Brochure

Varistors Introduction
TECHNICAL NOTE
Technical Note
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Vishay BCcomponents
Revision: 04-Sep-13
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Document Number: 29079
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DEFINITIONS
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.
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.
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
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 (t
1
) and the virtual time to half value (t
2
) 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 = (V
peak
x I
peak
) x t
2
x K
where:
I
peak
= peak current
V
peak
= voltage at peak current
 = given for I = ½ x I
peak
to I
peak
K is a constant depending on t
2
, when t
1
is 8 μs to 10 μs
(see table on page 8).
A low value of  corresponds to a low value of V
peak
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
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
Examples
When:
C = 230 V at 1 A
 = 0.035 (ZnO)
I = 10
-3
A or 10
2
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 10
2
A: V = 230 x (10
2
)
0.035
= 270 V