Computer Power Supply to Bench Power Supply Adapter

There are a lot of ways that you can re-purpose and reuse old electronics. For instance, a computer power supply can make a great bench power supply for your workshop.

There are already a lot of tutorials online that show how to convert an old computer power supply into a bench power supply. But most of these designs require you to permanently modify the power supply itself. 

Another option is to make an external adapter for your power supply. Any ATX power supply can be plugged into the adapter. The adapter then connects the appropriate wires to a series of output jacks that can be easily accessed to provide power for your projects. This design lets you utilize the power supply without having to modify it. 

Here are the materials that you will need for this project.

Materials
ATX Computer Power Supply
20 Pin (or 24 Pin) Female ATX Connector (these can be purchased online or salvaged from an old motherboard)
Plastic  Project Housing
Jumper Wires
8 Female Banana connectors/ Bind Posts
JB Weld
Heat Shrink Tubing
Crimp-On Spade Terminals
12V DC Power Outlet (optional)
USB Extension Cord (optional)

Tools
Drill and Bit Set
Sharp Knife
Soldering Iron and Solder
Colored Markers
Pencil
Ruler

A computer power supply converts the AC power from the wall outlet into smaller DC voltages that power the various components of the computer. They regulate the voltages by rapidly connecting and disconnecting the load circuit (Switched-mode power supply). Most modern computer power supplies follow the ATX convention. They output +3.3V, +5V, +12V and -12V on a series of color coded wires.

Computer power supplies have a number of safety features that help to protect you and the power supply itself. Here are a couple that you need to know about. 

Turning on the Power Supply
A power supply is designed to not turn on unless it is connected to a computer motherboard. This is controlled by the green "Power On" wire. Connecting this wire to ground (any black wire) will allow the power supply to tun on.

Minimum Load Requirement
Many power supplies require a minimum load current in order to stay on. Without this load the output voltages may vary significantly from the specified voltages or the power supply might shut itself off. In a computer the current that is used by the motherboard is sufficient to meet these requirements. If your power supply has a minimum output requirement, you can meet this by connecting a large power resistor across the output terminals. This is discussed in a later step.

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. This allows you to power any circuit that might control the ON/OFF signal. 

The gray "Power Good" indicator. This wire 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 a wire 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 am using 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 were drilled I inserted the switch and the power terminals and fastened them in place. 

To hold the main connector in place I used 2 part J-B 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. Then let it sit overnight to fully cure. 

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

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

I inserted the post of each terminal into holes and tightened them in place with their screws.

The output terminals need to connect to multiple wires. The easiest way to keep this neat and organized was to make the connections with crimp spade connectors. So I attached a spade connector to the end of each wire. 

I connected the black wire to the first black terminal. Then I connected 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. On the far left, I connected the 3.3V wire (orange). Then I connected 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. Then 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 as well. I added a 12V DC outlet. Start by drilling a hole in the side of the housing that is 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. Then 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, you need to 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 then glue the USB connector to the side of the housing. I recommend using J-B Weld just like you used for the 20 pin power supply connector. 

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

Now your power supply converter is complete.