What is the Resistance, RZ, of a Zener Diode?

The resistance, RZ, of a zener diode is the value of impedance across the diode that acts like a small resistance in series with the zener.

Zener diodes are not perfect diodes. No matter how great they are designed, they will always have some value of impedance.

The zener resistance, RZ acts like a small resistance in series with the zener.

The effect that the zener resistance has on a zener diode is that small changes in VZ will occur wehn IZ varies. This means that the voltage across a zener diode does not remain perfectly constant as the zener current varies.

Normally, the smaller the value of the zener impedance, the better the impedance rating. Usually on datasheets, the zener resistance, RZ will be specified as a maximum rating, meaning it will tell you the maximum impedance that will be present across the zener diode for that specific zener in use.