Hi. Hope you are having a good life.
A power supply is the very basic circuit in learning electronics. Almost every electronics beginner tries to make it. And I can not tell you how much fun it is when you finish your first power supply and it works fine. In this post, I share my knowledge on how to design 5V DC power supply.
The power supply which we will try to design here is the very basic. It is not like copying the circuit from the internet and implement it on a breadboard. It is like, learning how to design one yourself. For example, if a capacitor is being used in the schematic, you should know why it is there, and how its value is calculated.
Hopefully, this post will help you
- 1 The design of 5V DC power supply
- 2 Circuit diagram of 5V DC power supply
- 3 Step by step method to design 5V DC power supply
- 4 Making the power supply safe
- 5 5V Power supply kit (DIY)
- 6 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.
So, lets try to understand each section one by one.
The input transformer
A transformer is a device which can step up or step down voltage levels, following the law of conversation of energy.
Depending on your country, AC coming to your home has the 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.
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 converts AC into DC voltage. Basically, there are two types of the rectifier circuit; half wave and full wave. However, the one which we are interested in is a full rectifier, as it is more power efficient than half rectifier.
The smoothing capacitor/filter
Nothing is ideal in practical electronics. The rectifier circuit converts the incoming mains to DC but unluckily it can not make it a pure DC.
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 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 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.
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.
A 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.
I hope you have developed some basic concept 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.
The block diagram is implemented in NI Multisim, a good simulation software for students and electronics beginners. I encourage spent some time playing with it.
As, in my opinion, you should have a strong knowledge of simulation software in order to do fun things in learning basic electronics.
Step by step method to design 5V DC power supply
Like I said, 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 lets 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.
Next thing is, we need to know voltage, current and power ratings of selected regulator IC. This is done by using the datasheet of regulator IC.
Following are the ratings, pin diagram for LM7805.
Datasheet of 7805 also prescribes to use 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.
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. So, we need a transformer to step down the main AC to at least this value.
But, between the regulator and 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.
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 5V DC power supply design, we can select a transformer of current rating 1A and secondary voltage of 9V or 12V.
Step 3: The selection of diodes for bridge
You see, the rectifier is made by arranging diodes in some pattern. To make 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.
Instead of individual diodes, you can also use one individual bridge that comes in IC package. But I don’t want you to use here, just for purpose of learning and playing with individual diodes.
[su_quote]The selected diode must have the current rating more than the load current. And peak reverse voltage (PIV) more than peak secondary transformer voltage.[/su_quote]
We select IN4001 diode because it has the current rating of 1A more than our desire rating, and peak reverse voltage of 50V.
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.
Rule of thumb is, the capacitor voltage rating must be at least 20% more than the secondary voltage.
[su_quote]So, if the secondary voltage is 17 V (Peak value), then your capacitor voltage rating must be at least 50V.[/su_quote]
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:
Io = Load current i-e 500mA in our design, Vo = Output voltage i-e in our case 5V, f = Frequency
In our case:
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. Then by using capacitor formula, the practical standard close to this value i-e 3.1847E-4 is 470uF.
Another important formula from the book “Electronic Devices by Thomas L. Floyd” is listed below. This can also be used to calculate the capacitor value.
In this case, R is load resistance. And Rf is ripple factor, which should be less than 10% for a good design. And with this, we just finish designing 5V power supply.
Making the power supply safe
Every design must have a safety feature to protect it from burning. Similarly, our simple supply must have a one i.e. the input fuse. The input fuse will protect our supply in case of overloading.
For example, our desire load can handle 500mA. If in case our load start 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 case 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.
5V Power supply kit (DIY)
So, we got some basic knowledge of how a simple 5V power supply is designed.
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 would be a very good decision. 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.
I recommend Elenco power supply kit (Amazon Link) to start with. It is affordable, high quality, and well documented to guide you with each step. Trust me you will learn a lot. You will learn how to solder, assemble, and make an end product like you always see in different stores.
This is what I know how to design 5v dc power supply.
The design power supply would be fine to power up your other small projects or brings you good grades/money if you are assigned to a similar project.
I do not know why, but I feel confident if you follow the same easy steps with me you will have your first designed power supply.
Please do not specify it to just 500mA supply. It can be your 5V DC power supply in general with up to 500mA current capability.
Just for extra knowledge, for positive voltage output use LM78XX. XX indicates the value of output voltage and 78 indicates positive output. For negative voltage output use LM79XX, 79 indicate negative voltage and XX indicates the value of output. To get positive 5V regulated output use LM7805. To get negative 5V regulated output use LM7905.
Thank you and have a good life.
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