Figure 12. The
protection level, surge
current, and maximum
operating voltage of
A varistor's schematic
An actual disc varistor
typically used in surge
arrestors and inline
AC line protectors.
Figure 15. The internal atomic structure of a varistor
showing a conduction path and its oxides.
Figure 16. A real actual V/I characteristic of a metal
schemes. Figure 13 shows the schematic symbol and
Figure 14 shows an actual varistor.
During manufacturing as they cool and while still in
the liquid phase, a rigid amorphous coating around each
zinc oxide grain forms. This yields a microstructure of zinc
oxide grains that are isolated from each other by a thin
continuous intergranular phase. It is this complex, two-phase microstructure that yields this desired purposely
desired nonlinear characteristic.
Figure 15 is a simplified illustration of its conduction.
The zinc oxide grains are highly conductive, and the
intergranular boundary formed with other oxides is highly
resistive. The points where zinc oxide grains meet causes
sintering or “microvaristors” — comparable to symmetrical
zener diodes at approximately 3.5V. The electrical
behavior in Figure 15 results from the number of
microvaristors connected in series or parallel.
Sintering is the welding together and subsequent growth
of contact areas between two or more initially distinct
particles. This occurs at from one-half up to the full melting
point of the materials. Sintering occurs more rapidly with
smaller particles. This is why the process is so popular in
powder metallurgy and in firing of ceramic oxides.
The V/I curve is a straight line between 10-4 and 103
amps (see Figure 16). Figure 17 shows the change in
static resistance R = V/I for a typical varistor. The
resistance is > 1 megohm in the range of the
permissible operating voltage, but can drop by 10 orders
of magnitude in cases of overvoltage.
Standard IEC 60060 defines the maximum
non-repetitive surge current by an 8/20 µs waveform (rise
time 8 µs/decay time to half value of 20 µs). Check out
Figure 18. This waveform approximates a rectangular
wave of 20 µs. The derating curves of the surge current
— defined for rectangular waveforms — show a knee
between the horizontal branch and slope at 20 µs.
The same international standard requires relatively
long durations for testing surge currents and maximum
energy absorption capability. A rectangular wave of 2 ms
is commonly used for this test (Figure 19).
The most common type of varistor is the Metal Oxide
Varistor (MOV). This contains a ceramic mass of zinc
oxide grains in a matrix of other metal oxides (such
as small amounts of bismuth, cobalt, manganese)
sandwiched between two metal plates (the electrodes).
The boundary between each grain and its neighbor
forms a diode junction which allows current to flow in
only one direction.