Product Top » Precautions for use » Varistors
Varistors
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Precautions for Use of Varistors | ||||||||||||||||||||||||||||||||||||||||
| Please click here for precautions of common matters for all products. 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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| Maximum Energy (E) | ||
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The maximum energy within the varistor voltage change of  10% when a single impulse of 2ms is applied. (NV73 2E•2J•2L are applied 100 times.) |
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| Maximum Peak Current (IP) | ||
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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.(NV73 2E•2J•2L are 100 times.) |
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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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| 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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Recommended value of varistor voltage for power line |
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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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Failure Mode |
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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 short-circuiting (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 80% or less 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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