Varistor Metal Oxide Varistor Products Page 15 Varistor Metal-Oxide Varistor Products

2017 Littelfuse, Inc. Specifications are subject to change without notice. Revised: 09/14/17 Metal-Oxide Varistors (MOVs) 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. V M(DC) V PM V N(DC) V X V C I P I X I N(DC) I PM I D CURRENT (A) VOLTAGE (V) VARISTOR SYMBOL V N(DC) I N(DC) 10 -6 1000 10 CURRENT (A) 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. Test Waveform 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 reactive circuits. The 8/20 s current wave (8 s rise and 20 s 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 TM ), where a longer waveform of 10/1000 s 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- age (V N ) 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 TM (watt-seconds) per 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 high temperatures. -55 50 60 70 80 90 100 110 120 130 140 150 AMBIENT TEMPERATURE (C) 100 90 80 70 60 50 40 30 20 10 0 PERCENT OF RATED VALUE ML, MLE, MHS, MLN CH,RA SERIES BA/BB, CA, DA/DB, LA, "C"III, HA, MA, UltraMOV, ZA SERIES FIGURE 22. DEVICE RATINGS AND CHARACTERISTICS 100 90 50 10 VIRTUAL START OF WAVE IMPULSE DURATION VIRTUAL FRONT DURATION = 1.25 x RISETIME FROM 10% TO 90% PEAK CURRENT (A) FIGURE 21. DEFINITION OF PULSE CURRENT WAVEFORM Varistor Characteristics, Terms and Consideration Factors (continued)

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