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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 ...

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## 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 :

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

## Recommendations

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## 26 Discussions

Great Project! Could you explain the input supply a little more? And what transformer you used? I understand the bridge rectifier

The input is 220V 50Hz main outlet. And the transformer I used here is 220V to multiple output, I am using its max output around 14V with 2A max. You can get a transformer up to 30V. For the maximum current you need to change the value of R4 with the calculation provided.

hello!
i made this circuit but find no use of P1 & P3 potentiometers. It is supposed to control current but its not working. Only P2 is working. Please advice.

P1 is supposed to control Current. You need to have some loads connected before you can change the current.

P3 is what we called "fine tuning", so let's say P2 jumps from 4.5V to 6V with a very light turning, then we can get 5V easier with this P3 :)

GOOD BUILD ! Not knowing the exact circuit , I am guessing that the old circuit appears to be unregulated . With no load on the output , the capacitor will charge up to whatever the peak value of the AC voltage of the winding you have selected , minus the voltage drop of the rectifier . If you set it for 12V then turn the switch to a lower setting , the capacitor will still be charged up to the higher voltage , and a high impedance digital meter will read the capacitor voltage . A " load resistor " on the output would have possibly fixed that problem by " draining " the charge in the capacitor . Another suggestion might be to keep the winding selector switch , and feed the output of the switch into your rectifier bridge , then use your control pots as " fine adjustment " controls . You may need to TWEEK the circuit a bit , but it would be much more efficient . The circuit you have now would cause the 2N3055 transistor to waste power by generating a lot of heat when using it at low voltages and higher current , you may even need a heat sink . Do the math . If you have a 10V drop across the transistor , at , say 2Amps , you will generate 20 watts of heat ! Some soldering irons are 25Watts . If you lower the voltage feeding the bridge rectifier , you will lower the voltage drop across the transistor , thus generating less heat !

Cheers , take care , and have a good day !....73

4 replies

I am a newbie in circuits. That's why I try Swagatam Majumdar's circuit and credit goes to him. I am using the metal case as heat sink, but yes, I don't notice the calculation of the power waste. Thank you for your detail explanation. I will learn more and more and build more and more hopefully ^_^

You are doing fine , you will be an expert at this when you experiment and build more projects . Thomas Edison once said that " an invention is 10% inspiration , and 90% perspiration " , keep working at it !

Cheers , take care , and have a good day !!.....73

Yes, this is my 10% inspiration Re-Think-of-Phone.

Only that my 90% perspiration is because it is getting hot in here, not because of working hard to make my #dreamPhone come true hahaha...

Sure I work hard, but experiment-on-projects comes as hobby. I need to earn some money for my family is the top priority :)

adequately heatsinked a 2N3055 can probably dissipate 20 Watts. At least a metal can TO-3 package could.

It looks good. Did you ever determine what may have been wrong with the supply originally? There does not seem like there was much to it to possibly go wrong to me. It was a multi tap transformer with a rotary switch to change which tap was connected to the output it looks like to me.

5 replies

The circuit itself was simple, and also had polarity switch. I had once give it a test but I cannot figure out what went wrong. My last suspicion is the winding of transformer. I was too lazy to put bridge diode on every output and test them. In this modification, I sealed all others output with heat shrink and leave only the highest one.

The other thing is, the rotary selector switch is harsh to turn and had limited selection of output voltage. This is another reason for me to change it into linear adjustment using pots.

You could have just tested the AC voltage of each tap. But I was thinking the switch could be the problem too. I am not seeing a fuse anywhere, so did you add one? I usually at least fuse the primary whenever I build a PSU.

Ah, my bad, you remind me. Yes there is a fuse, somehow I forgot to add in the circuit diagram along with the on/off switch. So AC plug, one wire directly goes to transformer primary. The other wire goes to on/off switch then to the fuse before it goes to the transformer primary.

In step 2 above you see there is a fuse in locating the position, then it disappeared in my board design because I added it the last.

In step four, photo number two, if you zoom it in the fuse is there between the capacitor and 2N3055 transistor :)

I tried to find some useful information about how to connect the external power to the meter , and didn't have any luck . If you get it figured out , you should be able to connect the small red wire to the + output of the bridge rectifier , the + of C1 , The data I did find said that the supply voltage needed to be below 30V , so you will be fine there .