Introduction: Dual Power Supply Circuit(+12V and -12V)

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Hi friend,

Hope you are very well. Today i will show you how to create a dual power supply easily. The objective of this project is to convert 220V AC supply in to +12V and -12v DC supply, that is why it is named Dual Power Supply as we get positive and negative 12v power supply at the same time


There are three step:

1. Firstly, 220V AC is converted into 12V AC by using simple step-down (220V/12V) transformer.

2. Secondly, output of this transformer is given to the rectifier circuit, which will convert the ac supply into dc supply. The output of the rectifier circuit that is DC contains the ripples in the output voltage. To filter out these ripples, capacitor of 2200 uf, 25V is used.

3. Lastly, the output of the capacitor that is pure DC is given to voltage regulator IC 7812 and IC7912 which will regulate the output voltage at 12V and -12V DC, despite the change in input voltage.

Step 1: Required Component

Centre tapped transformer (220V/12V)

Power Diodes (6A) – 4No.

Capacitor (2200μF, 25V) – 2No.

Voltage regulator (IC 7812 & 7912)

Toggle switch DC load (DC motor)

Connenct those according to circuit diagram

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Step 2: Constructing Dual Power Supply Circuit:

Converting 220v AC into 12v AC using Step Down Transformer

The primary terminals of the centre tapped transformer is connected with household supply (220V ac, 50Hz) and output is taken from secondary terminals of the transformer. The centre tapped describes the voltage output of a center tapped transformer.

For example: A 24V centre tapped transformer will measure 24V ac across the outer two taps (winding as a whole), and 12V ac from each outer tap to the center-tap (half winding). These two 12V ac supplies are 180 degrees out of phase with each other, thus making it easy to derive positive and negative 12 volt dc power supplies from them.

Step 3: Converting 12v AC Into 12v DC Using Full Bridge Rectifier

The outer two terminals of the centre tapped transformer are connected to the bridge rectifier circuit. Rectifier circuit is a converter, which converts ac supply in to dc supply. It is generally made up of diode switches as shown in Circuit Diagram.

To convert ac into dc, we can make two types of rectifiers, one is half bridge rectifier and second is full bridge rectifier. In half bridge rectifier, output voltage is half of the input voltage. For example, if input voltage is 24V, then output dc voltage is 12V and number of diode used in this type of rectifier is 2. In full bridge rectifier, number of diodes is 4 and it is connected as shown in figure and output voltage is same as the input voltage.

Filter the Ripples from the output:
Now, 24V dc output which contains peak to peak ripples can’t be connected directly to the load. So, to remove ripples from the supply, filter capacitors are used. Now, two filter capacitor of rating 2200uF and 25 V are used as shown in circuit diagram. The connection of both capacitors are such that the common terminal of the capacitors is connected directly to the centre terminal of the centre tapped transformer. Now, this capacitor will get charged upto 12V dc as both are connected with the common terminal of a transformer. Furthermore, the capacitors will remove the ripples from the dc supply and give a pure dc output. But, the output of both the capacitors are not regulated. So, to make the supply regulated, output of the capacitors are given to the voltage regulator ICs which is explained in next step.

Regulate the 12v DC Power Supply
The next important thing is to regulate the output voltage lab of the capacitors which will otherwise be varying as per the input voltage change. For that depending upon the output voltage requirement, regulator ICs are used. If we need the output voltage +12V then IC 7812 is used. If required output voltage is +5V, then 7805 IC is used. Last two digits of the IC gives output voltage rating. Third last digit shows voltage is positive or negative. For positive voltage (8) and for negative voltage (9) number is used. So IC7812 is used for +12v regulation and IC7912 is used for -12v voltage regulation.