Introduction: Variable Voltage Power Supply <$15

About: The Western Center Academy is a STEM Charter in Hemet, CA. Our high school students take a course called "STEM Studio" in which they first build a project from Instructables, then they create a project to upl… sells a kit to build a power supply that is useful for breadboard, Arduino, and Raspberry Pi projects but the instructions are not in English. I will take you step by step how to build the project and for less than $15, you can build this power supply with a case. The website says that it is also a function generator and logic-level analyzer, but I have not been able to figure out those features yet. Any budding electronics hobbyist needs a variety of power supplies and this one is inexpensive, easy to build, and feature-packed.


Power Supply Kit:

Soldering Iron, I use this one:

Lead-free Solder: (don’t use thick solder)

Wire Cutters/Strippers

Multimeter (optional):

Step 1: Organize Your Materials

The kit comes with resistors, capacitors, screws, spacers, etc. Your job will be much easier if you separate then and organize them. Get your soldering iron heated up, wet your sponge to clean the tip of the soldering iron, and find your lead-free solder. If you don’t know how to determine the value of a resistor from the color code, you should use your multimeter to measure the resistance of each resistor. Here’s a link to an app that will determine the value of a resistor based upon the resistor color code in case you need a refresher or the bands are too small for your eyes to see (like me).

Step 2: Start Soldering the Small Components

When you solder into a board like this, you always want to start with the smallest, lowest components. If you solder in one of the huge capacitors first, they will get in the way of everything else. So, I started with the LEDs on the front edge of the board. The LEDs are labelled on the board R, Y, B, and G for the red, yellow, blue, and green. It is VERY important that you put the LEDs in the right way or they will not work and your circuit might not work either. Notice that the symbol on the board shows a circle with one side flat. The LEDs are the same way, round with one flat side. As long as you make sure that the flight side of the LED is aligned with the flat side of the diagram, you’ll have them in the right way.

Step 3: Soldering Tips

The secret to good soldering is to understand that solder does not stick to cold components. Do NOT try to melt solder on the soldering iron and then drip it onto cold components. You must heat up the components hot enough to melt the solder and let it flow evenly. With this circuit board, there is a metal ring to solder the component legs to. So, hold the soldering iron such that it touches the metal ring and the component leg for 10-15 seconds and then touch the solder to the junction between the iron, the ring, and the leg. It should flow smoothly and look flat and shiny. The image on this page, although blurry, shows a good, shiny solder joint.

Cut the legs off of the components to prevent short circuits.

Step 4: Keep Soldering

There is an integrated circuit as part of this power supply. It has 14 short, metal legs. Integrated circuits are sensitive to heat so you don't want to solder the IC directly into the board and potentially damage it, so they included a socket. I soldered the socket in next. The legs are very close together, so be sure not to use too much solder or it could bridge across the gap and create a short circuit. I was soldering by myself on this project so the socket kept falling out before I could finish the first solder joint. So, I used a little piece of clear tape to hold it in until the first solder joint was done.

ICs have a semi-circle at one end to make sure that you put them in the right way. The socket also has a semi-circle at one end. The diagram on the circuit board also shows a semi-circle on one end. Make sure that these semi-circles are all aligned.

Step 5: Solder the Diodes

Diodes are small and lay flat against the board, so they are a good candidate to go next. It is very important that diodes go in the right way. On the circuit board, there is a white line at one end. On the diode, there is a white/gray/black line at one end. Make sure that the lines are aligned. D3, D4, D5, and D6 all face the same direction. But D1 and D2 face opposite directions.

Solder in the 6 diodes and trim the legs.

Step 6: Solder the Resistors

Resistors are also flat and low, so they are up next. There are six resistors and it does not matter which way they go in. Use the colored lines or the multimeter to ensure that you put them in the right place. Solder them in and cut the legs off.

Step 7: Solder the Capacitors

Now, we can move on to some of the bigger, taller components. The capacitors are up next. There are 6 capacitors in this circuit. Four are shaped like canisters and they are polar, meaning that you must put them in the right way. The other two are not polar and can be put in either way. The diagram on the circuit board shows a positive on one side and the other side is shaded in with cross-hatchings. The cross-hatched side is the negative side. The polar capacitors have a line along one side with negative signs on it. Make sure that the negative side of the capacitors is aligned with the cross-hatched symbol on the diagram.

Step 8: Solder the Variable Resistor and the Speaker

The variable resistor has 3 legs and cannot be put in the wrong way. The small speaker has a positive symbol on the sticker on top that should be aligned with the positive sign on the circuit board. Solder them next.

The variable resistor is used to calibrate the reading on the 3-digit display. Use a multimeter to see the actual voltage output and then turn the resistor until the display matches the multimeter reading.

Step 9: Fixing Mistakes

I put a component in the wrong place and in removing it, I damaged the board. The metal ring lifted off of the board and therefore, there would be nothing to solder the correct component to. On the bottom of the board, the yellow stripes are embedded wires, so you can see what that component lead was supposed to be connected to. So, I used a small piece of wire (actually, one of the legs that I cut off) to connect to the next solder joint in the line. See the picture if that doesn't make any sense.

Step 10: Solder the Potentiometer, Transistor, and Screw Terminals

The potentiometer will connect to the knob so that you can adjust the voltage. The transistor has 3 legs: the emitter, the base, and the collector. They are not clearly marked on the transistor, but the component is flat one one side and rounded on the other. The diagram on the circuit board is flat on one side and somewhat rounded on the other. If you put it in that way, then it will be correct.

There are two screw terminals. One has 3 connections and the other has 2. The board is clearly marked where to put them. Make sure that the connections are facing outward so that you can connect the wires later.

Step 11: Large Transistor and Heat Sink

The large transistor will generate a great deal of heat, so it needs to be bolted to a heat sink before soldering. Use a small screw and nut to connect it to the heat sink before inserting it into the board and soldering it. It can really only go in one way. Solder carefully so that you don't damage the transistor. The heat sink will help protect it, but be careful anyhow.

Step 12: The 3-Digit Display

To prepare the 3-digit display, first trim the wires to be around 3/4 of an inch long and then pull the insulation off. You don't really need wire strippers to do this, they pull right off with your fingers. Be careful because it is multi-strand wire and if the strands start going all over the place, they will short out with neighboring wires and cause all kinds of havoc.

Bend the wires through the associated holes, screw the display to the board, and then solder the wires from the other side.

While you're at it, you can solder the blue wires from the transformer into the board. It doesn't matter which way the wires go. If the colors of your wires are different, it is the wires coming from the bottom of the transformer that connect to the circuit board. The rest of the transformer wires will be dealt with later.

Step 13: Put the IC In

Now that you're pretty much finished with the soldering, you're ready to put the IC into the socket. Make sure that the semi-circle is facing the right way and that you don't bend any legs as you put it in. Look carefully to make sure that all of the legs are aligned before pressing lightly. Mine had a couple of bent legs and the legs were a little farther apart than they should have been. So, I squeezed them closer together and aligned them all.

Step 14: Build the Case

The case is held together with some ingenious little slots in which a screw and nut are tightened. See the picture of the closeup of this connector.

See the second picture to see how the pieces are aligned. There are no instructions for how they go together, so the photo will come in handy.

First, use the spacers provided to bolt the circuit board to the bottom of the case. Then, put the side on that covers the potentiometer. That one goes on easiest first. The side that is near the transformer should go on last because some wires need to pass through it.

Step 15: Finish Wiring the Transformer

The red wires on the other side of the transformer connect to the plug. There is some shrink tubing included to seal the connection to avoid contact with 110V AC. Slide the shrink tubing on before soldering, solder the wires together, then use the soldering iron to shrink the tubing to seal the connection.

Put the wires inside of the case, put the wire holder in the slot in the case, and screw it together.

You now have a working power supply, but there are still a few things that you need to do.

Step 16: Wire Up the Outputs

There were no instructions for where the wires are to be connected in order to use the device as a power supply. So, before I put it together, I measured the outputs to find one that had around 6 volts. Counting from the top, the positive output comes from the first connector and the negative output comes from the fourth.

First, however, you need to connect the alligator clips to the red and black wires. Strip both ends of the wires and coat them with melted solder. Pass the wire through the hole in the alligator clip and solder it to the clip. Then crimp the end of the alligator clip to hole the wire in place.

Now, feed the wires through the side of the box and use the screw terminals to put the red wire in the first terminal and the black wire in the fourth terminal.

Step 17: Calibrate the Display

The small, blue variable resistor is for calibrating the 3-digit display. Connect your multimeter to the output alligator clips and then turn the resistor with a small flat screwdriver until the displays match close enough for you to be satisfied.

Step 18: Close It All Up and Use It to Power Your Projects

Now, you're finished. Screw the lid on, test the device, and use it to power new, cool projects!

If you figure out how to use the signal generator or the logic analyzer, please post it in the comments. I also have not yet figured out exactly what the LEDs mean. The blue and red seem to always be on and the yellow flickers occasionally.

Congratulations! Feel free to ask any questions in the comments section.