Zener Diode
Zener diodes look very similar to general purpose diodes in terms of
size and shape, but zener diodes have got a very unique characteristic
which makes them special.
It is known as zener voltage.
The zener voltage is a constant and is given by manufacturer.
Zener Voltage :
In reverse bias a zener diode tries to hold a constant voltage across it.
This constant voltage is characteristic of diode and known as zener
voltage.
It should be noted that in forward bias Zener diodes works similar to a general
purpose diode. In forward bias, the drop across Zener diode is 0.7V like any other
general purpose diode, as shown above.
Analyze both the above circuits in fig1 and fig2. You can see that we have used zener diode of "zener voltage 3.3V". As you can see in fig1, voltage drop across zener is 3.3V since it is its zener voltage. Now if we change the input voltage to 12V (fig2), the voltage across zener still remains same 3.3V. Interesting isn't it? Thats the beauty of zener diodes, "Voltage always Remains Constant", but there is a catch here..
The catch is that the input voltage should be more than the zener voltage in order for the zener to get the constant voltage. Otherwise zener will behave same as a general purpose diode in reverse mode(bias). You might be wondering where the excess voltage has gone? Has zener eaten it up? If you see both the circuits and voltage drops closely, you will find that voltage drop across resistor has increased by 2V. This is the place where voltage has got compensated. This property of zeners is used to make constant voltage regulators. It means that no matter how much the input voltage fluctuates, the voltage across zener will remain constant. One can easily tap voltage across zener and provide it to another circuit which works on 3.3V. And for that circuit input voltage will always be constant 3.3V (independent of supplied voltage fluctuation)
Analyze both the above circuits in fig1 and fig2. You can see that we have used zener diode of "zener voltage 3.3V". As you can see in fig1, voltage drop across zener is 3.3V since it is its zener voltage. Now if we change the input voltage to 12V (fig2), the voltage across zener still remains same 3.3V. Interesting isn't it? Thats the beauty of zener diodes, "Voltage always Remains Constant", but there is a catch here..
The catch is that the input voltage should be more than the zener voltage in order for the zener to get the constant voltage. Otherwise zener will behave same as a general purpose diode in reverse mode(bias). You might be wondering where the excess voltage has gone? Has zener eaten it up? If you see both the circuits and voltage drops closely, you will find that voltage drop across resistor has increased by 2V. This is the place where voltage has got compensated. This property of zeners is used to make constant voltage regulators. It means that no matter how much the input voltage fluctuates, the voltage across zener will remain constant. One can easily tap voltage across zener and provide it to another circuit which works on 3.3V. And for that circuit input voltage will always be constant 3.3V (independent of supplied voltage fluctuation)