No matter what project we are working on,a power supply is always needed to power it or test it. The required voltage will always be different for different projects.So here is an easy way to built your own compact yet powerful linear variable voltage power supply which can produce any voltage between 1.2 to 20 volt with 2 ampere current but still takes less space than a coffee mug on your work table.It is also very useful in cases were the adapter go missing for different electronics like desktop speakers etc and it can also be used for charging batteries.
Step 1: Things You Will Need:-
1. LM338K voltage regulator IC X1
2. 4700uf 50v capacitor X 2
3. Variable resistance potentiometer 10k X 1
4. 220 ohms resistor (1 watt) X 1
5.12K resistor (1 watt) X 1
6. 2.7K resistor ( for led) X 1
7. LED X 1
8. Diode 1N4007 X 2
9. Capacitor 0.1uf X 1
10. Transformer ( with 15 volt at transformer output) X 1
11. 2 amp diodes X 2 (in case of central tap transformer)
X 4 (in case of ordinary transformer)
12. Mini volt meter X 1
13. Small plastic connector for taking output voltage X1
14. Any metallic watch can.
15. A power chord with ground.
16. A 2 ampere poly fuse.
Step 2: Making the Transformer Rectifier Circuit
The rectifier filter circuit for both central tap and ordinary transformer is given above.
Here I have used a 15v 2 amp central tap transformer which is over 25 years old which I scavenged from one of my dad's old college project. The rectifier circuit consist of 2 diodes of 2 amp rating and a 4700uf capacitor (50v).
The voltage output after the rectifier and filter stages will be nearly 20 volt.
Step 3: Mounting the Transformer Inside the Can
1. The inside of the can is insulated using insulation tape just in case .
2. Two holes are drilled under the can for the nuts and bolts
3. One hole is drilled on the side or back to take the power cable out.
4. The power cable is taken in through that hole and the line and neutral are soldered to the primary coil of the transformer.The earth wire is connected with the body of the can(note:rubber padding should be used around the hole to prevent cutting of wired due to sharp edges)
5. The transformer is positioned and fastened tightly to the bottom of the can.
6. The rectifier is placed in the can in one of the gaps.
Step 4: THE MAIN VOLTAGE REGULATION CIRCUIT
1. All the necessary holes are made on the can lid.
2. The IC, potentiometer and voltmeter are fixed to the lid of the can.
3. All the components are soldered according to the above circuit inside the lid.
4. The 2 ampere poly fuse is connected in series with the final variable output terminals.This is important as it protects the IC from accidental short circuits and over current.
5. The transformer rectifier circuit and the voltage regulation circuits are connected together.
6. A hole is made on the top or front of the can and the final variable output wires are taken through it and connected to a plastic two wire connector which is bolted to the body of the can.
Step 5: Testing and Finishing
1. before the lid is closed the circuit is tested by switching the power supply on and turning the knob to see if the voltage is changing
2. If all is fine close the lid carefully and there you have it , your own variable power supply.