A voltage regulator is a device used to provide pure DC supply to electronic devices. In other words a voltage regulator maintains a constant voltage level.
What is the need of Voltage Regulator?
As you have learned during the lecture of rectifier circuits that an AC is converted to DC. But this DC got fluctuations which were not suitable for proper working of a DC load.
After rectifier circuit we used capacitor-input filter to remove those fluctuations. Although we removed large fluctuations but still there were small ripples in the output. (See Fig-1)
Fig-1: Comparison of Rectifier & Filter outputs
You can observe still small fluctuations remain. We need to remove these fluctuations to get a constant voltage level. That is why we need a voltage regulator.
In this lecture i will focus on how zener diode works as voltage regulator.
Zener as Voltage Regulator
As we learned before that zener diode works in reverse bias. For your convenience i copy the zener curves again below. But i recommend you first read the lecture on zener diode.
Observe above zener curves. VZB is the zener breakdown voltage. At this voltage zener diode goes into breakdown region. After that for a very large flow of current for example upto IZ_max it maintains a nearly constant voltage across its terminals. But on the other hand we have to maintain at least IZ_min to keep it in breakdown region. Otherwise it could not be able to maintain regulation. The efficiency of zener voltage regulator is not good so it is only used for applications need small load currents.
Consider the following circuit which will further help you to understand its working as regulator
Fig-2: Zener Voltage Regulator
In the above circuit the zener diode is connected parallel to the load. The resistor limits the current. We used 1N4733A zener. The values of Vin are set so that zener can operate within Iz_min and Iz_max. When Vin varies zener maintains almost constant values of voltage at its terminals.
For 1N4733A Iz_min is 1mA. VZB is 5.1V.
From the circuit you can see
Assume Pzener (Power of diode) is 1W. So maximum zener current would be
This means that zener can regulate the voltage between 5.2V to 24.7V. Here zener will maintain almost a constant voltage of 5.1V.