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What is the dynamic impedance of a Zener diode (voltage regulator diode, constant voltage diode)?

Zener impedance is the equivalent series resistance of a Zener diode when it is conducting current. Zener impedance should ideally be zero, but in reality, a Zener diode has some impedance. Thus, a Zener diode acts as such only in a limited current range.

A Zener diode is a type of diode designed to use Zener breakdown in the reverse-biased state. Reverse current suddenly increases at Zener voltage. In other words, Zener voltage is only slightly dependent on reverse current. Still, Zener voltage changes in response to a change in current. The rate of this change is specified as dynamic impedance.
Zener diodes exhibit higher impedance than the specified dynamic impedance in the lower and higher current regions as shown Fig. 1. In these regions, Zener voltage might change considerably in response to a change in current or temperature. Care should be exercised when Zener diodes are used in constant-voltage circuits.