5V dc power supply for mobile phone and other devices.
Welcome to my site, in this article How to make a 5V dc power supply for mobile phone and other related devices. I will help you learn how to use power supply through the AC mains without damaging your circuit.
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| 230V AC to 12V DC Power Supply |
So First we need a transformer, as in a stepdown transformer. As the name itself indicates, a transformer transforms voltage. A step-down transformer meaning from higher voltage to a lower voltage. We need a 5V DC power supply for mobile phone charging and other devices.
So I can use a step down transformer from 230V to 5V AC. however I want to use a transformer of 12V as output voltage. That will get to this point later, so a step down transformer with 12V ratings is what we will be using now. If you have any previous experience in electronics, you may know a transformer’s input and output are both AC voltages. So next step is to convert this 12V AC signal to DC.
To do this we need some diodes, let’s use a simple 1N4007 diode. A diode basically conducts in forward biased condition and acts as an open source during reverse biased.
I will illustrate this in a more simple way. The above image basically is a symbol for diodes, this here one end is positive other end negative. Now if I have a circuit like this, my LED will glow, if I reverse the polarity then my LED does not receive any voltage, since my diode is reverse biased.
Basically forming an open connection like this as shown image which does not allow the current to flow through the LED.
Now since our source voltage is AC, let me just replace my battery with an AC source, since AC stands for alternating current, this is how the voltage varies with respective to time. Now in our case, during the time interval of the positive half cycle this diode is forward biased. Meaning your LED will be glowing, but during the negative half of the cycle this diode is reverse biased resulting an open circuit here consequently turning OFF the LED.
So technically your LED will be blinking every 0.02 seconds assuming your source frequency is 50HZ. The blinking frequency may be small but it’s still noticeable. We don’t want this to happen. So to get an output which has voltage in both positive as well as negative cycle we need to have not one but four such diodes in a proper sequence.
Its opposite I guess for few this may ring a bell, a combination of 4 diodes is mostly used in a bridge rectifier. Now the question is how to I combine it to form a bridge rectifier, well that’s easy, we just need to draw a square box like this and mark the terminals on its four corners.
Let the top and the bottom be for the AC source. In the right let’s have the positive terminal and in left negative. So I need to place my diodes on these two ends so that positive voltage comes out of this. So see as shown the image I place the diode. In this direction positive half cycle would no doubt flow through this, same is true on the other end of the AC terminal. This way we would get positive half cycle in one side and for negative cycle. We just need to place the diodes in the reverse direction with respect to diodes. Now if we just follow the waveform we still haven’t got a DC output yet.
It’s still pulsating! To remove this pulsating effect we need to place a filter capacitor parallel to the pulsating output. For frequency of50Hz, we can use a filter capacitor of capacitance 1000 Micro farad. Now you will get a proper DC output of 12V on these terminals.
How to use LM7805 Voltage regulator,
We are not done yet, our aim was to get 5V dc power supply output. For this we need a voltage regulator placed at this 12V output. So let’s use LM7805 voltage regulator. LM7805 is basically a three terminal device. This three terminals is input, output and common ground. The last 2 digits here imply the output voltage. so we have a voltage of +5V as output, now having said that your voltage regulator will constantly supply +5V output provided. Your input is always above +5V.
Why we need a transformer of 12V
Guess now is the time to explain why we need a transformer of 12V instead of 5V dc power supply. Basically the main part of the transformer involves copper windings or commonly known as coils along with an iron loop. There’s a primary winding and then there’s a secondary winding. The two leads of the primary winding are for source voltage and the two ends of the secondary windings connect to the bridge rectifier.So how exactly a transformer works? To know this we need to know the concept of electromagnetic induction. Basically there is no actual physical contact between the coils.
So technically when an alternating current flows through the primary coil it induces electricity in the secondary coil through the principle of electromagnetic induction.
The output voltage or current at the secondary coil is solely dependent on number of turns. Thickness of the secondary coil provided the EMF induce remains constant.
Now the question is when the EMF changes? The electromagnetic field changes as the primary coil voltage changes, i.e. the source voltage in your AC mains. So imagine I have a transformer of 230V to12V, 2A ratings. What it means is that output will be 12V as long as the source voltage remains at 230V. This is not always true.
There are chances that your source voltage varies whenever there is voltage fluctuation in the power plant or for many other reasons. This is the reason why we need to have a transformer of slightly higher range than required.
So that when the source voltage comes down to a certain level, the transformer output may still remain in voltage higher than actually required. So that the input of the voltage regulator always remain higher than its output. Please note the fact that a voltage regulator is a passive device, it cannot generate extra voltage. So if the input for the regulator falls down below 5V then obviously output will come down as well.
5V dc power supply output.
I hope I’m clear why have we used a transformer of 12V instead of 5V. Now the question is why only 12V? Are there no other transformer with other ratings between 5-12V? The answer is yes, there are and of course you can use them. But most of the time we prefer 12V and sometimes 9V as well. So now we have 5V dc power supply output here.
Wait there’s one more small modification you may be interested in, you might want to have an LED indication to know whether your circuit is properly working or no. i.e. place a 1K ohm resistor in series with an LED between the positive and your negative terminal. And if the LED turns ON then your circuit is ready to use.
Be careful while you make the connections try not to touch the AC supply once you have turned ON your circuit, use proper insulation wherever necessary. Thank you for reading, if you liked my article How to make a 5V dc power supply for mobile phone, please share to others.
