Technical Note

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Technical Note
Varistors Introduction
GENERAL
FEATURES
Varistors provide reliable and economical protection against
high voltage transients and surges which may be produced,
for example, by lightning, switching or electrical noise on AC
or DC power lines. They have the advantage over transient
suppressor diodes in as much as they can absorb much
higher transient energies and can suppress positive and
negative transients.
When a transient occurs, the varistor resistance changes
from a very high stand-by value to a very low conducting
value. The transient is thus absorbed and clamped to a safe
level, protecting sensitive circuit components.
Varistors are manufactured from a non-homogeneous
material, giving a rectifying action at the contact points of
two particles. Many series and parallel connections
determine the voltage rating and the current capability of the
varistor.
• Wide voltage range selection - from 14 VRMS to 680 VRMS.
This allows easy selection of the correct component for
the specific application.
• High energy absorption capability with respect to size of
component.
• Response time of less than 20 ns, clamping the transient
the instant it occurs.
• Low stand-by power - virtually no current is used in the
stand-by condition.
• Low capacitance values, making the varistors suitable for
the protection of digital switching circuitry.
• High body insulation - an ochre coating provides
protection up to 2500 V, preventing short circuits to
adjacent components or tracks.
• Available on tape with accurately defined dimensional
tolerances, making the varistors ideal for automatic
insertion.
• Approved to UL 1449 edition 3 (file number: E332800) and
manufactured using UL approved flame retardant
materials.
• Completely non flammable, in accordance with IEC, even
under severe loading conditions.
• Non porous lacquer making the varistors safe for use in
humid or toxic environments. The lacquer is also resistant
to cleaning solvents in accordance with IEC 60068-2-45.
VARISTORS MANUFACTURING PROCESS
In order to guarantee top performance and maximum
reliability, close in-line control is maintained over the
automated manufacturing techniques. The manufacturing
process flow chart shows each step of the manufacturing
process, clearly indicating the emphasis on in-line control.
Each major step in the manufacturing process shown in the
Manufacturing process flow chart is described in the
following sections:
GRANULATION
A binder is added to produce larger granules for processing.
Manufacturing process flow chart
Revision: 04-Sep-13
PRESSING
The surface area and thickness of the disc help to determine
the final electrical characteristics of the varistor, therefore
pressing is a very important stage in the manufacturing
process. The granulated powder is fed into dies and formed
into discs using a high speed rotary press.
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TECHNICAL NOTE
MILLING AND MIXING
Incoming materials are checked, weighed, milled and mixed
for several hours to make a homogeneous mixture.
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Varistors Introduction
FIRING
QUALITY
The pressed products are first pre-fired to burn out the
binder. They are then fired for a controlled period and
temperature until the required electrical characteristics are
obtained. Regular visual and electrical checks are made on
the fired batch.
APPROVALS
• UL 1449 ed. 3 according file E332800
• VDE following IEC 61051-1/2 according file 40002622 or
40013495
• CSA file 219883 and cUL according file E332800
The term ‘QUALITY ASSESSMENT’ is defined as the
continuous surveillance by the manufacturer of a product to
ensure that it conforms to the requirements to which it was
made.
METALLIZATION
The fired ceramic discs are metallized on both sides with a
silver content layer to produce good low resisitive electrical
contacts. Metallization is achieved by screen printing. Visual
checks are made regularly and a solderability test is carried
out in each production batch.
PRODUCT AND PROCESS RELEASE
Recognized reliability criteria are designed into each new
product and process from the beginning. Evaluation goes
far beyond target specifications and heavy emphasis is
placed upon reliability. Before production release, new
varistors must successfully complete an extended series of
life tests under extreme conditions.
ATTACHING LEADS
Leads are automatically soldered to the metallized faces
and regular strength tests are made. Three types of lead
configuration are available; one with straight leads, one with
straight leads and flange, and one with kinked leads.
MONITORING INCOMING MATERIALS
Apart from carrying out physical and chemical checks on
incoming raw materials, a very close liaison with material
suppliers is maintained. Incoming inspection and product
results are gradually fed back to them, so ensuring that they
also maintain the highest quality standards.
LACQUERING
The components are coated by immersing them in a special
non flammable ochre epoxy lacquer. Two coats are applied
and the lacquer is cured. Regular tests to check the coating
thickness are made.
IN-LINE CONTROL
The manufacturing centre operates in accordance with the
requirements of IEC 61051-1 and UL 1449 . Each operator is
actively engaged in quality checking. In addition, in-line
inspectors and manufacturing operators make regulated
spot checks as a part of our Statistical Process Control
(SPC).
ELECTRICAL TESTING (100 %)
The voltage of each component is normally checked at
two reference currents (1 mA and another according to the
application). Any rejects are automatically separated for
further evaluation.
MARKING
FINAL INSPECTION AND TEST (100 %)
At the end of production, each varistor is inspected and
tested prior to packing.
All components are laser marked with type identification,
voltage rating and date code.
encapsulation
intergranular boundary
electrodes
LOT TESTING
Before any lot is released, it undergoes a series of special lot
tests under the supervision of the Quality department.
PERIODIC SAMPLE TESTING
Component samples are periodically sent to the Quality
laboratory for rigorous climatic and endurance tests to
IEC/UL requirements. Data from these tests provide a
valuable means of exposing long term trends that might
otherwise pass unnoticed. The results of these tests are
further used to improve the production process.
TECHNICAL NOTE
leads
V
(V)
3
100
Revision: 04-Sep-13
FIELD INFORMATION
The most accurate method of assessing quality is
monitoring performances of the devices in the field.
Customer feedback is actively encouraged and the
information is used to study how the components may be
further improved. This close relationship with customers is
based on mutual trust built up over many years of
co-operation.
I (µA)
2
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DEFINITIONS
In order to calculate the energy dissipated during a pulse,
reference is generally made to a standardized wave of
current. The wave prescribed by IEC 60 060-2 section 6 has
a shape which increases from zero to a peak value in a short
time, and thereafter decreases to zero either at an
approximate exponential rate, or in the manner of a heavily
damped sinusoidal curve. This curve is defined by the virtual
lead time (t1) and the virtual time to half value (t2) as shown
in the maximum energy curve (page 5).
The calculation of energy during application of such a pulse
is given by the formula: E = (Vpeak x I peak) x t2 x K
where:
Ipeak = peak current
Vpeak = voltage at peak current
 = given for I = ½ x Ipeak to Ipeak
K is a constant depending on t2, when t1 is 8 μs to 10 μs
(see table on page 8).
A low value of  corresponds to a low value of Vpeak and then
to a low value of E.
The maximum energy published does not represent the
quality of the varistor, but can be a valuable indication when
comparing the various series of components which have the
same varistor voltage. The maximum energy published is
valid for a standard pulse of duration 10 μs to 1000 μs giving
a maximum varistor voltage change of ± 10 % at 1 mA
When more than one pulse is applied, the duty cycle must
be so that the rated average dissipation is not exceeded.
Values of the rated dissipation are:
0.1 W for series VDRS05/VDRH05
0.25 W for series VDRS07/VDRH07
0.4 W for series VDRS10/VDRH10
0.6 W for series VDRS14/VDRH14
1 W for series VDRS20/VDRH20
MAXIMUM CONTINUOUS VOLTAGE
The maximum voltage which may be applied continuously
between the terminals of the component. For all types of AC
voltages, the voltage level determination is given by the
crest voltage x 0.707.
VOLTAGE AT 1 mA OR VARISTOR VOLTAGE
The voltage across a varistor when a current of 1 mA is
passed through the component. The measurement shall be
made in as short a time as possible to avoid heat
perturbation.
The varistor voltage is essentially a point on the V/I
characteristic permitting easy comparison between models
and types.
MAXIMUM CLAMPING VOLTAGE
The maximum voltage between two terminals when a
standard pulse current of rise time 8 μs and decreasing time
20 μs (8 μs to 20 μs) is applied through the varistor in
accordance with IEC 60060-2, section 6.
The specified current for this measurement is the class
current.
TECHNICAL NOTE
MAXIMUM NON REPETITIVE SURGE CURRENT
The maximum peak current allowable through the varistor is
dependent on pulse shape, duty cycle and number of
pulses. In order to characterize the ability of the varistor to
withstand pulse currents, it is generally allowed to warrant a
‘maximum non repetitive surge current’. This is given for one
pulse characterized by the shape of the pulse current of 8 μs
to 20 μs following IEC 60060-2, with such an amplitude that
the varistor voltage measured at 1 mA does not change by
more than 10 % maximum.
A surge in excess of the specified withstanding surge
current may cause short circuits or package rupture with
expulsion of material; it is therefore recommended that a
fuse be put in the circuit using the varistor, or the varistor be
used in a protective box
If more than one pulse is applied or when the pulse is of a
longer duration, derating curves are applied (see relevant
information in the datasheet); these curves guarantee a
maximum varistor voltage change of ± 10 % at 1 mA.
ELECTRICAL CHARACTERISTICS
Typical V/I characteristic of a ZnO varistor
The relationship between voltage and current of a varistor
can be approximated to: V = C x I
where:
V = Voltage
C = Varistor voltage at 1 A
I = Actual working current
 = Tangent of angle curve deviating from the horizontal
MAXIMUM ENERGY
During the application of one pulse of current, a certain
energy will be dissipated by the varistor. The quantity of
dissipation energy is a function of:
• The amplitude of the current
• The voltage corresponding to the peak current
• The rise time of the pulse
• The decrease time of the pulse; most of the energy is
dissipated during the time between 100 % and 50 % of
the peak current
• The non-linearity of the varistor
Revision: 04-Sep-13
Examples
When:
C = 230 V at 1 A
 = 0.035 (ZnO)
I = 10-3 A or 102 A
V = C x I
so that for current of 10-3 A: V = 230 x (10-3)0.035 = 180 V and
for a current of 102 A: V = 230 x (102)0.035 = 270 V
3
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SPCIFICATION OF A VARISTOR CURVE
log V
max. leakage
current region
β = 1 = fixed resistor
max. clamping
voltage region
up-turn
region
log V
β = 0.4 (SiC)
β = 0.03 (ZnO)
+ 10 %
max.
leakage current
β = 0 = ideal varistor
max.
clamping
voltage
- 10 %
tolerance band
1 mA
log I
Working points on a varistor curve
The drawing below shows the various working points on the
varistor curve using the series VDRS07, 60 V type as an
example. The electrical characteristic values are shown in
the Electrical Characteristics table below.
log I
Varistor characteristics using different  values
Ipeak
(%)
max. leakage current
100
90
max. clamping voltage
103
V
(V)
50
102
10
10 -5
10
t
t1
10-4
10-3
10-2
10-1
1
102
10
I (A)
103
t2
Curve for varistor type VDRS07H060
Maximum energy curve
pre-breakdown region
10
normal operating region
up-turn region
ELECTRICAL CHARACTERISTICS
3
β
V
(V)
V = Cl + IRS
9
10 Ω
PARAMETER
SLOPE = β
Maximum RMS voltage
V = Cl
102
β
Maximum DC working voltage
TECHNICAL NOTE
RS = 0.05 Ω to 0.5 Ω
10 -8
10
10-7
10-6
10-5
10-4
10-3
10-2
10-1
1
10
102
103
Varistor voltage
104 105
I (A)
Typical V/I curve
Pre-breakdown region: V  I;
dependent
Normal operating region: V = C x I
Up-turn region: V = C x I + I x Rs
Revision: 04-Sep-13
2 x 60 V = 85 V
100 V ± 10 %
165 V
Maximum non-repetitive current
1200 A
Transient energy
temperature
60 V
Maximum clamping voltage at 10 A
Leakage current at 85 VDC
highly
VALUE
4
10-5 A to 5 x 10-4 A
10 µs to 1000 µs: 8.3 J
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log V
normal
working
condition
(no spike)
protection zone
A
B
Given the small value of  (0.03 to 0.05), it is evident that the
modification of C x I will be very small compared to the
variation of R x I when VI is increased to VI + VI.
A large increase of VI will induce a large increase of VR and
a small increase of VO.
C
Examples
The varistor is a typical component of the series
VDR05C275 (C = 520;  = 0.04) and R = 250 .
For VI = 315 V (crest voltage of the 220 V supply voltage):
I = 10-5 A, VR = 2.5 x 10-3 V and VO = 315 V
10 µA
300 µA
10 A
100 A
1000 A
For VI = 500 V: I = 10-1 A, VR = 25 V and VO = 475 V
I
For VI = 1000 V: I = 1.88 A, VR = 470 V and VO = 530 V
Definitions of the varistor curve
The influence of a series resistance on the varistor drawing
shows the influence of different values of series resistors on
the varistor efficiency.
The points A, B and C shown on the curve are defined in the
Varistor Curve Definitions table.
By drawing the load line, it is also possible to estimate the
variation of the voltages VR and VO when VI is increased to
500 V or 1000 V. This effect is shown in the graphs below.
VARISTOR CURVE DEFINITIONS
POINT
DESCRIPTION
Normal working zone: current is kept as low as
possible in order to have low dissipation during
continuous operation (between 10 μA to 300 μA).
A
VO
(V)
Maximum clamping voltage: the maximum voltage
for a given (class) current (peak current based upon
statistical probability determined by standardization
authorities).
B
2500
1500
Maximum withstanding surge current: the maximum
peak current that the varistor can withstand (only)
once in its lifetime.
C
R=0Ω
2000
0.1 Ω
1Ω
1000
10 Ω
100 Ω
500
TRANSIENT VOLTAGE LIMITATIONS WITH ZnO
VARISTORS
1000 Ω
Principles of voltage limitation
500
R
1000
1500
2000
2500
VI (V)
Influence of a series resistance on the varistor
I
VR
VI
1000
VO
-U
V
(V)
800
TECHNICAL NOTE
Voltage limitation using a varistor
VR
In the Voltage limitation using a varistor drawing above, the
supply voltage VI is derived by the resistance R (e.g. the line
resistance) and the varistor (-U) selected for the application.
600
400
VI = VR + VO
V1
VI = R x I + C x I 
VO
200
If the supply voltage varies by an amount of VI the current
variation is I and the supply voltage may be expressed as:
0
Revision: 04-Sep-13
0
2
4
6
8
10
I (A)
(VI + VI) = R (I + I) + C (I + I)
Influence on varistor when V1 is 500 V (R = 250 )
5
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Nevertheless, due to the structural characteristic of the zinc
oxide varistors, the capacitance itself decreases slightly
with an increase in frequency. This phenomenon is
emphasized when the frequency reaches approximately 100
kHz. See the effect of HF alternating current on the varistor
type VDRS14T250; C = 480 pF drawing.
1000
V
(V)
VR
800
600
V1
10
400
3
V
(V)
VO
200
50 Hz
100 Hz
1 kHz
10 kHz
102
0
0
2
4
6
8
10
I (A)
Influence on varistor when V1 is 1000 V (R = 250 )
100 kHz
EQUIVALENT CIRCUIT MODEL
10
10-2
A simple equivalent circuit representing a metal oxide
varistor as a capacitance in parallel with a voltage
dependent resistor is shown in the Equivalent circuit model
drawing. Cp and Rp are the capacitance and resistance of
the intergranular layer respectively; Rg is the ZnO grain
resistance. For low values of applied voltages, Rp behaves
as an ohmic loss.
-1
10
1
I (mA)
10
Effect of HF alternating current on varistor type VDRS14T250;
C = 480 pF
ENERGY HANDLING
Maximum allowable peak current and maximum allowable
energy are standardized using defined pulses:
• Peak current (A); 8 μs to 20 μs, 1 pulse
I
• Energy (J); 10 μs to 1000 μs, 1 pulse
Rg
INTERNATIONALLY ACCEPTED PULSES
Rp
-U
Ipeak
(%)
Cp
100
t1
t2
8 µs
20 µs
10 µs 1000 µs
Equivalent circuit model
CAPACITANCE
50
TECHNICAL NOTE
Depending on area and thickness of the device, the
capacitance of the varistor increases with the diameter of
the disc, and decreases with its thickness.
In DC circuits, the capacitance of the varistor remains
approximately constant provided the applied voltage does
not rise to the conduction zone, and drops abruptly near the
rated maximum continuous DC voltage.
t2
In AC circuits, the capacitance can affect the parallel
resistance in the leakage region of the V/I characteristic. The
relationship is approximately linear with the frequency and
the resulting parallel resistance can be calculated from 1/C
as for a usual capacitor.
Revision: 04-Sep-13
t
t1
Standard pulse for current and maximum allowable energy
calculation
6
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Examples
Example of calculation of energy for a VDRS07H250 type,
at the maximum peak current (33 A) for a duration
t2 = 1000 μs (K = 1.4)
Pulse life time rating of VDRS07H060, 60 V type.
Energy capability: E = K x Vp x Ip x t2
1 pulse; 8 µs to 20 µs: 1200 A = 1 x 8 J
Ipeak
10 pulses; 8 µs to 20 µs: 300 A = 10 x 1.45 J
I
(A)
1 pulse; 10 µs to 1000 µs: 33A = 1 x 8.3 J
10 pulses; 10 µs to 1000 µs: 11 A = 10 x 2.5 J
33
The maximum specified energy is defined for a maximum
shift (V/V) 1 mA  10 %:
Ip = Pulse current
102
10
Vp = Corresponding clamping voltage
103
104
105
tp (µs)
Maximum energy (10 x 1000 μs): 1 pulse
K DEPENDS ON t2 WHEN t1 IS 8 μs TO 10 μs
t2
(μs)
Example: VDRS07H250 (250 V)
Example of selection of the maximum peak current as a
function of pulse duration.
K
20
1
50
1.2
100
1.3
1000
1.4
DISSIPATED POWER
DC DISSIPATION
The power dissipated in a varistor is equal to the product of
the voltage and current, and may be written:
W = I x V = C x I + 1 or K x V + 1
When the coefficient  = 30 ( = 0.033), the power
dissipated by the varistor is proportional to the 31st power of
the voltage. A voltage increase of only 2.26 % will, in this
case, double the dissipated power. Consequently, it is very
important that the applied voltage does not rise above a
certain maximum value, or the permissible rating will be
exceeded.
This is even more cogent as the varistors have a negative
temperature coefficient, which means that at a higher
dissipation (and accordingly at a higher temperature) the
resistance value will decrease and the dissipated power will
increase further.
AC DISSIPATION
When a sinusoidal alternating voltage is applied to a varistor,
the dissipation cannot be calculated from the same formula
as in a DC application. The calculation requires an
integration of the V x I product.
The instantaneous dissipated power is given by:
Typical surge life rating curves (number of surges allowed as
a function of pulse time and maximum current) are shown in
drawing below.
1
1
reduction factor
of rated pulse
peak current
10-1
10
100
1000
106
10-2
10-3
10
10
2
103
tp (µs)
104
TECHNICAL NOTE
Maximum peak current for various number of pulses as a function
of pulse duration

P INST = V x I = V  K x V  = K x V
In the above equation, the value V = Vpeak x sin t.
During a half cycle, the dissipated power is given by:
1000
Vpeak 690
(V)
429

P RMS
1
= --
 Kx
+1
+1
V peak x  sin t 
xd
0
100
Since Vpeak = VRMS x 2x
P RMS
10-3
10-2
10-1
1
I (A)
10

+1
a+1
1
+1
= --- x K x V RMS x  2 
x   sin t 

x dt
0
2
33 10
This integration is not easy to solve because of the exponent
 + 1) of sin t.
E = K x Vpeak x Ipeak x t2 = 1.4 x 700 x 33 x 10-3 = 32 J
Revision: 04-Sep-13
+1
7
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
It is generally easier to use the quotient of the AC power on
the DC power:
+1
1
a + 1   2
+1
--- x K x V rms
x2
x   sin t 
x dt


0
P = ---------------------------------------------------------------------------------------------------------------------------a+1
K
x
V

P = PAC/PDC
This quotient depends only on the value of  and not more
on the K value as shown in the formula:




+1
a + 1   2
1
P = --- x 2
x   sin t 

x dt
0
P has been calculated by successive application of a
reduction formula; see Power Ratios table.
POWER RATIOS

P

P

P

P

P
1
2
3
4
5
6
7
8
9
10
1.0
1.2
1.5
1.92
2.5
3.29
4.375
5.85
7.875
10.64
11
12
13
14
15
16
17
18
19
20
14.4
19.6
26.8
36.7
50.3
69
95
131
180
249
21
22
23
24
25
26
27
28
29
30
344
477
658
915
1264
1763
2439
3404
4715
6587
31
32
33
34
35
36
37
38
39
40
9135
12 776
17 734
24 822
34 482
48 301
67 149
94 126
130 941
183 660
41
42
43
44
45
46
47
48
49
50
255 646
358 778
499 673
701 611
977 622
1 373 365
1 914 510
2 690 675
3 752 439
5 275 834
TEMPERATURE COEFFICIENT
SURGE PROTECTION
In the leakage current region of the V/I characteristic, the
normal equation V = C x I of the varistor becomes less
applicable.
Varistors provide protection against surges which may be
generated in the following ways:
ELECTROMAGNETIC ENERGY
Atmospheric, lightning
Switching of inductive loads:
• Relays
• Pumps
• Actuators
• Spot welders
• Thermostats
• Fluorescent chokes
• Discharge lamps
• Motors
• Transformers
• Air conditioning units
• Fuses
This is due to a parallel resistance which shows a very
important temperature coefficient, created by thermal
conduction. This temperature coefficient decreases when
the current density increases. Then, the temperature
coefficient at 1 mA is higher for a large varistor than for a
small varistor.
This phenomena induces an increase in leakage current
when the varistor is used at high temperatures. The
relationship between the temperature and the current at a
given voltage can be expressed by:
I = I0 x eKT
where:
TECHNICAL NOTE
I0 is the limiting current at 0 K
K is a constant including the band gap energy of the
zinc oxide and the Boltzmann’s constant.
ELECTROSTATIC DISCHARGES
For example, discharges caused by synthetic carpets
(approximately 50 kV), due to the inductance of the
connecting leadwires, the reaction time of leaded VDR’s
might be too slow to clamp properly fast rising ESD pulses.
Practically, the maximum temperature coefficient is
guaranteed on the voltage for a current of 1 mA in % per K.
SOURCE OF TRANSIENT
The energy dissipated by switching of an inductive load is
completely transferred into the capacitance of the coil which
is generally very low.
E = ½ x L x I 2 = ½ x C x V2
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Varistors Introduction
VARISTORS APPLICATIONS
Examples, using the following values:
Varistors may be used in many applications, including:
• Computers
• Timers
• Amplifiers
• Oscilloscopes
• Medical analysis equipment
• Street lighting
• Tuners
• Televisions
• Controllers
• Industrial power plants
• Telecommunications
• Automotive
• Gas and petrol appliances
• Electronic home appliances
• Relays
• Broadcasting
• Traffic facilities
• Electromagnetic valves
• Railway distribution/vehicles
• Agriculture
• Power supplies
• Line ground (earth protection)
• Microwave ovens
• Toys, etc.
Mains voltage = 220 VRMS;
allowable peak voltage = 340 V
Line inductance: L = 20 μH = 20 x 10-6 H
Line capacitance: C = 300 nF = 0.3 x 10-6 H
Line resistance: 0.68 
In the event of a short circuit:
V
340 V
Load current: I L = ---- = ------------------ = 500 A
R
0.68 
Energy stored: E = ½ x 20 x 10-6 x 25 x 104 = 2.5 J (Ws)
In the event of a fuse going open circuit:
The energy goes from inductance L towards line
capacitance:
2E
2 x 2.5
V C = ------- = -------------------------- = 4082 V
-6
C
0.3 x 10
Vpeak
Ipeak
Ri
Vpeak
5
U
LOAD
t (µs)
APPLICATION EXAMPLES
Source of transient
For suppression of mains-borne transients in domestic
appliances and industrial equipment, see Suppression via
load, Suppression directly across mains, Switched-mode
power supply protection and Protection of a thyristor bridge
in a washing machine drawings.
Type VDRS05 or VDRS07.

The line impedance becomes high when the fuse goes open
circuit (resistance against high voltage peak in a very short
time).
Ri = L = 2 f L
Since the rise time of the pulse is 5 μs, the frequency
f = 50 kHz.
Ri = 6.28  x 50 x 103 x 20 x 10-6 = 6.28 
LOAD
Zi = 6.28  + 0.68  = 6.96 
VRi = 6.96 V x 500 V = 3480 V
VVDR = 4082 V - 3480 V = 602 V
TECHNICAL NOTE
U
Suppression via load
U
ELECTRONIC
CIRCUIT
or motor,
computer,
radio
Suppression directly across mains
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Varistors Introduction
For suppression of internally generated spikes in electronic
circuits, see Varistor used across a transistor or coil in a
television circuit and Varistor used across a switch or coil
drawings.
POWER
SUPPLY
fuse
U
In both examples shown in the drawings Varistor used
across a transistor or coil in a television circuit and Varistor
used across a switch or coil, type VDRS05 should be used
for up to approximately 50 A, and type VDRS07 up to
approximately 120 A.
Switched-mode power supply protection
S
heater
RH = 24 Ω
Rp
33 Ω
220 V
50 Hz
L
0.4 H
back
e.m.f.
U
U
Varistor used across a transistor or coil in a television circuit
PUMP
MOTOR
to drum motor
U
Protection of a thyristor bridge in a washing machine
BEHAVIOUR OF THE CIRCUIT WITHOUT
VARISTOR PROTECTION
The measured peak current through the pump motor when
S is closed is 1 A (see protection of a thyristor bridge in a
washing machine drawing). The energy expended in
establishing the electromagnetic field in the inductance of
the motor is therefore:
2
L
0.4
I x --- = -------- = 200 mJ
2
2
Varistor used across a switch or coil
dangerous voltage
(without VDR)
VAB
TECHNICAL NOTE
Without varistor protection, an initial current of 1 A will flow
through the thyristor bridge when S is opened, and a voltage
sufficient to damage or destroy the thyristors will be
developed. Arching will occur across the opening contacts
of the switch.
safe voltage
(with VDR)
mains
short
circuit
BEHAVIOUR OF THE CIRCUIT WITH
VARISTOR VDRS07H250 INSERTED
line
inductance
fuse
opens
line
capacitance
220 V I
mains
On opening switch S, the peak voltage developed across
the varistor is: V = Cmax. x I = 600 V
The thyristors in the bridge can withstand this voltage
without damage.
U
VAB
HOME
COMPUTER
fuse
1.5 A
The total energy returned to the circuit is 200 mJ. Of this
200 mJ. 15.1 mJ is dissipated in the heater, and 184.3 mJ is
dissipated in the varistor. The varistor can withstand more
than 105 transients containing this amount of energy.
Revision: 04-Sep-13
clamping
voltage
short
circuit
M
washing machine motor
220 W
Influence of a transient on the mains voltage
10
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SELECTION OF THE CORRECT VARISTOR TYPE
In order to select a ZnO varistor for a specific application,
the following points must first be considered:
To ensure correct selection of varistor type, two multi choice
selection charts have been prepared, see charts below.
1. The normal operating conditions of the apparatus or
system, AC or DC voltage?
The first chart determines the necessary steady state
voltage rating (i.e. working voltage) and the second chart
determines the correct size (i.e. correct energy absorption).
2. What is the maximum RMS or DC voltage?
TECHNICAL NOTE
Multi choice selection chart to determine the necessary steady state voltage rating (i.e. working voltage)
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WHICH PARAMETER
OF LINE IS KNOWN?
ORIGIN OF THE
PULSES NOT KNOWN
ORIGIN OF THE
PULSES KNOWN
LIGHTNINIG OR
INDUSTRIAL INDUCTIVE
LOAD ON LINE
SHORT CIRCUIT
CURRENT
VALUE KNOWN
SHORT CIRCUIT
CURRENT
VALUE NOT KNOWN
RCL LINE
IMPEDANCE KNOWN
VALUE OF
REPETITIVE
PEAK CURRENT
EQUALS SHORT
CIRCUIT CURRENT
VALUE
RCL LINE IMPEDANCE
NOT KNOWN
MULTIPLY NOMINAL
VAOLTAGE BY 10,
DIVIDE RESULT
BY RCL LINE
IMPEDANCE VALUE
TO FIND THE
REPETITIVE
PEAK CURRENT
LINE CONFORMS
TO CATEGORY A ACC.
ANSI/IEEE C62.41.1-2002
OR TYPE 3 LOCATION
SPD UL 1449 ED. 3
(Long branch
circuits and outlets)
LINE CONFORMS
TO CATEGORY B ACC.
ANSI/IEEE C62.41.1-2002
OR TYPE 2 LOCATION
SPD UL 1449 ED. 3
(Feeders and short
branch circuits, distribution
panel devices, lightning
systems in large buildings)
SURGE CONDITIONS
1.2/50 µs 6 kV,
8/20 µs 500 A
SURGE CONDITIONS
1.2/50 µs 6 kV,
8/20 µs 3 kA
WHEN THE REPETITIVE PEAK
CURRENT IS MAX.
VDR
50 A
S05/H05
80 A
S07/H05
120 A
S07/H07
175 A
S10/H07
250 A
S10/H10
350 A
S14/H10
500 A
S14/H14
700 A
S20/H14
1000 A
H20
SOLENOID
(e.g. transformer,
electromagnet etc.)
REPETITIVE PEAK
CURRENT EQUALS
VALUE OF PEAK
CURRENT PASSING
THROUGH SOLENOID
(do not forget to
calculate the dissipation
when the recurrent time
is short i.e. < 5 minutes)
WHEN THE SHORT CIRCUIT SURGE
CONTITION IS
VDR
6 kV/0.4 kA
S07/H05
6 kV/1.0 kA
S10/H07
6 kV/1.5 kA
S10/H10
6 kV/3.0 kA
S14/H14/S20/H20
6 kV/5.0 kA
S20/H20
TECHNICAL NOTE
Multi choice selection chart to determine the correct size (i.e. correct energy absorption)
Revision: 04-Sep-13
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Document Number: 29079
For technical questions, contact: [email protected]
THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT
ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000