In this instruction, I will explain how I built an adjustable (voltage/current limit) DC benchtop power supply using some components of an old PC ATX power supply.
My goal was a small and cheap unit with great power performance. I needed adjustable wide range output 1V...30V with current control and high power.
I found some great instruction about benchtop DIY power supplies, some of that modify the original electronics of the PC PSU and give fix 5V and 12 V, other instructions using power supply modules and build it into a new housing, I combined all these and created my version.
Step 1: BOM List
I bought components from aliexpress see BOM list. Total cost 25$
6mm Knurled Shaft Potentiometer 2 pcs 0,148 $/pc 0,296 $/total
500k potentiometer 1 pc 0,336 $/pc 0,336 $/total
200k potentiometer 1 pc 0,217 $/pc 0,217 $/total
AC to 12VDC adapter 10A 120W power supply 100V -265V 1 pc 9,91 $/pc 9,91 $/total
DC 5V-32V to 1V-30V 10A LTC3780 Automatic Step Up Down Regulator Charging Module 1 pc 10,43 $/pc 10,43 $/total
Banana connector socket 1 pc 1,63 $/pc 1,63 $/total
Voltmeter Ammeter Dual LED Display 1 pc 1,96 $/pc 1,96 $/total
Switch 1 pc 0,125 $/pc 0,125 $/total
Spacers and screws 12 pc 0,02 $/pc 0,24 $/total
ATX PSU from my old computer 1 pc 0 $/pc 0$/total . Any old, broken PSU can be suitable. I do not recommend to buy a new one just find a PSU from an old computer. Requirements for the PSU: the fan should work and the fun size should be 120 mm and placed on the top side of PSU. (this will give you space and cooling)
Step 2: Recycling Old PSU
I opened the PSU and removed all electronics, except the board soldered to the power socket. This board is an EMI filter. I decided to leave intact because it will give better power input to my adapter and has an already established grounding to the house and has cable outputs that can be used for connection to other modules.
I cut all cables at the PSU main board and used them for wiring.
Next, to the power connector, there is the main AC switch. I left the switch there, but I disconnected from the circuit and put a new switch on the other side of the house, because this side will be the front panel of the PSU, and the main switch should be there. The cable entry of the original PSU was perfect for the round AC switch. I did not need to drill a new hole for the switch, and I did not need to fill the hole left by the removed cables.
Step 3: House Modification
A made holes for the potentiometers, display and for the banana connector socket. Before drilling I checked that the fan will take some space from the front panel and the potentiometers, the display can be placed only on the right side of the front panel, not in the center as it would be logical. I drilled small holes around the outline of the display and used a rasp to make it rectangular. I fixed all components temporarily into the box to found the right place for the AC/DC and for the DC/DC converters. At the first time, I made small, tight holes for the banana connector socket and at 30 V pierced throw and created a huge spark and burned the connector (picture attached), after
that I widened the holes to have better isolation.
I installed spacers to the house with screws for the converters. I used nylon spacers and screws to insulate the house from the converter. I checked the height of the converters to see if the fan still has enough space.
Step 4: Circuit
Some words about how this circuit works. The 230 V is transferred into 12V DC. The 12 VDC will power the fan, which will cool the whole unit. The same 12VDC will be the input for the DC/DC step up and down converter and also power the display, which is measuring the Voltage and current. The DC/DC converter is controlled by two potentiometers. The DC/DC converter has three originally soldered potentiometers. I had to replace two from them by external linear potentiometers. I connected DC/DC converter’s Vout-set pins to a 500 Kohm and the CC-set pins to a 200 Kohm potentiometers. The UV-set potentiometer should be left intact, only set to maximum.(In this application will not be used.) The current measuring unit is placed between the ground output connector and the OUT- pin of the DC/DC converter. The voltage measuring yellow cable of the display module should be connected to OUT+ pin of the DC/DC converter
First I connected all low voltage components to test the units. I connected them as you can see at the attached schematic (instead of AC/DC converter I used my old PSU). I tried the circuit with load as well but only for a short time, because the unit can be damaged without active cooling.
Step 5: Final Assembly
After I know that everything is working correctly. I installed components to their final place in the following order. Display, banana connector socket, Potentiometers already cabled to DC/DC converter, DC/DC converter, AC/DC converter. After installation, I made the remaining cabling. I fixed one side of the cable, measured the correct size, cut the length and fixed this side of the cable as well. The length of the cables should be as short as possible. Longer cables should be fixed to the lower part of the housing because long, free cables can get caught by the fan.
The cables should be thick enough, I used 1 mm^2 wire for the main power lines. Careful with cabling, WARNING! 230VAC is dangerous. Before plug it in, I used a multimeter to check that “L” and “N” lines are isolated from each other and form the house. I checked the connection between the grounding PIN at the AC/DC converter and the house. Double check all connection and cables before plugging it in.
Step 6: End Words
I am still testing the unit with a loads. It works like a charm. The maximum performance I tried 13V 9A (114 W)
The only annoying part is the noise made by the fan.
My plans for the future are: Placing a fuse next to the power connector, this will make the whole unit safer. I ordered some vinyl to cover the house, to look better. Not arrived yet. I plan to conduct more tests with other loads.
Have a nice day!