CURRENT FROM
A VOLTAGE SOURCE TRANSIENT
2,000
RATED PEAK PULSE CURRENT (A)
1,000
500
1
2
NUMBER OF
PULSES
10
200
MODEL SIZE 7mm
V130LA1 - V300LA4
103
104
102
100
105
50
20
106
10
5
2
1
INDEFINITE
NUMBER OF
PULSES
The varistor nominal voltage (VNOM or VN) represents the
applied voltage where the varistor transitions from its
“standby” mode to its low impedance “clamping” mode. It is
measured at the 1mA conduction point. The minimum and
maximum limit values are specified in the ratings table.
Power Dissipation Requirements
Transients generate heat in a suppressor too quickly to be
transferred during the pulse interval. Power dissipation
capability is of concern for a suppressor if transients will be
occurring in rapid succession. Under this condition, the power
dissipation required is simply the energy (watt-seconds) per
pulse times the number of pulses per second. The power so
developed must be within the specifications shown on the
ratings tables for the specific device type. It is to be noted that
varistors can only dissipate a relatively small amount of
average power and are, therefore, not suitable for repetitive
applications that involve substantial amounts of average
power dissipation (likewise, varistors are not suitable as
voltage regulation devices). Furthermore, the operating
values need to be derated at temperatures above the absolute
maximum limits as shown in Figure 5.
CH, CP CS, RA SERIES
20
100
1,000
IMPULSE DURATION (µs)
10,000
FIGURE 4. PEAK CURRENT DERATING BASED ON PULSE
WIDTH AND NUMBER OF APPLIED PULSES
Then:
E = (.5)(10)(340)(500)(10-6)
= 850mJ
The equivalent exponential waveform of equal energy is then
found from:
ETRIANGULAR = EEXP
850mJ = 1.4 VCIτEXP
The exponential waveform is taken to have equal VC and I
values. Then,
850mJ
1.4 (340) (10)
= 179µs
100
PERCENT OF RATED VALUE
LOG VARISTOR (V)
Application Note 9771
90
80
70
60
50
40
30
BA/BB, CA, DA/DB,
LA, “C” III, HA, NA, MA,
UltraMOV, PA, ZA SERIES
20
10
0
-55
50
60
70
80
90
100
110
120
130
140 150
AMBIENT TEMPERATURE (oC)
FIGURE 5. CURRENT, ENERGY, POWER DERATING vs
TEMPERATURE
τEXP =
Voltage Clamping Selection
Or:
τEXP =
K*τ∗
1.4
Where: K* and τ* are the values for the triangular waveform
and τEXP is the impulse duration for the equivalent
exponential waveform.
10-123
Transient V-I characteristics are provided in the
specifications for all models of varistors. Shown below in
Figure 6 are curves for 130VAC rated models of the LA series.
These curves indicate the peak terminal voltage measured with
an applied 8/20µs impulse current. For example, if the peak
impulse current applied to a V130LA2 is 10A, that model will
limit the transient voltage to no higher than 340V.
Application Note 9771
The ability of the varistor to limit the transient voltage is
sometimes expressed in terms of a clamp ratio. For example,
consider a varistor applied to protect the power terminals of
electrical equipment. If high line conditions will allow a rise to
130VAC , then 184V peak would be applied. The device
selected would require a voltage rating of 130VACRMS or
higher. Assume selection of a V130LA2 model varistor. The
V130LA2 will limit transient voltages to 340V at currents of
10A. The clamp ratio is calculated to be,
1000
8000
6000
5000
4000
MAXIMUM CLAMPING VOLTAGE
COMPARED BY MODEL SIZE
VM(AC) = 130V RATING
TA = -55 TO 85oC
MAXIMUM PEAK (V)
3000
2000
UL1449 CORD CONNECTED
AND DIRECT PLUG-IN
CATEGORY
1500
V130LA2
Clamp Ratio =
1000
800
V130LA10A
400
(IMPLIED) UL1449 PERMANENTLY
CONNECTED CATEGORY, AND
ANSI/IEEE C61.41 (IEEE587)
CATEGORY B
100
101
102
103
= 1.85
The clamp ratio can be found for other currents, of course,
by reference to the V-I characteristic. In general, clamping
ability will be better as the varistor physical size and energy
level increases. This is illustrated in Figure 7 which
compares the clamping performance of the different
Littelfuse Varistor families. It can be seen that the lowest
clamping voltages are obtained from the 20mm (LA series)
and 60mm (BA series) products. In addition, many varistor
models are available with two clamping selections,
designated by an A, B, or C at the end of the model
number. The A selection is the standard model, with B and
C selections providing progressively tighter clamping
voltage. For example, the V130LA20A voltage clamping
limit is 340V at 100A, while the V130LA20B clamps at not
more than 325V.
V130LA20A
300
100
340V
184V
=
600
500
200
VC at 10A
Peak Voltage Applied
V130LA5
104
PEAK AMPERES 8/20µs WAVESHAPE
FIGURE 6. TRANSIENT V-I CHARACTERISTICS OF TYPICAL
LA SERIES MODELS
If the transient current is unknown, the graphical method of
Figure 3 can be utilized. From a knowledge of the transient
voltage and source impedance a load line is plotted on the
V-I characteristic. The intersection of the load line with the
varistor model curve gives the varistor transient current and
the value of clamped peak transient voltage.
MAXIMUM CLAMP RATIO AND
MAXIMUM INSTANTANEOUS VOLTAGE
1000
4
800
MA4
LA4
600
3
2
500
400
300
1.5
LA10
PA, LA20
BA
200
1
RATIO
NOTE: CLAMP RATIO EQUALS VARISTOR VOLTAGE DIVIDED
BY VNOM OR 184V FOR 130VACRMS
100
0.01
0.05 0.1
0.5
1.0
5
10
50 100
500
1K
5K 10K
INSTANTANEOUS CURRENT (A)
FIGURE 7. VARISTOR V-I CHARACTERISTICS FOR FOUR PRODUCT FAMILIES RATED AT 130VAC
10-124
Application Note 9771
PEAK
CURRENT ENERGY
(A)
(J)
80 500
0.5 - 5.0
30 1000
0.1 - 25
40 - 100
0.07 1.7
50 - 6500
0.1 - 52
100 - 6500
0.4 - 160
1,200 10,000
11 - 400
6500
70 - 250
25,000 40,000
270 1,050
50,000 70,000
450 10,000
30,000 40,000
270 1050
20,000 70,000
200 10,000
65,000 100,000
2,200 12,000
VOLTS AC RMS
4 10 25
150
130
264
250 275
460
660 750
1,000 2,800
6,000
VOLTS DC
3.5 14 35
200
175
365
330 369
615
850 970
1,200 3,500
7,000
22, 20,
16 GAUGE
CP, SERIES
AUML †, ML †, MLE †, MLN
CH SERIES
0603 0805
1206 1210
1812 2220
5 x 8mm
†,
MA SERIES
3mm
5, 7, 10,
14, 20 (mm)
ZA SERIES
5 x 8, 10 x 16,
14 x 22 (mm)
RA SERIES
7, 10, 14,
20 (mm)
C-III, LA, UltraMOV SERIES
20mm
PA SERIES
32, 34
40 (mm)
HA, HB, DA/ DB SERIES
BA/ BB SERIES
60mm
NA SERIES
34mm SQ.
CA SERIES
32, 40, 60 (mm)
AS ††
SERIES
† Littelfuse multilayer suppression technology.
FIGURE 8. VARISTOR PACKAGE STYLES AND RATINGS RANGE
10-125
DISC SIZES/
PACKAGES
32, 42, 60 (mm)
Application Note 9771
Varistor Ordering Information
The varistor part number includes ratings information. Some
types include the working voltage, others indicate the
nominal voltage. See the varistor ordering nomenclature
guides below.
ULTRAMOV TYPES
V
XX
E
XXX
LX
X
X
DEVICE FAMILY:
(DO NOT ADD IF STANDARD) (NOTE 2):
Varistor
5
7
1
DISC DIAMETER:
07, 10, 14, or 20 (mm)
PACKAGING:
ENCAPSULATION:
E = Epoxy
L1
L2
L3
L4
VM(AC)RMS:
130 to 625 (V)
=
=
=
=
Straight
Crimped
In-Line
Trim/Crimp
(Bulk pack only)
B = Bulk Pack
T = Tape and Reel
A = Ammo Pack
OTHER VARISTOR TYPES
BA, BB, CA, CP, CS, DA, DB, HA, HB, LA, NA, PA, VARISTOR SERIES
V
130
LA
20
A
CH, MA, ZA, VARISTOR SERIES
V
220
MA
4
A
Selection - Clamping
Voltage (A or B)
Selection - Clamping
Voltage (A or B)
Relative Energy Indicator or Disc Size
Product Series
Max RMS Applied Voltage
V = Metal-Oxide Varistor (MOV)
The five major considerations for varistor selection have
been described. The final choice of a model is a balance of
these factors with device packaging and cost trade-offs. In
some applications a priority requirement such as clamp
voltage or energy capability may be so important as to force
the selection to a particular model. Figure 8 illustrates the
Littelfuse varistor package styles in a matrix that compares
energy and current ratings to the working voltage range.
10-126
Relative Energy Indicator
Product Series
VN(DC) Nominal Varistor Voltage
MOV Varistor