©2009 Littelfuse, Inc.
Revision: November 5, 2009
Specifications are subject to change without notice.
Please refer to www.littelfuse.com for current information.
Definitions (IEEE Standard C62.33, 1982)
A characteristic is an inherent and measurable property of a
device. Such a property may be electrical, mechanical, or ther-
mal, and can be expressed as a value for stated conditions.
A rating is a value which establishes either a limiting capa-
bility or a limiting condition (either maximum or minimum)
for operation of a device. It is determined for specified
values of environment and operation. The ratings indicate a
level of stress which may be applied to the device without
causing degradation or failure. Varistor symbols are defined
on the linear V-I graph illustrated in Figure 20.
FIGURE 20 I-V GRAPH ILLUSTRATING
SYMBOLS AND DEFINITIONS
Voltage Clamping Device
A clamping device, such as an MOV, refers to a characteristic
in which the effective resistance changes from a high to low
state as a function of applied voltage. In its conductive state,
a voltage divider action is established between the clamping
device and the source impedance of the circuit. Clamping
devices are generally "dissipative" devices, converting much
of the transient electrical energy to heat.
Choosing the most appropriate suppressor depends upon
a balance between the application, its operation, voltage
transient threats expected and sensitivity levels of the
components requiring protection. Form factor/package
style also must be considered.
At high current and energy levels, varistor characteristics
are measured, of necessity, with an impulse waveform.
Shown in Figure 21, is the ANSI Standard C62.1 wave-
shape, an exponentially decaying waveform representative
of lightning surges and the discharge of stored energy in
The 8/20s current wave (8s rise and 20s to 50% decay of
peak value) is used as a standard, based on industry practic-
es, for the characteristics and ratings described. One excep-
tion is the energy rating (W
), where a longer waveform of
10/1000s is used. This condition is more representative of
the high energy surges usually experienced from inductive
discharge of motors and transformers. Varistors are rated for
a maximum pulse energy surge that results in a varistor volt-
) shift of less than +/-10% from initial value.
Power Dissipation Ratings
When transients occur in rapid succession the average
power dissipation is the energy W
pulse times the number of pulses per second. The power
so developed must be within the specifications shown
in the Device Ratings and Characteristics Table for the
specific device. Certain parameters must be derated at
Varistor Terms and Definitions
-55 50 60 70 80 90 100 110 120 130 140 150
AMBIENT TEMPERATURE (ºC)
T OF RA
ML, MLE, MHS, MLN
BA/BB, CA, DA/DB,
LA, "C"III, HA, MA,
UltraMOV, ZA SERIES
FIGURE 22. DEVICE RATINGS AND CHARACTERISTICS
VIRTUAL START OF WAVE
VIRTUAL FRONT DURATION
= 1.25 x RISETIME FROM 10% TO 90%
PEAK CURRENT (A)
FIGURE 21. DEFINITION OF PULSE CURRENT WAVEFORM