15
Varistor Products
©2009 Littelfuse, Inc.
Revision: November 5, 2009
O
VERVIEW
Ov
er
vie
w
Specifications are subject to change without notice.
Please refer to www.littelfuse.com for current information.
Leakage Region of Operation
At low current levels, the V-I Curve approaches a linear
(ohmic) relationship and shows a significant temperature
dependence. The varistor is in a high resistance mode
(approaching 10
9
) and appears as an open circuit. The
nonlinear resistance component (R
X
) can be ignored be-
cause (R
OFF
) in parallel will predominate. Also, (R
ON
) will be
insignificant compared to (R
OFF
).
For a given varistor device, capacitance remains approxi-
mately constant over a wide range of voltage and frequen-
cy in the leakage region. The value of capacitance drops
only slightly as voltage is applied to the varistor. As the
voltage approaches the nominal varistor voltage, the ca-
pacitance decreases. Capacitance remains nearly constant
with frequency change up to 100 kHz. Similarly, the change
with temperature is small, the 25ºC value of capacitance
being well with +/-10% from -40ºC to +125ºC.
The temperature effect of the V-I characteristic curve in the
leakage region is shown in Figure 13. A distinct tempera-
ture dependence is noted.
The relation between the leakage current (I) and tempera-
ture (T) is
The temperature variation, in effect, corresponds to a
change in (R
OFF
). However, (R
OFF
) remains at a high resis-
tance value even at elevated temperatures. For example, it
is still in the range of 10M to 100M at 125ºC.
Although (R
OFF
) is a high resistance it varies with frequency.
The relationship is approximately linear with inverse fre-
quency.
If however, the parallel combination of (R
OFF
) and (ºC) is pre-
dominantly capacitive at any frequency of interest. This is
because the capacitive reactance also varies approximately
linearly with 1/f.
At higher currents, at and above the mA range, tempera-
ture variation becomes minimal. The plot of the tempera-
ture coefficient (dV/dT) is given in Figure 14. It should be
noted that the temperature coefficient is negative (-) and
decreases as current rises. In the clamping voltage range
of the varistor (I > 1A), the temperature dependency ap-
proaches zero.
C
L
R
OFF
FIGURE 12. EQUIVALENT CIRCUIT AT LOW CURRENTS
10
-8
10
-7
10
-6
10
-5
10
-4
10
-3
10
-2
10
-9
25 50
100
SPECIMEN V130LA10A
125
o
C
VARISTOR CURRENT (A
DC
)
V
ARIST
OR
V
O
L
T
A
GE IN PERCENT OF
100
80
60
50
40
30
20
10
V
NOM
V
ALUE A
T
25
o
C (%)
75
FIGURE 13. TEMPERATURE DEPENDENCE OF THE CHARACTER-
ISTIC CURVE IN THE LEAKAGE REGION
-V
B
/kT
I = I
O
where:
I
O
= constant
k
= Boltzmann's Constant
V
B
= 0.9eV
V22ZA3
SAMPLE TYPE
V130LA10A
LEAKAGE REGION
NORMAL
OPERATION
10
3
10
2
10
1
10
0
10
-1
10
-2
10
-3
10
-4
-0.5
10
-5
-0.4
-0.3
-0.2
-0.1
0
0.1
TEMPERA
TURE COEFFICIENT (%/
o
C)
CURRENT (A)
NOTE: Typical Temperature Coefficient of Voltage vs Current, 14mm
Size, 55
o
C to 125
o
C.
FIGURE 14. RELATION OF TEMPERATURE COEFFICIENT
DV/DT TO VARISTOR CURRENT