Littelfuse C-III Varistor Series - Manual (Page 7)

© 2013 Littelfuse, Inc. 119
Revised: June 5, 2013
Varistor Products
C-III Varistor Series
Radial Lead Varistors > C-III series
Specifications are subject to change without notice.
Please refer to www.littelfuse.com/series/c-iii.html for current information.
C–III Series
NUMBER OF SURGES
(RATED FOR 120 PULSES)
0 100 50 150 250 300 200
600
550
500
450
400
350
300 CLAMPING V
O
L
T
A
GE
AT 7
5
0A V175LA20C
750A (8/20μs)
AC Bias Reliability
The C-III Series MOVs were designed for use on the AC
line. The varistor is connected across the AC line and is
biased with a constant amplitude sinusoidal voltage. It
should be noted that the definition of failure is a shift in the
nominal varistor voltage (V
N
) exceeding +/-10%. Although
this type of varistor is still functioning normally after this
magnitude of shift, devices at the lower extremities of V
N
tolerance will begin to dissipate more power.
Because of this possibility, an extensive series of
statistically designed tests were performed to determine
the reliability of the C-III type of varistor under AC bias
combined with high levels of temperature stress. To
date, this test has generated over 50,000 device hours of
operation at a temperature of 125ºC, although only rated at
85ºC. Changes in the nominal varistor voltage, measured at
1mA, of less than 2% have been recorded, as displayed in
the diagram at right.
Transient Surge Current/Energy Transient Capability
The transient surge rating serves as an excellent figure
of merit for the C-III varistor. This inherent surge handling
capability is one of the C-III varistor’s best features.
The enhanced surge absorption capability results from
improved process uniformity and enhanced construction.
The homogeneity of the raw material powder and
improved control over the sintering and assembly
processes are contributing factors to this improvement.
In the low power AC mains environment, industry
standards (UL, IEC, NEMA and IEEE) all suggest that the
worst case surge occurrence will be 3kA. Such a transient
event may occur up to five times over the equipment life
time (approximately 10 years). While the occurences of
five 3kA transients is the required capability, the rated,
repetitive surge current for the C-III Series is 20 pulses
for the 20mm units and 10 pulses for the 14mm Series.
As a measure of the inherent device capability, samples
of the 20mm V130LA20C devices were subjected to
a worst case repetitive transient surges test. After 20
pulses, each of 3kA, there was negligible change in
the device characteristics. Changes in the clamping
voltage, measured at 100A, of less than 3% were
recorded, as shown in the upper diagram at right.
Samples of the 20mm Series V175LA20C were subjected
to repetitive surge occurrences of 750A. Again, there was
negligible changes in any of the device characteristics
after 120 pulses, as shown in the lower diagram at right.
In both cases the inherent device capability is far
in excess of the expected worst case scenario.
300
250
200
150
100
0 100 200 300 400 500 600 700 800 900 1000 1100
TIME (HOURS)
V
NOM
A
T
1mA (V)
V130LA20C
500
450
400
350
300
NUMBER OF SURGES
CLAMPING V
O
L
T
A
GE
AT 3kA
V130LA20C
3kA (8/20μs)
(RATED FOR 20 PULSES)
0 10 5 15 25 30 20
High Temperature Operating Life 125ºC for 1000 Hours
at Rated Bias
Typical Repetitive Surge Current Capability of
C-III Series MOVs
Figure 12
Figure 13
Figure 14