Design of 5v Power Supply Using 7805 Pdf

Looking for some help to design a 5V power supply on your own? Well then, welcome. In this post, we will just not design a power supply but also get to know about design calculations that you can do all by yourself.

A power supply circuit is a very basic circuit in learning electronics. Almost everyone in electronics tries to make it. And I can not tell you how much fun it is when you finish your first power supply design, test it, and it works fine.

Alright!

The power supply which we will design here is very basic. It is a linear technology-based design, will go through you each design step, try to present everything in simple language, will carry out some design mathematics i.e. if a capacitor is being used in the schematic, you should know why it is there, and how its value is calculated.

Hopefully, you will enjoy this post and will learn something. Just in case you enjoy electronics by doing DIY, then this Variable power supply DIY kit (Click here) will be the right fit for you. Have fun 😀

Table of Contents

The design of 5V DC power supply
- The input transformer
- The rectifier circuit
- The filter
- The regulator
Circuit diagram of 5V DC power supply
Step by step method to design 5V DC power supply
- Step 1: The selection of regulator IC
- Step 2: The selection of transformer
- Step 3: The selection of diodes for the bridge
- Step 4: The Selection of smoothing capacitor and calculations
- Step 5: Making the power supply safe
More fun with electronics
Conclusion

The design of 5V DC power supply

The design of any circuit begins with a well made general block diagram. It helps us to design the sections of the circuit individually and then at the end put them together to have a complete circuit, ready for use.

The general block diagram for this project is given below. It is very simple. It has the following four main sub-blocks.

The Transformer
The Rectifier Circuit
The Filter
The Regulator

First, I will explain each block in general and then we will go for designing. I think you need to understand which block is doing what first.

block diagrame of 5V Dc power supply

So, let's try to understand each section one by one.

The input transformer

A transformer is a device that can step up or step down voltage levels, following the law of conversation of energy.

The question is, why we need it in our supply design?

Well, depending on your country, AC coming to your home has a voltage level of 220/120 V. We need the input transformer to step down the incoming AC to our required lower-level i.e. close to 5V (AC). This lower level is further used by other blocks to get the required 5V DC.

A transformer is a device that is used to step up or step down the AC voltages level, keeping the input and out power the same.

Be careful when playing with this device.

As you are using the main supply voltage which can be too dangerous. Never touch any of the terminals with bare hands or with bad instruments. Have a good and decent non-contact voltage tester, and use it to always be sure of which line is the live wire coming to the transformer.

The rectifier circuit

If you are thinking the transformer just stepped down the voltage to 5V DC. I am sorry, you are wrong like once I was. The stepped-down voltage is still AC. To convert it into DC, you need a good rectifier circuit.

A rectifier circuit is the combination of diodes arranged in such a manner that converts AC into DC voltage.

Without the rectifier circuit, it is not possible to have the required output 5V DC voltage. This circuit comes in nice integrated packages or you can make it using four diodes as well. You will see how we design it in later sections.

Basically, there are two types of rectifier circuits; half-wave and full-wave. However, the one which we are interested in is a full rectifier, as it is more power-efficient than the first one.

The filter

Nothing is ideal in practical electronics. The rectifier circuit converts the incoming AC to DC but unluckily it does not make it a pure DC. The output of the rectifier is pulsating and is called pulsating DC. This pulsating DC is not considered good to power up sensitive devices.
So, the rectified DC is not very clean and has ripples. It is the job of the filter to filter out these ripples and to make the voltage compatible for regulation.

A capacitor filter is used when we need to convert a pulsating DC into pure or to remove distortion from signal

A rule of thumb is DC voltage must have less than 10 percent ripples to be regulated perfectly.

The best filter in our case is the capacitor. You may have heard, a capacitor is a charge storing device. But actually, it can be best used as a filter. It is the most inexpensive filter for our basic 5V power supply design.

The regulator

A regulator is the linear integrated circuit use to provide a regulated constant output voltage. Voltage regulation is very important because we do not need a change in output voltage when the load changes.

An output voltage independent of the load is always required. The Regulator IC not just makes the output voltage independent of varying loads, but also from line voltage changes.

A regulator is the integrated circuit used to give a constant output voltage regardless of input voltage changes.

I hope you have developed some basic concepts of power supply design. let's go further with the actual circuit diagram for our specific 5V DC power supply design.

Circuit diagram of 5V DC power supply

Below is the circuit diagram for the said project. You get the main supply; voltage and frequency can depend on your country, fuse; to protect the circuit, transformer, rectifier, capacitor filter, an LED indicator, and the regulator IC.

Design 5V 500mA power supply

The block diagram is implemented in NI Multisim software, good simulation software for students and electronics beginners. I encourage spent some time playing with it.

Now, let get into the actual design.

Step by step method to design 5V DC power supply

Here is the deal, we will design each section first, and then put together each of them to have our DC power supply ready to power up our projects.

So let's get started step by step.

You are thinking, I would start the design explanation from the transformer but it is not the case. A transformer is not selected at the very first.

Step 1: The selection of regulator IC

The selection of a regulator IC depends on your output voltage. In our case, we are designing for the 5V output voltage, we will select the LM7805 linear regulator IC.

In the design process, the next thing is, we need to know the voltage, current, and power ratings of the selected regulator IC. This is done by using the datasheet of the regulator IC.

The following are the datasheet provided ratings and pin diagram for LM7805.

Design 5V power supply

The datasheet of 7805 also prescribes to use of a 0.1μF capacitor at the output side to avoid transient changes in the voltages due to changes in load. And a 0.1μF at the input side of the regulator to avoid ripples if the filtering is far away from the regulator.

Just for extra knowledge, for positive voltage output, we use LM78XX. XX indicates the value of output voltage and 78 indicates positive output. For negative voltage output use LM79XX, 79 indicates negative voltage and XX indicates the value of output.

Step 2: The selection of transformer

The right transformer selection means saving a lot of money. We got to know, the minimum input to our selected regulator IC is 7V (See above datasheet values). So, we need a transformer to step down the main AC to at least this value.

But, between the regulator and secondary side of the transformer, there is a diode bridge rectifier too. The rectifier has its own voltage drop across it i.e. 1.4V. We need to compensate for this value as well.

transformer and transformer symbol

So mathematically:

This means we should select the transformer with a secondary voltage value equal to 9V or at least 10% more than 9V.

From these points, for the 5V DC power supply design, we can select a transformer of current rating 1A and a secondary voltage of 9V. Why 1A current? Because the regulator IC has a current rating of 1A, meaning we cannot pass more current than this value. Selecting a transformer with a current rating more than this will cost extra money. And we don't need it.

Step 3: The selection of diodes for the bridge

You see in the circuit diagram, the rectifier circuit is made by arranging diodes in some patterns. To make a rectifier we need to select proper diodes for it. When selecting a diode for the bridge circuit. Keep in mind the output load current, and maximum peak secondary voltage of the transformer i-e 9V in our case.

rectifier circuit with symbol

Instead of individual diodes, you can also use one individual bridge that comes in an IC package. But I don't want you to use it here, just for the purpose of learning and playing with individual diodes.

The selected diode must have the current rating more than the load current (i.e. in this case is 500mA). And peak reverse voltage (PIV) more than peak secondary transformer voltage

We select the IN4001 diode because it has a current rating of 1A more than our desire rating, and a peak reverse voltage of 50V. Peak reverse voltage is the voltage a diode can sustain when it is reverse biased.

Step 4: The Selection of smoothing capacitor and calculations

Things we need to keep in mind while selecting a proper capacitor filter are, its voltage, power rating, and capacitance value. The voltage rating is calculated from the secondary voltage of a transformer.

The rule of thumb is, the capacitor voltage rating must be at least 20% more than the secondary voltage. So, if the secondary voltage is 13 V (Peak value for 9V), then your capacitor voltage rating must be at least 50V.

capacitors for basic electronics

Second, we need to calculate the proper capacitance value. It depends upon the output voltage and the output current. To find the proper value of capacitance, use the formula below:

Where,

Io = Load current i.e. 500mA in our design, Vo = Output voltage i.e. in our case 5V, f = Frequency i.e 50Hz

In our case:

The frequency is 50Hz because in our country mains AC is 220 @ 50Hz. You might have 120V @ 60Hz mains AC. If so then put the values accordingly.

By using capacitor formula, the practical standard value close to this value i-e 3.1847E-4 is 470uF.

Another important formula is listed below. This can also be used to calculate the capacitor value.

In this case, R is the load resistance. Rf is the ripple factor, which should be less than 10% for a good design. And with this, we almost finished with a 5V power supply design.

Step 5: Making the power supply safe

Every design must have a safety feature to protect it from burning. Similarly, our simple supply must have one i.e. the input fuse. The input fuse will protect our supply in case of overloading.

fuse and fuse symbol

For example, our desire load can handle 500mA. If in case our load starts to miss behave, there is a chance of burring of components. The fuse will protect our supply.

A rule of thumb for selecting the fuse rating is, it must be at least 20% more than the load current.

The simple power supply we designed has the capability to deliver 1A current, which in some cases you can use it for. If you decide to use it for such cases, then don't forget to attach a heat sink to the regulator IC.

More fun with electronics

Electronics is much fun. You got new things to do all the time once you step into the world of electronics.

If you enjoy making DIY electronics, enjoyed this post, learned all the design concepts, and now are curious to make your own DIY power supply project. You want to solder and play around with all the above mentioned components, then check it out, the Elenco power supply kit (Amazon Link), you will find itinteresting .

Also, there is a fun book called Make Electronics: Learning through discovery (Amazon link), which will teach you a lot of cool electronics with practices. If you find this book interesting, give it a try you will learn a lot.

Conclusion

For me, if you are an electronics hobbyist or beginner, learning some basic electronics, I would recommend you to design your own lab power supply.

It will help you learn electronics as well as give you the best lab power supply.

I call it the best one because you will make it yourself. And I cannot put it in words how much fun it is to play with electronics in a safe environment. It is like learning from doing

Please do not specify it to just a 500mA supply. It can be your 5V DC power supply in general with up to 500mA current capability. And this was what I know how to design a 5v dc power supply.

Hopefully, it was some kind of help to you.

Thank you and have a good life.