Unlike LEDs, Zener diodes use a breakdown voltage property for their usage. They are specially designed for the breakdown voltage to be strictly defined, low and constant. On the left bottom side of the figure below it can be seen that after a certain current level (Iz(min)) that causes a breakdown on the diode, the voltage drop becomes constant and remains almost the same until the maximum allowable current (Iz(max)), defined by the specification. This special feature will help us to reduce or regulate the voltage when making the power supply, later in the manual.
(Source: referenced)
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