Three Terminal Fixed Voltage Regulators 78XX, 79XX

In this lesson, we'll talk about the operation of three terminal fixed voltage regulators (78XX, 79XX).

First, we should know that, what is the purpose of a voltage regulator?
The purpose of a voltage regulator is to provide a constant dc output that is practically independent of the variations of the input voltage, output load current and temperature.

Now here, I’m showing you the circuit diagrams of three terminal fixed voltage regulators (78XX, 79XX).

All linear regulators require an input voltage at least some minimum amount higher than the desired output voltage. That minimum amount is called the dropout voltage.

For example, in this case, 7812 has an output voltage of +12V but can be maintained, if the input voltage remains above about 14.5V before the output voltage begins sagging below the rated output. Its dropout voltage is therefore 14V − 12V = 2.5V.

The 79XX series of fixed output voltage regulators are complements to the 78XX series devices.

For example, in this case, 7905 has an output voltage of -5V, but can be maintained, if the input voltage remains above about -7.3V, before the output voltage, begins sagging below the rated output.

In the next session, I will explain to you how to implement and test the voltage regulator circuits on the bread board using Autodesk circuits.
Thank you.