12 V DC POWER SUPPLY

Today I am going to make here a 220 Volt AC to 12 Volt DC  Power supply.

For that we must be familiar with the conversion of  high voltage AC to low Voltage AC.A transformer is used for the conversion, which is the combination of two coils mutually coupled together using a ferrite core, Which is a soft iron core.One coil is of large numbers of turns with high resistance of small diameter of copper wire, which is called high voltage coil and another coil is of low resistance ,low number of turns but with comparatively large diameter of copper wire,which is called low voltage coil.

TRANSFORMER TEST-

Before using the transformer we must know which coil's terminals should be connected to mains or circuitry.It is important since this knowledge would protect your home wiring as well as protects your transformer  from being burned or damaged.

Now have a multimeter  (analog or digital) with you and set it to ohm measurement,now test each coils terminals,see that what readings you get.

If you are getting a reading in few kilo-ohms then it must be the high voltage coil or primary coil.

If you are getting a reading in only ohms then it must be the low voltage coil or secondary coil.

Also test for the shorting of the coils by checking the resistance of both the coil is infinite or not,If not then the transformer is faulty, just change the transformer.

Also test for the coil and core resistance is infinite or not,If so then be aware that you can get shock if you don't take care of it.It is recommended that you must change the transformer in such condition.

BUILDING THE DEVICE-

Here is the list of components-

:: Four 1N4001 diodes

:: one 9-0-9,500mA transformer

:: one LM7812 IC

:: one 470uF,50v Electrolytic capacitor

:: one 10uF,25v Electrolytic capacitor

:: one 0.1uF ceramic capacitor

:: wires

:: shoulder and shouldering iron

:: A perforated copper board or a copper clad board with 5cm By 5cm                  dimensions.  

:: A 10 cm by 10 cm cardboard

Make a bridge rectifier (see here how to make bridge rectifier) on the perforated board now take the transformer secondary or low voltage coil terminals as the rectifier AC input.The 9-0-9 has three terminals of the low voltage coil,leave the zero potential or common terminal of the coil and take the other two terminals.Always remember that a transformer output gives an AC voltage which has no polarity so don't worry of the connection at the rectifier input,it can be connected at any polarity as the input to rectifier.

Now the output of the bridge rectifier is always fluctuating DC, for knowing the polarity test it with the multimeter.You can also know the polarity by seeing that the same p terminals connected together will have negative output and vice-versa in the bridge rectifier.

Now after knowing the polarity, connect the 470uF capacitor in parallel to the bridge rectifier output.Before connecting also see the polarity of the electrolytic capacitor and connect with the same polarity as the bridge output.The black line near the terminal shows that it is negative terminal.

If the electrolytic Capacitor is connected with wrong polarity then the capacitor may explode.Also connect another 10uF electrolytic capacitor in parallel with the 470uF capacitor with correct polarity.

Now connect the positive terminal of the capacitor to the input or first terminal of the LM7812 IC and other terminal to the ground terminal or second/middle terminal of the IC. Now take the output at third/Output terminal of the IC w.r.t. the ground terminal.For stability Connect 0.1uF capacitor in parallel to the output terminal and ground terminal.Take the output across the 0.1uF capacitor.

Now fit all the circuitry on the cardboard and use it for any electronic application, Which requires 12v dc and 400mA of current.For high current applications just add a heat sink to the LM7812 IC.

You can also mount the components on a copper clad board PCB (See here how to make PCB).

Here are some pics of the Device-