Electronic Circuits Design For Beginners - Chapter 4

It has a temperature compensated precision voltage reference, short circuit protected output, and differencial feedback. It looks like a very complex circuit. Fortunately we don't need to worry about components and mounting, because the whole circuit is available as a tree-legs integrated circuit, with the size of a transistor, and in several packages.

Depending on our tension needs, we will choose the appropriate one from typical values like 5V, 6V, 9V, 10V, 12V, 15V, 18V and 24V and current capability as 0.5A, 1A and 3A. They are generically called 78XX, where XX field can be 05, 12, etc, identifying the nominal output voltage. There may be other characters indicating other characteristics like current capability and package,  for example 78TXX where "T" stands for 3 Ampers.
Next we have some information from its datasheet. First of all the pinout diagram and typical connection.

We see that there is an input, where unregulated tension is connected, a regulated output, and the "Ground" which is the common zero for input and output.

Pay special atention to "Absolute Maximum Ratings" section, there you can see that there is a limit for the input voltage. If we feed the regulator with a higher tension, it will get burnt.


Some tips about electrical characteristics:
Output Voltage: shows the range of values that different regulators can have from manufacture.  For example one regulator may result of 4.85 Volt, and another of 5.2, etc, and these are fixed values for each one. It does not mean that each regulator can vary from 4.75 to 5.25 Volt.
Line Regulation: is the output tension variation when the input voltage varies on the range indicated below. For example, a 7812 at 25ºC withan input voltage varying between 14.5 and 30 Volt, shows an output variation of 4mV typical, and 120 mV maximum (wonderful!! compared with 1 or 2 volt ripple on a just filtered power supply).
Values on different temperature and current conditions are too shown
Load Regulation: is the output tension variation when the output current varies on the range indicated on the left. Of course this happens when the load changes its state and take more or less current. For example, a 7815 with an output current varying between 5 mA and 1.5 A, shows an output variation of 12mV typical, and 150 mV maximum (wonderful too).Continuing with the datasheet, we find a very important parameter which is "Input voltage required to maintain line regulation" (autoexplained). For example if we have a 7812 with an input signal whith ripple, and the tension descends to 13 Volt, then the output becames non regulated, and its value can go down the regulated value. That's why we always must design the previous stage in order to assure a higher Vin, with a margin.


Regulated Power Supply  Design

How do we use a 78XX?
Let's suppose that we wont to design a 12 V x 500 mA regulated power supply. We will need a power transformer with a full wave rectifier and a capacitor filter, for providing the Vinput to the 7812. This is the circuit.

Now we must choose those componentes in order to accomplish the "Input voltage required to maintain line regulation" with a safe margin.
Does our filtered rectifier from Chapter 3 give the required tension for the regulator? (14.6 Volt )
Let's start by the transformer, a 110 to 12 volt. The secundary rms voltage is 12 Volt, we can calculate the Vpeak after the diodes, using the formula from Chapter 1



This is the maximum value, for knowing the minimum we need to substract the ripple:



It is not enough, so we will need a higher voltage, let's recalculate Vmin with a 110 to 15 Volt transformer and the same capacitor:



Now there is a 3.3V safe margin, just in case any line variation. That's perfect. A higher margin is not desirable, it would result in a higher power and heat dissipation. We can see below how to calculate the power dissipated on the 7812, it is the voltage difference between input (average) and output, multiplied by the output current:






We will need to use a dissipator to reduce heating, on next chapter we will explain how do we do to calculate it. By now, let's know that a little dissipator of 10ºC/W will work fine.

The last thing we must calculate is the transformer's power, which is obtained multiplying nominal rms voltage by output dc current:



We can buy it as a "15V x 7.5W transformer" or, in other words, a "15V x 0.5A transformer".

This is the circuit with complete components' characteristics.


The 100nF capacitor is recommended in the manual for regulator stability.

Applying the same procedure you can design power supplies for other voltages and configurations. For example, below we have a +-15 split power supply, typically needed for operational amplifiers applications.

There you see a 7915 negative regulator. 79XX series are simmetric to 78XX, appropriated for negative or split power supplies.
If current capability needs to be enhanced, there is a variation of this design which is explained on chapter 14.
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On the next chapter we will learn how to calculate heat dissipators, and after that, some basic circuit blocks based on resistors, diodes and capacitors.