My work bench power supply

I am going to need a power supply for my upcoming product development work. I decided to make one from an old ATX power supply – I have lots of them because one of my queer hobbies is to salvage old computers that people dump in the garbage and take them apart. So I have many old power supplies, and incidentally, here is my collection of old CPUs:

CPU Collection

So I read some tutorials like this one, went to the one and only electronics store in Jerusalem and bought some stuff for about $11:

Parts for converting an ATX power supply

I also used some stuff that I had at home: LEDs, a SPDT switch and some heat shrink.

Stuff I had at home

The first step is to test if the power supply works at all. One possible method is to test if it outputs a standby voltage (called +5VSB). This signal is used by a PC motherboard for functions such as “wake up on LAN” and other stuff it has to do when the PC is turned off. The standby cable is purple, so if I measure the voltage between the purple cable and one of the black cables I should see 5V. As you can see in the next photo, the power supply seems to be ok.

Testing the standby voltage

The next step is planning. I decided to install two LEDs. One to indicate that the power supply is  connected to the mains and the second to indicate that the power supply is on. The power supply will be turned on with a switch. I also decided to provide the following voltages:

  • Ground
  • +5V
  • +12V
  • -12V

These voltages will be useful for various purposes, including for charging my LiPo batteries where the charger input voltage is 11-18V. I decided not to use the 3.3V line.

I cut a piece of paper to the size of the side of the power supply and drew all the connectors.

Connector plan

I started with soldering the LEDs to the purple (+5VSB) and the grey line (Power OK). I soldered a 330 Ohm resistor to the other leg and connected to resistor to a black (ground) line.


I connected the switch between the green line (Power on) and one of the black (ground) lines.

Power switch

I connected a load resistor – 10 Ohm 10 Watt – between one of the +5V lines (red) and ground. The purpose of the load resistor is to maintain a load on the power supply because the power supply shuts down if there is no load. I spread a small amount of heat sink to the back of the power resistor and attached it to the ventilated part of the power supply case.

Load resistor

The I grouped the lines by color – black (ground) , red (+5V)  and yellow (+12V). I folded the orange lines (+3.3V) and secured them with a cable tie. I connected the black, red and yellow cables together and into a connector.

Output lines

The additional voltage of -12V is provided on a singe blue line that can be seen at the bottom of the photo above.

I decided to install a 1A fuse on each voltage line. I’m not totally sure it is necessary because these power supplies are protected, but it seemed to be a good idea. I will have to change the 1A fuse to a higher current one when I will start to use the power supply to power my LiPo charger – I still didn’t check the current requirements for the charger, but its display shows 4.5A when it is charging.


I covered each fuse in a wide heat shrink sleeve.

The next challenge was to drill the holes in the cover. I tried various drill bits and various ways to hold the cover in place until I finally found this to work – holding a piece of wood in the clamp, placing the power supply cover over it and drilling with my power drill.

Drilling Holes

I inserted the binding posts and all the other components, soldered the wires and drew two nice labels.


I closed the power supply and started testing. All seemed to be ok. The SB LED came on when I connected the power supply to the mains. The Power LED came on when I flipped the switch. The voltage between GND and -12V and between GND and +5V was ok, but there was no voltage between the +12V and GND. I also noticed that there was a spark when I touched the power supply cover to the body and the +12V fuse was burned. I burned another fuse before I realised that there is a short circuit.

It took me some time to understand that I bought two different types of binding posts. One type had a plastic sleeve that went through the hole. The other type had a narrow metal connector. This connector touched the cover and was shortening.

The solution was to cover the metal connected with a heat shrink tube as can be seen in the next photo (the one on top is covered in green heat shrink)

Covering the binding post connector

I had to enlarge the hole in the power supply cover and drill through the red plastic connectors.

When I assembled the power supply again it worked perfectly.


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