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The Zener diode is a special type of diode that allows the flow of the current to run on the opposite direction of the arrow of the diode.
This diode does not conduct until the applied voltage reaches the breakdown voltage or until it is greater. The so called "zener voltage".
Note: The zener diode behaves like a common semiconductor diode when it is forward biased
 The following picture shows the symbol of the zener diode (A - anode, K - cathode) and the direction of the electric current when the zener diode operates in the region of voltage regulation.
The Zener diode will be analyzed, not as an ideal diode, but as a real one. When the zener diode is in reverse biased a small amount of electric current flows throughout the diode.
 When we analyze the zener diode curve, we can see that when the voltage applied to the diode is increasing (negatively), the flowing current does not increase a lot.
However, once the applied voltage reaches a voltage value called Zener voltage (Vz), the voltage does not increases a lot so it can be considered constant.
The current that passes through the diode (zener current), can vary in a large range of values, but the voltage on the diode is almost the same. This region is called: region of voltage regulation.
This is the feature of the zener diode that we can take advantage of, because the voltage stays practically constant for a great current variation. See the picture.
What can a voltage regulator do with a zener diode?
A regulator with a ideal zener diode maintains a fixed predetermined output voltage, regardless of the variation of voltage on the power supply and / or the load.
Note: The output voltage doesn't change as the load changes in an ideal voltage source.
However the voltage source is not ideal and it is common that the ouput voltage decreases as the load goes up. (the electrical current raises)
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