I bought a power supply with 3 to 12 volts variable voltage selector last year. But guess what, it never work as expected. Any voltage I chose, it threw out 12 volts. So long it was laying in its box until I decided to mod it last month.. er.. actually it has been months until I finish this iBle :D

I replaced the whole circuit and turn it into a 2.2V to 14V Linear Adjustable Voltage Adjustable Current Power Supply. The circuit I used is taken from Homemade Circuit Projects so credit goes to Swagatam Majumdar.

I am too late to post this and enter the "Before and After Contest", but never mind, sharing is my top priority :)

Let's walk through the journey of the making, shall we ...

Step 1: Circuit and Materials

This is the circuit posted in Homemade Circuit Projects.

Then I redraw it into my version :

The materials in this project :

R 56
R 120Ω
R 330Ω
R 1KΩ
R 1K5Ω
R 2K2Ω
R 5K6Ω
R 1KΩ; 5 watt
R 0.2Ω; 2 watt
1 x Bridge Diode PBL406 4A
2 x Transistor BC547
1 x Transistor 2N3055
2 x Capacitor 1,000uF 35V
2 x Potentiometer 10K
1 x Potentiometer 1K
3 x Potentio-knob
1 x Universal PCB board
1 x Dual Display Voltage-Current meter

The old parts from my previous (before modification) power supply that I use are:

Power Supply Case
On/Off switch
LED for power on indicator
Multiple output transformer which has max output around 14 volts

Well, for the resistors if you can't find the exact value of the resistor or wattage, you can use some in series or parallel to get the closest value as needed in the circuit diagram.

Step 2: Align and Soldering Parts on PCB

I really love this part. It's like playing with Lego blocks :D

First I put one of every kind of parts needed on the PCB board, to figure out how many holes it takes on the board. Then I design the parts' locations on a piece of paper. I really need to do this because I want it as compact as possible. I have a limited space on my not-so-big case. I also need to place them neatly so that there will be no crossing path on the board. Then I re-draw it using vector app, mark the parts with different colors.

Tip : 
You better print a horizontally-flip version of your drawing so that it is easier to trace the tracks on the back of the PCB.

Double check your circuit. The last two pictures show that I miss one connection and my circuit fails. It took me one day to find what I was missing. Yeah, I didn't print the flipped circuit so it was hard for me to compare my circuit's line and my drawing. I figure out this method after everything is done and my mind gets clear, so I present you as a "tip" ^_^

Step 3: Drill and Drill ...

Now we are drilling to place the pots, on-off switch in front and 2N3055 transistor at the back. First I wanted to drill only two holes to hold the transistor. Later I thought that it would be better to place it outside of the case to save some space inside. Then I added another two holes at the back of the case. Then I drill some other holes on the bottom for the new position of the transformer. Previously it was located horizontally in the middle of the case. I moved it vertically on one side, so that I can put my board on the other side.

Step 4: More Drills ....

Now it is crowded in this tiny container. I was frighten that it will get too hot in there, but it will be used for testing and not continuously on for a long time, so be it.

I set the dual display digital meter on top of the case. Then I need more drills. A lot of drills to make a big hole for the display since I have no Dremel cutter. Use a file to clean the cut.

About wiring of the display, there is little documentation or manual of the product I used. But somehow it measure good after some testing. There are 3 large wires : Red-Yellow-Black.

Here are how I wire them :

Red to the positive output of power supply.
Yellow to the negative output of power supply.
Black to the negative output of the Bridge Diode.

There are also two thin wires : Red-Black; which are supposed to connect to external power for the display, but I have no luck with them, so I unplugged the wires.

Step 5: Give It a Test

Well, there is slightly different between my multimeter and the power supply display. None of them is top-of-the-line product. On the video below, you can see that below 4 volts the display will goes off, but the power supply is still working. I have no idea how to connect it to external power because when I try to connect the small wires to external battery, the measurement goes ridiculous.

Finally I conclude that I really need a better multimeter. My current multimeter cannot measure Calorie. I need to know how much calories were gone in this project so that I can charge ME back to initial condition. I'll put that kind of measurement tool in my upcoming projects list ... who knows :D