TPC 3
TPC 3
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 curr ent range, the curr ent-volt-
age relationship can be expressed by the equation:
I = KVa
In which K represents a constant depending on the geome-
try of the part and the technology used and athe 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 a= ∞
whereas a linear material has an equation of I = f(V) obeying
the well-known Ohm’s law (a= 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.
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 (Bi2O3,MnO,Sb2O3, 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. V1mA ø3 N where N = —
in which L represents the average dimension of a zinc oxide
grain and t the thickness of the material.
In other words: V1mA ø3 —
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 tr eatment
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).
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
Figure 4 explains the behavior of a zinc oxide varistor. r rep-
resents the equivalent resistance of all semiconducting
grains and rthat 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 rdecreases (region II in Figure 5).
When a certain voltage value is reached, rbecomes 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.
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.