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Hobby Power Supply

Do you need a power supply for your Model Railroad?

I may be able to help you out.

While surfing the web I ran up on the following web site that explain how to build a Electronics "Lab Power Supply" out of a cheap computer power supply to save money as most collage kids have better thing to spend their money on then a $400 power supply just to use during testing.

While discussing using a computer power supply for model railroad use, it was brought up that not everyone is comfortable opening the case of said unit. They worry about being shocked. The following is a test unit (unit #2) convert by me to find out if I could convert a PC power supply for hobby use without opening the case,

(thus no shock hazard!)

The 20pin Connector and 12Volt light can be installed in a project box from RadioShack.

First I gathered up the items needed

1 - PC power supply, new or used. (In my case a New ATX power supply that  I had on hand that had a blown fuse.)

1 - 1157 automotive tail light bulb, new or used. (In my case used)

Wire cutters

1 - Volt-ohm meter. (for testing) (Optional)

1 - Soldering Iron and Solder. (used)

about a foot of 18 gage wire. (Optional)  because I need to replace the blown fuse.

1 - Case mounted fuse holder. (Optional)

1 - power drill with 1/2" drill bit. (Optional)

1 - hour of your time.

And here I go,

 With the PC power unit on my worktable, I located the 20 pin connecter that is normally plugged into the motherboard.

 Bottom row, connect pin8 (gray) [Power Ok] to pin 9 (Purple or Brown) [Power Sensing],

Top row, connect pin 14 (Green) to the switch, then the switch to pin 13 (black) [Ground],

To make the switched power circuit. (turn the unit on or off)

Connect a 12V light from pin 10  (yellow) [+12V] to any pin

with a black ground wire, to make the load circuit.

 You should have full power to all leads.

Unit must be plugged in for testing.


The color code for the wires are: Red = +5V, Black = Ground (0V), White = -5V, Yellow = +12V, Blue = -12V, Orange = +3.3V, Purple = +5V Standby, Gray = power is on (not used), and Brown or green = Sense wire.

The power supply that I used is a "Hercules switching power supply" for a "ATX" mother board. The unit is rated at 500 watts max output.

Now one amp = 1000ma

The three power outputs I plan to use on the power supply are,

+3.3V at 28 amps max output, 28 amps divided by a 30ma (load) = 933 loads

+5V at 38 amps max output, 38 amps divided by a 30ma (load)    = 1,266 loads

+12V at 17 amps max output, 17 amps divided by a 30ma (load)   = 566 loads

                                                                                      Totals = 2,765 loads or lights at 30ma each

Note: To get the max output from each set of power group leads I would bundle the wires together by color to keep from over loading a single wire. Three or four wires of the same color in a bundle should do it.



The power supply can easily power 1382 load items at 30ma each at a 50% of the load on the power supply. By centrally locating the unit under the layout and installing a central terminal strip from which power buses can be run through out the layout. A single power supply could power a small to midsize layout with ease. I would however install fuses just before the main power bus for added safety. 

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