Turn a Computer Power Supply into Bench Power

Computer power supply as a bench supply

There are a lot of ways that you can repurpose 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 it.

This design for an external adapter lets you use the power supply without modifying it. Any ATX power supply can be plugged into the adapter. The result is a high capacity power supply that can output 3.3V, 5V, 12V and -12V.

Before we begin, here is some background information on computer power supplies.

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A computer power supply converts the AC power from the wall outlet into smaller DC voltages that power the various components of the computer. It regulates 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 It 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 turn 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 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 the steps below.