Turn a Computer Power Supply into Bench Power

The wires on the main 20 pin (or 24 pin) connectors are color coded. These are the same for all ATX power supplies: 3.3V wires are orange; +5V wires are red; -5V wires (if they are present) are white; +12V wires are yellow; -12V wires are blue; ground wires are black.

The green wire is the “power on” sensor. This wire is internally connected to 5V with a pull-up resistor. If you connect this wire to ground (any black wire) the power supply will turn on.

The purple wire is the +5 “stand by” power. This outputs a 5V signal even if the rest of the power supply has not yet turned on, allowing you to power any circuit that might control the ON/OFF signal.

The gray “power good” indicator is at 5V if each of the output wires is operating at the correct voltages.

To make the connections easier to identify, I used colored markers to color code each slot on the 20 pin connector.

Next you need to attach wires to some of the pins on the connector. I connected to the 3.3V, 5V, +12V, -12V, Ground and the “power on” wire. Solder the wires to each connector. It is easier if you select pins that are a little spaced out from each other. I also used the same colored wires to help keep track of them.

In order to attach the connector to the housing, you first need to cut a slot for it. It is easiest to do this at the edge where the two parts come together. Hold the connector up to the side of the housing and mark the edges. Then using a sharp knife, cut along the outline.

I used four pairs of Banana jack connectors for the outputs, so I marked eight locations evenly spaced on the top of the housing. Then I drilled holes that where just big enough for the mounting screws. I also drilled a hole for the power switch on the right side.

Once all the holes are drilled, insert the switch and the power terminals and fasten them in place.

To hold the main connector in place, I used 2-part JB Weld. This compound comes in two parts; squeeze out an equal amount from each tube, then mix them together thoroughly. Apply the mixture all around the connector and let it sit overnight to fully cure.

In order to turn on the power supply we need to connect the green wire with the ground (one of the black wires). I used a small push button switch to make this connection. Solder the green wire from the 20 pin connector to one side of the switch, then solder the second green wire to the other side of the switch. To help keep them insulated, I cover the connections with heat shrink tubing.

I attached the banana jack terminals to the top of the housing in two rows. The red terminals are for the positive 3.3V, 5V and 12V connections; the black terminals are for ground.

Insert the post of each terminal into the holes and tighten them in place with their screws.

The output terminals need to connect to multiple wires. The easiest way to keep this neat and organized is to make the connections with crimp spade connectors. Attached one to the end of each wire.

Connect the black wire to the first black terminal, then connect the other black terminals to the first one with short jumper wires. I also connected the wire from the power switch to the nearest black terminal.

I connected the rest of the wires in ascending order according to their voltages. Begin with the 3.3V wire (orange) on the far left, then the 5V wire (red), the +12V wire (yellow), and the -12V wire (blue).

Many power supplies will not stay on unless the output reaches a certain minimum power requirement. To test if your power supply has this requirement, press the power button and wait for a few minutes. If it shuts itself off, then you know that your power supply has a minimum output requirement.

To take care of this, you can add a power resistor between the 5V terminal and ground. In most cases, a 10 watt 10 ohm resistor will work. In very rare cases, you may also have minimum output requirements on the 12V pin and the 3.3V pin. This will require additional power resistors.

These power resistors create a lot of heat, so if you add a power resistor, make sure that your project housing has adequate ventilation. In some cases you may even need to add a small PC fan to help dissipate the heat.

In addition to the banana jack connectors, you can add other kinds of connectors. I added a 12V DC outlet. Start by drilling a hole in the side of the housing big enough for the barrel on the DC outlet. Then connect the center terminal on the 12V DC outlet to the +12V terminal (the yellow wire) from the power supply. Connect the outside terminal on the 12V DC outlet to one on the ground terminals (black wire) from the power supply.

Another optional connector that you can add is a USB outlet. This will let you run any device that is powered by a USB port. To add this kind of connector, I recommend using a USB extension cord. Cut the cord in half — we want to use the piece with the female connector on the end. Next, separate the internal wires at the end. Strip the insulation off of the ends of the red and black wires and add a spade connector to each one. Connect the red wire from the USB cable to the red wire from the power supply. Connect the black wire from the USB cable to the black wire from the power supply.

Once the wires are connected, mount it to the side of the housing. Trace the outline of the USB connector onto the side of the housing, then use a sharp knife or a rotary tool to cut it out. You can glue the USB connector to the side of the housing. I recommend using JB Weld, just like you did for the 20 pin power supply connector.

To help keep track of the different voltage outlets, I labeled each pair of terminals. You can print out a simple label and attach it to the top face of the housing between the positive and negative terminals.