Varistor Metal Oxide Varistor Products Page 14 Varistor Metal-Oxide Varistor Products

2017 Littelfuse, Inc. Specifications are subject to change without notice. Revised: 09/14/17 Metal-Oxide Varistors (MOVs) Speed of Response and Rate Effects The varistor action depends on a conduction mechanism similar to that of other semiconductor devices. For this reason, conduction occurs very rapidly, with no apparent time lag - even into the nanosecond (ns) range. Figure 18, shows a composite photograph of two voltage traces with and without a varistor inserted in a very low inductance impulse generator. The second trace (which is not synchro- nized with the first, but merely superimposed on the oscil- loscope screen) shows that the voltage clamping effect of the varistor occurs in less than 1.0 ns . In the conventional lead-mounted devices, the inductance of the leads would completely mask the fast action of the varistor; therefore, the test circuit for Figure 18, required insertion of a small piece of varistor material in a coaxial line to demonstrate the intrinsic varistor response. Tests made on lead- mounted devices, even with careful attention to minimizing lead length, show that the voltages induced in the loop formed by the leads contribute a sub- stantial part of the voltage appearing across the terminals of a varistor at high current and fast current rise. Fortu- nately, the currents which can be delivered by a transient source are invariably slower in rise time than the observed voltage transients. The applications most frequently encountered for varistors involve current rise times longer than 0.5 s. Voltage rate-of-rise is not the best term to use when dis- cussing the response of a varistor to a fast impulse (unlike spark gaps where a finite time is involved in switching from nonconducting to conducting state). The response time of the varistor to the transient current that a circuit can deliver is the appropriate characteristic to consider. The V-I characteristic of Figure 19A, shows how the re- sponse of the varistor is affected by the current waveform. From such data, an "overshoot" effect can be defined as being the relative increase in the maximum voltage appear- ing across the varistor during a fast current rise, using the conventional 8/20 s current wave as the reference. Figure 19B, shows typical clamping voltage variation with rise time for various current levels. TRACE 2 LOAD VOLTAGE CLAMPED BY VARISTOR TRACE 1 LOAD VOLTAGE WITHOUT VARISTOR 500ps/DIV. 100V/DIV. FIGURE 18. RESPONSE OF A ZnO VARISTOR TO A FAST RISE TIME (500ps) PULSE FIGURE 19B. OVERSHOOT DEFINED WITH REFERENCE TO THE BASIC 8/20s CURRENT PULSE DEVICE: V130LA20A (LEAD AREA <1cm 2 ) (NOTE) 100% 8s AT 1000A/cm 2 (2.5kA) 100A/cm 2 (250A) 10A/cm 2 (25A) PULSE RISE TIME (s) 0.2 0.4 0.6 0.8 1 2 4 6 8 10 90 100 110 120 130 140 % OF CLAMPING VOLTAGE AT 8s FIGURE 19A. V-I CHARACTERISTICS FOR VARIOUS CURRENT RISE TIMES FIGURE 19. RESPONSE OF LEAD-MOUNTED VARISTORS TO CURRENT WAVEFORM WAVESHAPE 0.5/1.5s 1/3s 8/ 20s DEVICE: V130LA20A (LEAD AREA <1cm 2 ) (NOTE) CLAMPING VOLTAGE (V) 10 200 400 800 1000 600 20 100 40 60 200 400 600 1000 2000 PEAK CURRENT (A) Transient suppressors can be exposed to high currents for short durations in the nanoseconds to millisecond time frame. Littelfuse Varistors are connected in parallel to the load, and any voltage drop in the leads to the varistor will reduce its effectiveness. Best results are obtained by using short leads that are close together to reduce induced voltages and a low ohmic resistance to reduce I R drops. Varistor Characteristics, Terms and Consideration Factors (continued)

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