Using a PC Power Supply Instead of Wall Warts

A typical model railroad, particularly DCC, has an over abundance of power supplies strewn around under the layout, due to the never ending number of accessories we all add. Many of these power supply / adapters, affectionately known as "wall warts", are unregulated, providing an approximation of the rated voltage shown on the body. Looking for regulated supplies can be an expensive proposition, but there is an inexpensive and ubiquitous solution - the PC Power Supply! I know many folks here have used this solution, but in looking through the archives of TB, I saw no "how to" written up here, so I thought I would provide this example for others

Modern PC power supplies can provide lots of well regulated power at +12V, +5V, +3.3V, -12V. Inexpensive, low wattage (for a PC - but ample for train accessories) ATX supplies can be bought at Fry's and other electronics stores for about $30. Of course, if you have an old, unused, PC at home (and who doesn't), there's a PS waiting to be used.

There are lots of cables and connectors coming out of the PS, but all you really need to know are the colors:

Black = common
Yellow = +12V
Red = +5V
Orange = +3.3V (great for LEDs)
Blue = -12V

There is one other wire on modern supplies that you need to be aware of - the Green one. This is actually the "power on" wire, and needs to be joined to ground for the power supply to switch on. You can do this via a SPST toggle switch, to let you control the supply, or just connect the green wire to a black one, for an "always on" brick.

The other item you need to do is put a constant load on the +5V line. The new power supplies need a load to actually run. This is accomplished by putting a 10 ohm, 10 Watt resistor (sandbar resistor - approx $1 each at RS) between a red and black wire. This resistor will get very hot if not attached to a heat sink. The metal case of the supply itself will suffice, though I actually took the heat sink off the processor in the PC I salvaged.

Run DC power bus lines from the supply around your layout, and with this simple technique, you can provide more than sufficient power to all your accessory needs. Just tap the correct voltage off the bus where needed. Typical amperage ratings are 4-5 amps on 12V, and even higher on 5V.

For a complete tutorial on turning a PC power supply into a bench supply, here's a great link that I found