Varistor Metal Oxide Varistor Products Page 13 Varistor Metal-Oxide Varistor Products

2017 Littelfuse, Inc. Specifications are subject to change without notice. Revised: 09/14/17 Metal-Oxide Varistors (MOVs) Nominal Varistor Region of Operation The varistor characteristic follows the equation: I = kV a , where ( k ) is a constant and the exponent ( a ) defines the degree of nonlinearity. Alpha is a figure of merit and can be determined from the slope of the V-I curve or calculated from the formula: In this region the varistor is conducting and R X will pre- dominate over C , R ON and R OFF . R X becomes many orders of magnitude less than R OFF but remains larger than R ON . During conduction the varistor voltage remains relatively constant for a change in current of several orders of mag- nitude. In effect, the device resistance, R X , is changing in response to current. This can be observed by examining the static or dynamic resistance as a function of current. The static resistance is defined by: and the dynamic resistance by: Plots of typical resistance values vs current ( I ) are given in Figure 16A and 16B. Upturn Region of Operation At high currents, approaching the maximum rating, the varistor approximates a short-circuit. The curve departs from the nonlinear relation and approaches the value of the material bulk resistance, about 1 -10 . The upturn takes place as R X approaches the value of R ON . Resistor R ON represents the bulk resistance of the Z N O grains. This resis- tance is linear (which appears as a steeper slope on the log plot) and occurs at currents 50A to 50,000A, depending on the varistor size. L R X FIGURE 15. EQUIVALENT CIRCUIT AT VARISTOR CONDUCTION L R ON FIGURE 17. EQUIVALENT CIRCUIT AT VARISTOR UPTURN R X V I --- - = Z X dv di ------ V I / R X / = = = 500 100 50 10 5 1 0.5 0.1 0.05 0.01 0 0 1 1 1 . 0 1 0 . 0 10 PEAK CURRENT (A) STATIC RESISTANCE ( PER RATED RMS INPUT VOLT) FIGURE 16A. R X STATIC VARISTOR RESISTANCE FIGURE I 2 I 1 / ( ) log V 2 V 1 / ( ) log ------------------------------- = 1 V 2 V 1 / ( ) log ------------------------------- for I 2 I 1 / 1 = = 5 1 0.5 0.01 0.05 0 . 01 0.005 0.001 0.01 0.1 1.0 10 100 DYNAMIC RESISTANCE ( PER RATED RMS INPUT VOLT) PEAK CURRENT (A) FIGURE 16B. Z X DYNAMIC VARISTOR RESISTANCE Varistor Characteristics, Terms and Consideration Factors (continued)

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