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Zinc Oxide Varistors
Zinc Oxide Varistors
Contents
Page
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Selection Guide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Ordering Code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
VE / VF Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Electrical Characteristics (VE / VF types) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
VN 32 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
VB 32. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Taping Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
Packaging. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
Quality. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Manufacturing Process and Quality Assurance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
Reliability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
As we are anxious that our customers should benefit from the latest developments in technology and standards,
AVX reserves the right to modify the characteristics published in this brochure.
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1
Zinc Oxide Varistors
General
Metal Oxide Varistors are ceramic passive components
made of zinc oxide sintered together with other metal oxide
additives.
They provide an excellent protective device for limiting surge
voltages and absorbing energy pulses.
Their very good price / performance ratio enables designers
to optimize the transient protection function when designing
the circuits.
Varistors are Voltage Dependent Resistors whose
resistance decreases drastically when voltage is increased.
When connected in parallel with the equipment to protect,
they divert the transients and avoid any further overvoltage
on the equipment.
Manufactured according to high level standards of quality
and service, our Metal Oxide Varistors are widely used as
protective devices in the telecommunications, industrial,
automotive and consumer markets.
2
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Zinc Oxide Varistors
Introduction
ZINC OXIDE VARISTORS.
PROTECTION FUNCTION
APPLICATION
Definition of the varistor effect
The varistor effect is defined as being the property of any
material whose electrical resistance changes non-linearly
with the voltage applied to its terminals.
In other words, within a given current range, the current-volt-
age relationship can be expressed by the equation:
I = KV
In which K represents a constant depending on the geome-
try of the part and the technology used and the non-lin-
earity factor.
The higher the value of this factor, the greater the effect. The
ideal (and theorical) case is shown in Figure1 where =
∞
whereas a linear material has an equation of I = f(V) obeying
the well-known Ohm’s law ( = 1).
The relationship between these two extreme cases is shown
in Figure 2. It should be pointed out that the I = f(V) curve is
symmetrical with respect to zero in the case of zinc oxide
varistors.
Current
=
Current
or, yet again, by changing the chemical composition of the
varistor.
The polycrystal is schematically represented in Figure 3. At
room temperature the semiconducting grains have very low
resistivity (a fews ohms/cm).
Intergranular
phase
Zinc oxide
grains
Figure 3
On the contrary, the resistivity of the second phase (or inter-
granular layer) basically depends on the value of the applied
voltage.
If the voltage value is low, the phase is insulating (region I of
the I = f(V) curve). As the voltage increases this phase
becomes conductive (region II). At very high current values
the resistivity of the grain can become preponderant and the
I = f(V) curve tends towards a linear law (region III).
The curve I = f(V) for the different types can be found in cor-
responding data sheets.
2 - Equivalent electrical circuit diagram
=1
0
Voltage
0
Voltage
Figure 1
Figure 2
ZINC OXIDE VARISTORS
1-Composition of the material
Zinc oxide varistors are a polycrystalline structured material
consisting of semiconducting zinc oxide crystals and a sec-
ond phase located at the boundaries of the crystals.
This second phase consists of a certain number of metallic
oxides (Bi
2
O
3
,MnO,Sb
2
O
3
, etc.). It forms the «heart»of the
varistor effect since its electrical resistivity is a non-linear
function of the applied voltage.
Thus, a zinc oxide varistor consists of a large number of
boundaries (several millions) forming a series-parallel net-
work of resistors and capacitors, appearing somewhat like a
multijunction semiconductor.
Experimentally, it is found that the voltage drop (at 1mA) at
each boundary is about 3V. The total voltage drop for the
thickness of the material is proportional to the number N of
boundaries.
t
V
1mA
3 N where N = —
L
in which L represents the average dimension of a zinc oxide
grain and t the thickness of the material.
t
In other words: V
1mA
3 —
L
Thus, with a thickness of 1 mm and average dimension of
L = 20 µ, we obtain a voltage of 150 V for a current of 1mA.
The desired voltage at 1mA can thus be obtained either by
changing the thickness of the disc or by controlling the aver-
age dimension of the zinc oxide grain through heat treatment
Figure 4 explains the behavior of a zinc oxide varistor. r rep-
resents the equivalent resistance of all semiconducting
grains and that of the intergranular layer (the value of which
basically varies with the applied voltage). Cp corresponds to
the equivalent capacitance of the intergranular layers.
When the applied voltage is low, the resistivity of the inter-
granular layer is quite high and the current passing through
the ceramic is low. When the voltage increases, the resis-
tance decreases (region II in Figure 5).
When a certain voltage value is reached, becomes lower
than r and the I = f(V) characteristic tends to become ohmic
(region III).
The equivalent capacitance due to the insulating layers
depends on their chemical types and geometries.
r
{
Zinc oxide
grains
III
Current
I
II
grains
Cp boundaries
{
>r
ρ=
f (V)
>r
r>
Voltage
Figure 4
Figure 5
Values of a few hundred picofarads are usually found with
commonly used discs.
Capacitance value decreases with the area of the ceramic.
Consequently, this value is lower when maximum permissi-
ble energy and current values in the varistor are low, since
these latter parameters are related to the diameter of the
disc.
Capacitance values are not subject to outgoing inspection.
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3
Zinc Oxide Varistors
Introduction
3 - Temperature influence on the I = f(V) characteristic
A typical I = f(V) curve is given in Figure 6.
Different distinct regions can be observed:
• The first one depends on the temperature and corre-
sponds to low applied voltages (corresponding currents
are in the range of the µA). Consequently, a higher leakage
current is noticeable when temperature is increasing.
• The second one shows less variation and corresponds to
the nominal varistor voltage region (Figure 7). The temper-
ature coefficient of the varistor voltage at
1 mA is:
K=
V/V
and has a negative value with
K
< 9.10
-4
/°C
T
The A versus
∝
curve
A
0.5
0.1
1 10 20
50
100
Figure 8
As the temperature coefficient decreases with increasing
current density, this curve also depends on the type of the
varistor.
• For higher voltages, the temperature has no significant
influence. Practically the clamping voltages of the varistors
are not affected by a temperature change.
(I)
For usual values of
∝
(30 to 40), the continuously dissipated
power is about 7 times greater than that dissipated by a
sinusoidal signal having the same peak value. For example,
a protective varistor operating at RMS voltage of 250 V has
a power dissipation of a few mW.
4.2 - Non-linearity coefficient
The peak current and voltage values basically depend on the
I = f(V) characteristic or, to be more precise, on the value of
the coefficient defined by:
=
log (I
1
/I
2
)
log (V
1
/V
2
)
∆
V 1 mA
V 1 mA
A
10
-3
10
-4
10
-5
0
10
-6
10
-7
10
-8
100°C
10
-9
10
10
2
10
3
75°C
25°C
-4
-2
+2
(%)
- 25
0
25
50
75 100
125
(V)
-9.10
-4
/
°C
In which I
1
and I
2
are the current values corresponding to
voltage values V
1
and V
2
.
The value of depends on the technology used (chemical
composition, heat synthesis, etc.). Nevertheless, the value is
not constant over the entire current range (several decades).
For example, Figure 9 shows the variation of this coefficient
for currents ranging from 100 nA to 100 A. It can be seen
that passes through a maximum value and always stays at
high values, even at high levels of current.
Log l
1
/ l
l
=
Log V /
2
V where l
1
= 10
1 2
2
60
Figure 6
Figure 7
4 - Varistor characteristics
The choice of a varistor for a specific application should be
guided by the following major characteristics:
1) Working or operating voltage (alternating or direct).
2) Leakage current at the working voltage.
3) Max. clamping voltage for a given current.
4) Maximum current passing through the varistor.
5) Energy of the pulse to be dissipated in the varistor.
6) Average power to be dissipated.
50
1.9
1.8
1.7
1.6
1.5
1.4
1.3
1.2
1.1
=
V
1
2
40
V
V
1
= Voltage for l
1
V
2
= Voltage for l
2
l
1
> l
2
l
1
= 10
6
l
2
30
l
1
3
l
2
= 10
4.1 - Max. operating voltage and leakage current
The maximum operating voltage corresponds to the “rest”
state of the varistor. This “rest” voltage offers a low leakage
current in order to limit the power consumption of the pro-
tective device and not to disturb the circuit to be protected.
The leakage currents usually have values in the range of a
few micro-amperes.
P
A
= AV .lp = AKVp
+
1
with P
A
= A
P
C
in which: A = a constant f( )
K = a constant
(I = KVa).
P
C
= dissipated power for a DC voltage Vp.
10
-6
10
-3
10
(I) A
10
2
10
20
30
Figure 9
Figure 10
The non-lineary of the varistor can be expressed in another
way by the ratio of the voltages corresponding to 2 current
values.
= V
1
V
2
b
a
Where:
V
1
voltage for current I
1
V
2
voltage for current I
2
The curve giving
versus the value of
Figure10 for 2 ratios of I
1
/I
2
=10
3
and 10
6
.
b
is shown in
4
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