Varistors Caution & Terms

Caution & Terms
varistors
Precautions for the Varistors
The reliability of the metal oxide varistors are dependant on the
ways of their use and could lead to accidents so please be aware.
Selection and Protection
• It is recommended that the steady-state circuit voltage
which remains at 80% or less of the max. allowable circuit
voltage (d.c., a.c., r.m.s.). Exceeding the specification will
cause deterioration, short-circuits, etc.
• Select proper parts according to the surge energy and the number
of the impressions if the varistors are used to absorb the surge for
an inductive loading.
• The rated surge endurance is defined in terms of shock wave
current waveform (starting up 8μs/wave-tail length 20μs).
The waveform in a practical use may be different from the
test conditions, in which case, select the proper parts from
the Voltage-Current Curves in the varistor catalog.
• Insert fuses or thermal fuses in series with varistors if the size of
the surge power cannot be estimated, in order to prevent varistors
from bursting due to an excessive surge over the rating.
• Give consideration on the layout to combustible materials and
to take measurements on the circuits (fuses or thermal fuses)
since there may be smoking or flaming if the varistor short-circuits
due to an excessive surge over the rating.
• Upon mold sealing, fully confirm the reliability and use the resin
which has small contractile stress at stiffening since the protection
coat may peel off, cracks may occur at the solder connection,
and the characteristics of the varistor may change.
• Perform the withstand voltage test and the insulation resistance test
with the varistors removed from the circuit since the test voltage
may exceed the varistor voltage.
Failure Mode
• The varistors will deteriorate and have a possibility of short-circuiting
if they are exposed to an excessive surge over the specification.
A short-circuit occurs when the load factor rises against the circuit
voltage due to the deterioration of a varistor voltage. Then the Joule
heat is generated by the leakage current and a thermal runaway
occurs. The varistors may burst and become open if a commercial
power supply of 200V r.m.s. is connected to a varistor for
100V r.m.s. (270V/1mA).
circuit
protection
Chip Varistors for Surface Mount
• The voltage that is constantly applied must be set within the
maximum allowable circuit voltage (d.c., a.c., r.m.s.) stated in the
catalog. (The recommended voltage is 80% or less of the rating.)
• Give a consideration to intercept the power supply by providing
a protective circuit as necessary upon designing since there might
be self heating and short circuits if an excessive surge over
the specification should intrude.
• Please perform damp-proofing on the surface of the varistors prior
to the use when installing in a high-humidity and high-temperature
environment.
Terms and Definitions
Maximum Allowable Circuit Voltage
• The maximum commercial frequency sinusoidal voltage effective
value or maximum DC voltage which can be applied continuously.
Maximum Energy (E)
• The maximum energy within the varistor voltage change rate of
±10% when a single impulse of 2ms is applied. (NV73 2E, 2J, 2L
are applied 100 times.)
Maximum Peak Current (IP)
• The maximum peak current within the varistor voltage change rate
of ±10%, when a single standard impulse of 8/20μs is applied in two
times with an interval of 5 min. (NV73 2E, 2J, 2L are 100 times.)
Operating Temperature (Topt)
• The allowable ambient temperature range while the device
is operating.
Storage Temperature (Tstg)
• The temperature range in which the elements do not deteriorate.
Varistor Voltage (VC)
• The terminal voltages on both ends of the varistor when the
specified current is applied.
Clamping Voltage (VP)
• The peak value of the voltage between two terminals of the varistor
when the specified standard waveform impulse current (8/20μs)
is applied.
Recommended value of varistor voltage for the power
supply voltage
Voltage of Power Line
Varistor Voltage
3.3V d.c.
8.2V
5V d.c.
8.2V, 12V
12V d.c.
24V, 27V
24V d.c.
47V, 56V
48V d.c.
82V, 100V, 120V
100V a.c. Line-to-Line
220V, 240V, 270V
100V a.c. Line-to-Earth
470V
200V a.c. Line-to-Line, Line-to-Earth
430V, 470V
Reference
• For basic precautions, please refer to the technical report of
JEITA EMAJ-R039 Safety application guide for varistors.
Specifications given herein may be changed at any time without prior notice. Please confirm technical specifications before you order and/or use.
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KOA Speer Electronics, Inc. • 199 Bolivar Drive • Bradford, PA 16701 • USA • 814-362-5536 • Fax: 814-362-8883 • www.koaspeer.com