DIY VARIABLE POWER SUPPLY
built from an ATX power supply from a computer
After enduring several semesters of heading to and from the lab just to use the equipment, I decided that I had to have my own equipment for my personal projects. I initially wanted to buy a power supply but later decided to build my own. It was cheaper and a lot more satisfying. The circuit was also simple enough to implement. My dad had an ATX power supply salvaged from an old computer that I could use. The common voltage output of 3.3V, 5V and +-12V were already available. So the main aim of the project is make the voltage output variable using the LM350K, a variable voltage regulator. I don't have the habit of documenting the process so pardon me if I missed any steps/photos.
Components and Tools
So here is a list of components and tools required:
Item No
Item Name
Model
Quantity
Cost (SGD)
Electronics
1
ATX power supply
-
1
-
2
Variable voltage regulator
LM350K
1
6
3
120ohm resistor
-
1
0.5
4
2kohm variable resistor
-
1
2.5
5
1uF capacitor (tantalum)
-
1
0.8
6
0.1uF capacitor (ceramic)
-
1
0.8
7
2-pin toggle switch
-
1
2.5
8
Banana jack socket
-
6
2.5 each
9
Crimps
-
6
0.8 each
10
DC voltage meter panel (0-50V, 1 decimal)
-
1
18
11
Heat sink for LM350K
-
1
4
12
Connector to power supply (male)
-
1
5
Housing
13
Plastic sheet (large enough to hold all components)
-
1
-
14
Aluminium sheet (as supports)
-
2
-
Tools
15
Soldering station
-
1
-
16
Crimping tool
-
1
-
17
Drills (handheld/electric)
-
1
-
18
Dremel
-
1
-
19
Screwdriver
-
1
-
Things to take note: Ensure that the voltage meter panel has separate terminals for power supply and power measurement (i.e. at least 3-4 terminals). Else the voltage meter will not be able to measure voltages less than 3V (the minimum voltage required to power the meter). Also, tantalum capacitors have polarities so make sure to connect them to the right terminals. Lastly, I got the ATX power supply from an old pc. The plastic and aluminium sheets were also spares that my dad kept in his tool box so these didn't contribute to the cost of the project.
The Circuit
LM350 pin diagram (diagram taken from the LM350 datasheet)
I had initially used the plastic packaging IC (TO-220 casing) as it is smaller and cheaper. However, it heated up very quickly (even with a heat sink attached to it) and eventually burnt out when I was testing it on breadboard. I switched to the metal can package (TO-3 casing). Even though it took up more space and cost twice as much as the other IC, it was more durable and I settled with this instead.
Variable voltage regulator circuit diagram (diagram taken from the LM350 datasheet)
The circuit is simple enough. There is however some minor changes to the resistor values. Since I am using the LM350 IC chip, R1 should be 120ohms instad of 240. Also, the voltage input to the IC will be 12V (the highest voltage supplied by the ATX power supply) instead of 25V. Hence, the variable resistor R2 can have a smaller range of about 2kohms instead of 5k.
So next, to understand the pin connections of the connector to the power supply.
I am using the ATX2 (24 pin). The important pins that will be using for this project are:
PS_ON which turns on the power supply when it is pulled down to ground; COM which is ground (0V); 12V which supplies the voltage input to the LM350 and can be used as a fixed voltage supply; 5V which is used to power the voltage meter panel and used as a fixed voltage supply; 3.3V which is used as a fixed voltage supply; -12V which is used as a fixed voltage supply.
To turn on the power supply, a toggle switch is used to short the PS-ON pin to ground. Before soldering all the components, I did a trail circuit on breadboard to ensure that all the components were working fine.
full circuit diagram
The Housing
For the housing, I used a plastic sheet to hold the main components in place. I used drills (for holes), filing tools (for better fitting) and a dremel (to cut the rectangular shape for the voltage meter panel).
Fitting everything together
After soldering all the components together and cleaning it up, the variable voltage supply looks something like this.
And of course, to test and show that it actually works.
