WARNING: because of the AC power supply cable needed I must warn that mains voltage is very dangerous and should be handled with EXTRA care. I am not responsible for what you do with the knowledge conveyed in this project posting.

Here is how I made my own Variable Lab Bench power supply for roughly $20. This particular design gives 2 channels variable from 1.2-12 volts and allows you to adjust the current. It also includes a display to monitor each channel, and a 5V channel which is handy for use with digital electronics. Some things such as the box, my switch, my display, my knobs, and my potentiometers are things I had laying around or from other projects but what I have included as the amazon links can give you the barebones for a power supply with my same specs and performance. Several youtubers have also done similar tutorials so if you look these parts up on amazon you can definitely find some alternatives. Get creative with it!

Step 1: Parts Needed / Some Background Info

Parts used:

-buck converters(2):

https://www.amazon.com/gp/product/B07ZZ1X262/ref=p...

-60W PSU:

https://www.amazon.com/PUSOKEI-Switching-Computer-...

-Banana plug binding posts:

https://www.amazon.com/hudiemm0B-Speaker-Terminal-...

-(optional) 5V voltage regulator from car phone charging adapter

-(optional)Voltmeter/Ammeter display module

-4* 10kOhm 3 pin potentiometer/ variable resistor (need 2 for every 1 buck converter)

-(optional)6 pin 2 position switch

-(optional) 4 knob covers for potentiometer (makes it look nicer but definitely not needed)

-some wires

Tools used:

-Dremel for the enclosure

-screwdriver (flathead, typically for phones or watches was handy)

-soldering iron with solder

-desoldering pump or solder wick if you have

-electrical tape for connections (you can use shrink wrap too, I do not suggest wire nuts cause these wires typically run smaller and more frail)

The buck converter works by taking voltage and current in and storing it in an inductor. A switching element such as an IC or a mosfet is used to switch the circuit on and off as the voltage level changes to increase or decrease the current at the output accordingly. A voltage regulator smooths the output effectively changing it from a DC input to a modified waveform, and then back to DC at the output. This maintains the overall power across the circuit because although the voltage level may vary, the current can be made constant, and if the current changes, the circuit can maintain the voltage. Both of these behaviors are dependent on the potentiometers connected to the circuit.

Step 2: Modifying the Buck Converter(s)

This step is the trickiest part and because these modules are manufactured so well that if the heat on your iron is too high it can damage the other parts or even the board. So handle with care, your future self will thank you for it!

Simply de-solder the blue potentiometers(pots from here on out) from the boards. Add some wires to our larger 10 kOhm pots so we can bind them to the enclosure later. Solder these leads back to the board and store the pots from the modules (blue fellas) for later.

If you only need one controllable channel, you only need to do this once with 2 pots. For me I did this twice with 4 pots.

Step 3: (optional) 'digital' Power Supply Channel

Use the screwdriver and disassemble the car phone charger adapter. I removed the USB port but if you're brave enough you can add a USB port to yours. Add some leads to the 12V side and the 5V side. It may be useful to color code your wires to tell which is which and connect them to the binding post tabs included with the banana plugs.

Step 4: (optional) Adding the Voltmeter/ammeter

The pinout for each meter will vary but one thing for sure is black is ground, so these can be twisted together for simplicity. In the case of my display, The longer red and black leads are for the display power, and the other three thicker leads are for sensing. The red is for voltage, the blue is connected to the ground plug for current sensing on the terminals of your PSU and the black is again grounded. Test this by turning the power on and connecting an LED or motor (something to complete the circuit that draws power and will physically show you it works) and test the module now that you've added the meter.

Step 5: (optional) Adding the Channel Display Switch

My switch used 2 channels and one display, so for each side of my switch (3 pins per side for a total of 6). The middle two (1 per side mind you) are grounded and the accompanying sides connect across the voltage from channel 1 and the voltage from channel 2. The same is done for the current (I will add a diagram eventually but if you choose to spring for another display to avoid this problem do so. This was very budget intensive for me so I had to make do).

Step 6: Final Assembly

This step is as simple as throwing all the parts in the box, gluing/screwing/zip tying things down, and making sure it still works from prototyping your build. If you notice a couple of the steps here really depend on the demands you need for your power supply but my build cost about $20 and I got two fully controllable power supply channels up to about 30 W each and an additional 5V 1A power supply useful for controlling digital devices such as arduino.

UPDATE (8/10/21): as an upgrade to my design I used an old laptop charger and it works just as well. The bigger your power supply the more voltage you can control but do not exceed the limits of the buck converters!

For a barebones supply with 1 buck converter and no display:

- test fit all your parts in your enclosure (plug nothing in until the end for safety!)

-Connect an AC cable to your PSU using the Live(usually black or brown wire) Neutral(usually white or blue) and ground(usually green or green/yellow) terminals.

-cut a hole into your enclosure for the AC cable using the dremel; if you're like me maybe chose something that can use some screws or can be press fit into it's place.

-introduce your PSU with the mains power cable to it's new home in the box

-Modify the buck converter as directed before

-connect the input terminals to the V+ and V- on the PSU

-make holes in your enclosure for your banana plugs and screw them in

-Connect the output wires to the banana plugs.

- power on and test by connecting some load to the output (note that the most immediate physical change will come from the voltage level knob) .

- Mark which side is 0 and which side gives the max power on the box if you did not use a meter. You may want to write 0-12 over this knob to remind yourself. For my first draft I simply drew little ramps over each as a symbol for the output power going from minimum to max.

**if it does not work, or if you smell smoke stop immediately and double check your connections and each component!

If it works as directed, congratulations you now have a variable voltage and current 12V power supply!

Some things I included for personal appeal:

-knobs for all the potentiometers (who doesn't like a nice set of knobs)

-the switch that goes from the first channel to the second (my 2nd display actually burnt out so I fixed it using a switch from an old oscilloscope)

-banana plug terminals for each channel

-5V supply because its handy just to have the extra supply seperate from the variable terminals.

-The main on off switch is in the back with the AC power supply cable, which I pulled from an old playstation 2 that had died and I used for a sleeper PC. You could always just use a stationary switch or just keep it unplugged until you need it, but get creative with parts like these; especially given they help with the aesthetic appeal of your new device.

So now that it's done, get to work with your handy new lab bench power supply!