Varistors are voltage dependent, nonlinear devices that behave electrically similar to back-to-back Zener diodes. The symmetrical, sharp breakdown characteristics shown here enable the varistor to provide excellent transient suppression performance. When exposed to high voltage transients, the varistor impedance changes many orders of magnitude—from a near open-circuit to a highly conductive level—thereby clamping the transient voltage to a safe level. The potentially destructive energy of the incoming transient pulse is absorbed by the varistor, thereby protecting vulnerable circuit components.
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