AT/ATX to Bench Power Supply Conversion

AT POWER SUPPLY CONVERTED TO A BENCH POWER SUPPLY

Having a good DC power supply capable of high currents is often quite useful. Buying such supplies ready-built can be expensive, and the cheaper ones often lack desired features.

Since obsolete desktop computers are quite abundant, it is easy to find one and convert the power supply for bench use. It will give DC voltages of +/- 12 V, +/- 5 V, and possibly +3.3 V as well. The positive voltages also will supply hefty amounts of current - the supply I have will produce 8 amps at 12 V and 20 amps at 5 V.

The conversion

The conversion is quite simple: remove the PC wiring harness (while possibly reusing some of its wires) and bring the various power rails out of the supply and terminate them with your favorite style of connection. I used binding posts since they will accept wires, banana plugs, and spades. I also installed a triple cigarette lighter socket on a pigtail as I often need to power devices which have cigarette lighter socket plugs.

CIGARETTE LIGHTER SOCKETS IN FOREGROUND ARE CONNECTED TO +12 V. BINDING POSTS IN BACKGROUND ARE CONNECTED TO +12 V, -12 V, AND +5 V.

I used an AT power supply from an old PC (usually pre-1998 PCs have these). AT supplies have the power switch inline with the incoming 120 volt line, so they truly cut the power to the supply rather than the "soft" system of the modern ATX supply which uses a usually green wire from the motherboard to turn the supply on and off. An ATX supply used by itself must have this (usually green) wire tied to ground to turn it on. A power switch can be made by putting a switch between this wire and ground.

I brought +12 V, -12V, +5 V, and ground out to four binding posts and +12 V to the cig sockets. I added an LED to display when the supply is powered on.

INSIDE OF AT BENCH POWER SUPPLY

Minimum Load Requirement

Many switching power supplies, including AT/ATX supplies, require a minimum load current to maintain regulation. In some cases the supplies will not even function without a minimum load. In most cases 250 mA on the +5 V rail is sufficient. I used a 15 ohm/3 watt ceramic resistor attached to the 5 volt rail. This resistor provides a load of 333 mA and the supply seems to run fine.

THE "MINIMUM LOAD" RESISTOR PROVIDES A BASELINE CURRENT FOR THE SUPPLY TO HELP IT MAINTAIN REGULATION. THE RESISTOR IS MOUNTED IN THE METAL SLEEVE IN THE CENTER OF THE PHOTO WITH THE RED/BLACK WIRES ATTACHED.

Words of Caution

Some kind of current limiting should be used if this supply is being used for sensitive electronics (e.g. a circuit you just built). Putting the +5 V of this supply in the wrong place with no current limiting could produce a room full of smoke and a mass of charred and molten plastic/metal on the workbench.

A 5-10 ohm resistor of a few watts or a 6 volt incandescent bulb works as a good current limiter for working with low-current logic circuits.

I also am obligated to provide the warning that the inside of PC power supplies are energized with line voltage (anywhere from 100 to 240 volts AC, depending on your country), if you failed to see the yellow stickers and lighting bolts plastered all over the power supply case. They also contain capacitors which may retain a charge with a few hundred volts for some time after the power is turned off.

CREATED/WRITTEN: 2011-04-07 20:32