16
Varistor Products
©2009 Littelfuse, Inc.
Revision: November 5, 2009
Specifications are subject to change without notice.
Please refer to www.littelfuse.com for current information.
Nominal Varistor Region of Operation
The varistor characteristic follows the equation:
I = kV
a
, where (k) is a constant and the exponent (
a
)
defines the degree of nonlinearity. Alpha is a figure of merit
and can be determined from the slope of the V-I curve or
calculated from the formula:
In this region the varistor is conducting and R
X
will pre-
dominate over C, R
ON
and R
OFF
. R
X
becomes many orders of
magnitude less than R
OFF
but remains larger than R
ON
.
During conduction the varistor voltage remains relatively
constant for a change in current of several orders of mag-
nitude. In effect, the device resistance, R
X
, is changing in
response to current. This can be observed by examining
the static or dynamic resistance as a function of current.
The static resistance is defined by:
and the dynamic resistance by:
Plots of typical resistance values vs current (I) are given in
Figure 16A and 16B.
Upturn Region of Operation
At high currents, approaching the maximum rating, the
varistor approximates a short-circuit. The curve departs
from the nonlinear relation and approaches the value of the
material bulk resistance, about 1-10. The upturn takes
place as R
X
approaches the value of R
ON
. Resistor R
ON
represents the bulk resistance of the Z
N
O grains. This resis-
tance is linear (which appears as a steeper slope on the log
plot) and occurs at currents 50A to 50,000A, depending on
the varistor size.
L
R
X
FIGURE 15. EQUIVALENT CIRCUIT AT VARISTOR CONDUCTION
L
R
ON
FIGURE 17. EQUIVALENT CIRCUIT AT VARISTOR UPTURN
R
X
V
I
----
=
Z
X
dv
di
------
V
I
/
R
X
/
=
=
=
500
100
50
10
5
1
0.5
0.1
0.05
0.01
0
0
1
1
1
.
0
1
0
.
0
10
PEAK CURRENT (A)
ST
A
TIC RESIST
ANCE
(
PER RA
TED RMS INPUT
V
O
L
T
)
FIGURE 16A. R
X
STATIC VARISTOR RESISTANCE FIGURE
I
2
I
1
/
(
)
log
V
2
V
1
/
(
)
log
-------------------------------
=
1
V
2
V
1
/
(
)
log
------------------------------- for I
2
I
1
/
1
=
=
5
1
0.5
0.01
0.05
0
.
01
0.005
0.001
0.01
0.1
1.0
10
100
D
YNAMIC RESIST
ANCE
(
PER RA
TED RMS INPUT
V
O
L
T
)
PEAK CURRENT (A)
FIGURE 16B. Z
X
DYNAMIC VARISTOR RESISTANCE