Aumov LV Ultramov Varistor Design Guide Page 20 Varistor Design Guide For DC & Automotive Applications

2015 Littelfuse DC Application Varistor Design Guide DC Application Varistor Design Guide Figure 22. Parallel operation of varistors by graphical technique 0.1 0.5 1 5 10 50 100 500 1000 5000 10000 Peak Current (A) 1000 800 100 Peak Voltage (V) 200 300 400 500 600 LIMIT SAMPLE MODEL V251BA60 T = -40C TO 85C A LOWER BOUND (20%) SAMPLE UNIT With this technique, current sharing can be considerably improved from the near worst-case conditions of the hypothetical example given in the preceding figure. In summary, varistors can be paralleled, but good current sharing is only possible if the devices are matched over the total range of the voltage-current characteristic. In applications requiring paralleling, Littelfuse should be consulted. The following table offers some guidelines for series and parallel operation of varistors. Series Parallel Objective Higher voltage capability. Higher energy capability. Non-standard voltage capability. Higher current capability. Higher energy capability. Selection Required No Yes Model Applicable All, must have same Itm rating. All models Application Range All voltage and currents. All voltages - only high currents, i.e., >100A. Precautions Itm ratings must be equal. Must be identical voltage rated models. Must test and select units for similar V-I characteristics. Effect on Ratings Clamp voltages additive. Voltage ratings additive. Current ratings that of single device. Energy Wtm, ratings additive. Current ratings function of current sharing as determined graphically. Energy ratings as above in proportion to current sharing. Clamp voltages determined by composite V-I characteristic of matched units. Voltage ratings that of single unit. Series and Parallel Operation of Varistors (continued) 19

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