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Varistor Products
©2009 Littelfuse, Inc.
Revision: November 5, 2009
O
VERVIEW
Ov
er
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Specifications are subject to change without notice.
Please refer to www.littelfuse.com for current information.
Terms and Descriptions
Symbol
Clamping Voltage. Peak voltage across the varistor measured under conditions of a specified peak V
C
pulse current
and specified waveform. NOTE: Peak voltage and peak currents are not necessarily coincidental in time.
V
C
Rated Peak Single Pulse Transient Currents (Varistor). Maximum peak current which may be applied for a single
8/20s impulse, with rated line voltage also applied, without causing device failure.
I
TM
Lifetime Rated Pulse Currents (Varistor). Derated values of I
TM
for impulse durations exceeding that of an 8/20s
waveshape, and for multiple pulses which may be applied over device rated lifetime.
-
Rated RMS Voltage (Varistor). Maximum continuous sinusoidal RMS voltage which may be applied.
V
M(AC)
Rated DC Voltage (Varistor). Maximum continuous DC voltage which may be applied.
V
M(DC)
DC Standby Current (Varistor). Varistor current measured at rated voltage, V
M(DC)
.
I
D
For certain applications, some of the following terms may be useful.
Nominal Varistor Voltage. Voltage across the varistor measured at a specified pulsed DC current, I
N(DC)
, of
specific duration. I
N(DC)
of specific duration. I
N(DC)
is specified by the varistor manufacturer.
V
N(DC)
Peak Nominal Varistor Voltage. Voltage across the varistor measured at a specified peak AC current, I
N(AC)
, of
specific duration. I
N(AC)
is specified by the varistor manufacturer.
V
N(AC)
Rated Recurrent Peak Voltage (Varistor). Maximum recurrent peak voltage which may be applied for a
specified duty cycle and waveform.
V
PM
Rated Single Pulse Transient Energy (Varistor). Energy which may be dissipated for a single impulse of maximum
rated current at a specified waveshape, with rated RMS voltage or rated DC voltage also applied, without causing
device failure.
W
TM
Rated Transient Average Power Dissipation (Varistor). Maximum average power which may be dissipated due to a
group of pulses occurring within a specified isolated time period, without causing device failure.
Varistor Voltage. Voltage across the varistor measured at a given current, I
X
.
V
X
Voltage Clamping Ratio (Varistor). A figure of merit measure of the varistor clamping effectiveness as defined by
the symbols (V
C
) ÷ (V
M(AC)
), (V
C
) ÷ (V
M(DC)
).
V
C
/V
PM
Nonlinear Exponent. A measure of varistor nonlinearity between two given operating currents, I
1
and I
2
, as
described by I = kV
a
where k is a device constant, I
1
I I
2
, and a
12
= ( logI
2
/ I
1
) ÷ ( logV
2
/ V
1
)
a
Dynamic Impedance (Varistor). A measure of small signal impedance at a given operating point as defined by:
Z
X
= ( dV
X
) ÷ ( dI
X
)
Z
X
Resistance (Varistor). Static resistance of the varistor at a given operating point as defined by:
R
X
= ( V
X
) ÷ ( I
X
)
R
X
Capacitance (Varistor). Capacitance between the two terminals of the varistor measured at C specified
frequency and bias.
C
AC Standby Power (Varistor). Varistor AC power dissipation measured at rated RMS voltage V
M(AC)
.
P
D
Voltage Overshoot (Varistor). The excess voltage above the clamping voltage of the device for a given current that
occurs when current waves of less than 8s virtual front duration are applied. This value may be expressed as a % of
the clamping voltage (V
C
) for an 8/20 current wave.
V
OS
Response Time (Varistor). The time between the point at which the wave exceeds the clamping voltage level (V
C
)
and the peak of the voltage overshoot. For the purpose of this definition, clamping voltage as defined with an 8/20s
current waveform of the same peak current amplitude as the waveform used for this response time.
-
Overshoot Duration (Varistor). The time between the point voltage level (V
C
) and the point at which the voltage
overshoot has decayed to 50% of its peak. For the purpose of this definition, clamping voltage is defined with an
8/20s current waveform of the same peak current amplitude as the waveform used for this overshoot duration.
-
TABLE 3. VARISTOR CHARACTERISTICS (IEEE STANDARD C62.33-1982 SUBSECTION 2.3 AND 2.4)