Varistor Metal Oxide Varistor Products Page 12 Varistor Metal-Oxide Varistor Products

2017 Littelfuse, Inc. Specifications are subject to change without notice. Revised: 09/14/17 Metal-Oxide Varistors (MOVs) Leakage Region of Operation At low current levels, the V-I Curve approaches a linear (ohmic) relationship and shows a significant temperature dependence. The varistor is in a high resistance mode (approaching 10 9 ) and appears as an open circuit. The nonlinear resistance component ( R X ) can be ignored be- cause ( R OFF ) in parallel will predominate. Also, ( R ON ) will be insignificant compared to ( R OFF ). For a given varistor device, capacitance remains approxi- mately constant over a wide range of voltage and frequen- cy in the leakage region. The value of capacitance drops only slightly as voltage is applied to the varistor. As the voltage approaches the nominal varistor voltage, the ca- pacitance decreases. Capacitance remains nearly constant with frequency change up to 100 kHz. Similarly, the change with temperature is small, the 25C value of capacitance being well with +/-10% from -40C to +125C. The temperature effect of the V-I characteristic curve in the leakage region is shown in Figure 13. A distinct tempera- ture dependence is noted. The relation between the leakage current (I) and tempera- ture (T) is The temperature variation, in effect, corresponds to a change in ( R OFF ). However, ( R OFF ) remains at a high resis- tance value even at elevated temperatures. For example, it is still in the range of 10M to 100M at 125C. Although ( R OFF ) is a high resistance it varies with frequency. The relationship is approximately linear with inverse fre- quency. If however, the parallel combination of ( R OFF ) and ( C ) is pre- dominantly capacitive at any frequency of interest. This is because the capacitive reactance also varies approximately linearly with 1/f . At higher currents, at and above the mA range, tempera- ture variation becomes minimal. The plot of the tempera- ture coefficient ( dV/dT ) is given in Figure 14. It should be noted that the temperature coefficient is negative (-) and decreases as current rises. In the clamping voltage range of the varistor ( I > 1A ), the temperature dependency ap- proaches zero. C L R OFF FIGURE 12. EQUIVALENT CIRCUIT AT LOW CURRENTS 10 -8 10 -7 10 -6 10 -5 10 -4 10 -3 10 -2 10 -9 25 50 100 SPECIMEN V130LA10A 125 o C VARISTOR CURRENT (A DC ) VARISTOR VOLTAGE IN PERCENT OF 100 80 60 50 40 30 20 10 V NOM VALUE AT 25 o C (%) 75 FIGURE 13. TEMPERATURE DEPENDENCE OF THE CHARACTER- ISTIC CURVE IN THE LEAKAGE REGION -V B /kT I = I O where: I O = constant k = Boltzmann's Constant V B = 0.9eV V22ZA3 SAMPLE TYPE V130LA10A LEAKAGE REGION NORMAL OPERATION 10 3 10 2 10 1 10 0 10 -1 10 -2 10 -3 10 -4 -0.5 10 -5 -0.4 -0.3 -0.2 -0.1 0 0.1 TEMPERATURE COEFFICIENT (%/ o C) CURRENT (A) NOTE: Typical Temperature Coefficient of Voltage vs Current, 14mm Size, 55 o C to 125 o C. FIGURE 14. RELATION OF TEMPERATURE COEFFICIENT DV/DT TO VARISTOR CURRENT Varistor Characteristics, Terms and Consideration Factors (continued)

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