Before explaining what a zener diode is, we are going to explain what a diode is. So, your basics knowledge will improve. A diode is an electronic component ( semiconductor ) that allows current to pass “only in one way.” Let’s see how the actual diode and its symbol looks like in the below picture. 
As we see in the above image, for the current to pass through the diode, the anode must be connected to the positive and the cathode to the negative. When the diode allows the passage of current, we say that it is directly polarized. If it is connected so that the current does not pass through it, we say that it is reverse polarized. Let’s see what happens when we connect a diode with a lamp in series. 
Now let’s get to know the Zener Diode
Table of Contents
What is a Zener Diode?
The zener is a highly doped, special pn (semiconductor) junction designed to drive in the reverse direction (reverse biased diode) when a certain specified voltage, called the zener voltage or voltage, is reached . Once the zener voltage is reached, the voltage at the zener terminals does not change, it remains constant even though the supply voltage increases. The Zener diode has a well-defined reverse breakdown voltage, at which it begins to conduct current and continues to operate continuously in reverse bias mode without damage. Then we will see all this in more detail, in the operation section. Here you can see the symbols: 
Here is the image of a real zener diode: 
How a Zener Diode Works
When we reverse bias it and reach Vz the diode conducts and maintains the constant voltage Vz even though we continue to increase the voltage in the circuit. The current through the zener diode under these conditions is called the reverse current (Iz).
For example: We have a Zener diode of 5.1V and 0.5w. What will be the maximum inverse current that it will support?
We remember P = V x I; I = P / V. In our case Izmax = Pz / Vz = 0.5 / 5.1 = 0.098A.
To prevent us from ever exceeding the maximum reverse current, zener diodes are always connected with a series resistor that we call “Drain Resistor”.
Let’s see what the basic connection of a zener diode would be like in a circuit:
The Rs (resistance in series with the zener) would be the drain resistance that serves to limit the flow of current through the zener and the RL is the Load to the output element that will have the constant zener voltage because it is in parallel with the Zener diode. Do you realize that the connection is reverse? Thus the zener diode is always connected.
In the above circuit, the output voltage will remain constant, as long as it is higher than Vz, and it will also be independent of the input voltage Vs. This ensures that the load will always be at the same voltage.
If we increase the input voltage Vs at the output above Vz we will always have the constant voltage equal to Vz.
The Rs absorbs the voltage difference between the input and the output. How is the Rs calculated?
Rs = (Vs- Vo) / (IL + Iz)
Being Vs the regulator input voltage, Vo the output voltage, which will be equal to Vz, IL is the maximum load current and Iz the current or current through the Zener diode.
The latter is always chosen from a value of 10% or 20% of the maximum current.
Zener diode is used for?
These diodes are used as voltage or voltage regulators for certain voltages and load resistors. With a zener we can ensure that a component (for example a loudspeaker) always receives the same voltage fairly accurately.
Another use of the zener is as a protection element of a circuit so that a certain voltage never exceeds the circuit load. Normally for this, instead of a Zener the Varistor is used , but we could use a zener.
LOOK the zener must be designed so that they are able to support the power of the load, otherwise they could get blocked or even burn.
Let’s give an example with an exercise .
You want to design a 5.1V zener regulator to supply a 5 ohm load, from a 9V input. For this we will use a Zener of 5.1V and 1w. Calculate:
a) The necessary drainage resistance, assuming a zener current of 10% of the maximum current.
b) The limits of variation of the input voltage within which the regulation is maintained. The load is assumed to be constant.
c) The nominal power of the drain resistance.
We are going to solve the problem:
To calculate the drainage resistance we already know that it is:
Rs = (Vs-Vo) / (Il + Iz); in our case:
Vs = 9V; Vo = 5.1V;
Il = Vo / Rl = 5.1 / 5 = 1.02A;
Iz = Il / 10 = 1.2 / 10 = 0.102A (10%)
If we put these values in the formula for Rs we will have:
Rs = (9V-5.1V) / (1.02A-0.102A) = 3.48Ω.
As this resistance value does not exist in reality, we will choose the value of a 3.3Ω resistance that does exist in reality and is marketed.
We will now solve section b).
The maximum and minimum values of the input voltage between which the circuit maintains the output voltage regulated, we can clear them from the previous formula, clearing Vs and taking into account that the Iz, current through the zener, cannot be greater than its maximum value Izmax or less than zero. We clear Vs:
Vs = (Il + Iz) x Rs + Vo;
The minimum value for Vs will be when Iz equals zero.
Vsminimum = Il x Rs + Vo;
The maximum value will be when Iz equals Izmax.
Vs = (Il + Izmax) x Rs + Vo;
The values for our exercise are:
Il = 1.02A; Rs = 3.3Ω; Vo = 5,1V and the Izmax will be:
Maximum Iz = Pz / Vz = 1 / 5.1 = 0.196A. If we put the values in the previous formulas, we have:
Vsminimo = 1.02 x 3.3 + 5.1 = 8.47V
Vmaxima = (1.02 + 0.196) x 3.3 + 5.1 = 9.11V.
What does this mean? Well, the input voltage can be between 8.47V and 9.11V so that there is voltage regulation of the zener diode.
If the zener has a voltage lower than 8.47V it stops driving and if it is higher than 9.11V it is destroyed by overheating. This will be the range that we talked about earlier and so the zener cannot be used for all cases.
In both cases there will be no voltage regulation and the circuit will behave like a normal voltage divider.
Conclusion to this is that zener diodes can only be used for a limited range of load voltages or load currents.
To handle high voltages it must be used together with a transistor , which will be in charge of transporting the load current without altering the voltage applied to it. But for that we would have to understand the transistor. If you are interested here you have the link: Transistor .
Finally, let’s calculate section c).
The minimum nominal power of the drain resistance is calculated with the formula:
Ps = (9.11V -5.1V) / 3.3Ω = 1,215w.
Accordingly, a resistance of 3.3Ω must be chosen as a minimum and that it can withstand a power of 2w or so.
You can see everything explained in Video as well as a Real Practice with Board Board of a Zener Diode.
Types of Zener Diodes
Currently you can find zener diodes with Vz values from 0.2V to 200V and from Pz up to 50 watts.
There are mainly two varieties of zener, the ZD or ZDP that are European and the 1N that are American.
The ZDP for example the ZPD12 means that they have a 12V zener voltage. For the rest we will have to look at its characteristics in the manufacturer table, although normally its breakdown voltage is printed on the same zener diode.
