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Precautions for Use of
Varistors |
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Please refer to precautions in use in the beginning part
for particulars common to all product types.
Although the reliability of metal oxide varistors is highly appreciated,
the ways of their uses greatly affect it and can cause an accident,
to which you are requested to pay attention. |
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Ratings |
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Maximum Allowable Circuit Voltage |
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The maximum sinusoidal RMS voltage or maximum DC voltage
which can be applied continuously. |
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The maximum energy within the varistor voltage
change of  10%
when a single impulse of 2ms is applied. |
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The maximum peak current within the varistor
voltage change of 10%,
when a single standard impulse of 8/20μs is applied in two times
with an interval of 5min. |
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Operating Temperature(Topt) |
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The allowable ambient temperature range while the device
is operating. |
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Storage Temperature(Tstg) |
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The temperature range that can store the elements without
deteriorating them. |
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Characteristics |
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Varistor Voltage(VC) |
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The terminal voltages on both ends of the varistor when
the specified current is flowed into it. |
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Maximum Clamping Voltage(VP) |
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The peak value of the voltage between two terminals of the
varistor when the specified standard wave form impulse current (8/20μs)
is flowed. |
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Precautions |
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Notes for Selection and Protective Ways |
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It is recommended that the circuit voltage that
is always applied be used at not more than 80 percent of the max.
tolerance circuit voltage (d.c.,a.c.r.m.s.). If it is used
over the specified rating, it will cause deterioration, short-circuits,
etc. of varistors. |
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If varistors are used to absorb the surge for
an inductive load (L), select proper products according to the surge
energy and the number of impressions. |
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The rated surge endurance volume is prescribed
on shock-wave current waveform (starting up 8μs/wave-tail length
20μs). In a practical use, the wave-tail length can differ from
the test method, in which case, select the proper products from
the Fig. for the characteristics of surge life put in the separate
varistor catalogs. |
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When the size of the surge power cannot be estimated,
insert fuses or thermo fuses in series with varistors ,or insert
arresters in parallel in order to prevent varistors from bursting
due to an excessive surge over the rating. |
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If the varistors short-circuit due to an excessive
surge over the rating, there is a possibility of smoking or flaming,
depending upon the conditions, so give consideration to the layout
of the ambient parts so as not to be fixed close to anything combustible,
as well as to the measures (fuses or thermo fuses) taken on the
circuits. |
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In case of resin coating/potting or molded sealing,
fully confirm the evaluation of reliability and use those that have
small contractile stress at a resin stiffening time. By the contractile
stress at the resin stiffening time, the protection coat may be
peeled off, cracks may occur at the solder connection, and the characteristics
of the varistor may change. |
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In case of making an insulation resistance test,
the test voltage can exceed a varistor voltage, accordingly conduct
the test in the condition of the varistor being removed from an
equipment. |
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If the varistors are impressed by an excessive surge over
the max. rating, they will deteriorate and have a possibility of
shortcircuiting (short) in the worst case. The process from the
deterioration to the short-circuit is that the overcurrent rate
to the circuit voltage rises by deterioration of a varistor voltage,
a Joule heat generates by a leakage current and a heat runaway happens,
therefore resulting in a short-circuit (short). If a commercial
power supply of 200Vr.m.s. is connected to a varistor for 100Vr.m.s.
(270V/1mA) by mistake, the varistor can burst and become in an open
condition. |
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Chip Varistors for Surface Mount |
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Set up a voltage for use at all
times that must not exceed the max. tolerance circuit voltage (d.c.,a.c.r.m.s.)
stated in the catalog. (The recommended voltage is not more than
80% of the rating.) |
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If an excessive surge over the rating
should intrude, there would be fear of self heat generation and
short circuits (short), so take into consideration an intercepting
a power supply by providing a protective circuit as necessary when
designing. |
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When setting up equipment in the
environment of high temperature and high humidity, be sure to use
varistors after processing the surfaces of them for protection of
humidity. |
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The flow system is available in
mounting parts with solder plated electrodes on the board. |
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Application Notes
Brush up your circuit
Basic Knowledge
