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Loveably weird
First, let me say right off the bat: Don't use this to power your DCC system. For that application, use the power source recommended by the manufacturer of the system you use.
Second, these power supplies can kill you! Did that get your attention? Good! They have some heavy duty capacitors in them that can hold quite a charge for several days, even if the power supply is unplugged the whole time. They can deliver a fatal shock if not discharged. One YouTube video I watched said that to discharge the capacitors, leave the on-off switch on and unplug the power supply from the wall. It will continue to operate for a short time, then stop. At that point the capacitors have discharged and the case can be opened up if necessary. You can also leave the power supply unplugged and turned off for a few days. The capacitors will discharge themselves, but it takes a while.
I take no responsibility for anything that happens if you decide to convert a computer power supply into a power supply for your layout. You do this taking responsibility for yourself.

The current need for a good DC power supply is that I want to convert my double crossover from manual operation to operation from a single Tortoise switch machine.
I plan to use Melvin Perry's idea, found here:
I bought a Tortoise from my LHS, and picked up a DPDT switch from Menards. The RC bellcranks are Dubro products, part #167. Powering the Tortoise requires a DC power supply. You can use a wall wart if you choose, but I wanted something more substantial that could power numerous things on my layout, so I choose to convert a computer power supply, hereafter referred to as the PS.
I have an old Windows XP computer that ran when I mothballed it, so I took the 300 watt ATX PS out of it. Nothing I did would get it power up. I twisted the green wire to a black wire. Nothing. I put a load resistor between a red (+5V) and black wire. Nothing. Methinks it be kaputsky! So I bought a new 600 watt PS for $26.95 including shipping from eBay.
I received it, jumpered the green wire to a black wire, plugged it in, and it powered right up!
The primary video I watched to figure out what to do was this one:

So this afternoon I got to work on it. I cut all the plugs off the wires and separated them into bundles based on their voltages and purposes.

From the top: The black wires are ground wires.
The orange wires are +3.3 volts. You will notice that there is a brown wire in the bundle. That is a sensing wire the power supply uses to help regulate the +3.3 volts. It will be included in the wire bundle later when I connect the wires to a terminal strip.
The red wires are +5 volts. These will be used for lights and LED's.
The yellow wires are +12 volts. +12 volts will be used more than any other.

There are also some wires that will be used by themselves.

From the top: The purple wire is +5 volts standby. This wire is energized anytime the PS is plugged in, even if the on-off switch is off. This will be connected to an LED as an indicator light.
The blue wire is -12 volts. This can be used in conjunction with a +12 volt wire to give 24 volts, but there are some special wiring guidelines that have to be followed. I have no plans at this time to use this, but I will connect it to the terminal strip anyway.
The green wire is the on-off switch. I will connect this to a SPST switch.
The gray wire is the power good wire. This wire will be connected to another LED as the indicator that the PS is running.

While not all PS's need to have a load on a +5 volt wire to operate, the PS will regulate the voltages better if a load is present. I am using a 50W 6 ohm resistor wired between a red an black wire. These resistors will generate a lot of heat, so I have mounted it to the metal case in the air-stream of the cooling fan. Mounting it to the metal case will help dissipate heat through the case of the resistor, and having it in the air-stream will also help carry away heat. I opened up the case and mounted it using 2-56 screws and nuts.

That's all for today, folks!


Section Hand
I used a Radio Shack for my DC power supply for my layout's 20+ Tortoise switch machines. I like the power pack since it has an adjustable power output from 3 to 12 volts in 3 volt increments. The lower the voltage, the slower the Tortoise operates.

Interesting post Flyboy.



Loveably weird
Thanks, Greg! I remember the Radio Shack stores that had all kinds of neat gadgets you didn't even know you needed! ;)
I stripped the ends of the wires and bundled them into groups. The groups then had a connector installed on them. They were then screwed to a 10 position terminal strip. I ended up with 3 +12 volt terminals, 1 -12 volt terminal, 2 +5 volt terminals,1 +3.3 volt terminal (I did it this way due to the 3.3 volt sensing wire), and 3 ground terminals.

My faithful assistant, Wally, is inspecting the job. His response: "Well, I guess it will do." He can be so hard to please sometimes.
I installed a SPST switch on the green wire. I turned on the PS and checked the voltages both with the resistor unconnected and connected.
With the resistor unconnected, the voltages were:
+3.3 = 3.43
+5 = 5.42
-12 = -11.89
+12 = 12.65

With the resistor connected, the voltages were:
+3.3 = 3.33
+5 = 5.12
-12 = -11.58
+12 = 12.34

This power supply does not have the -5 volt wire that older PS's might have.
Given the generally accepted tolerance for + DC voltages of +/- 5%, without the resistor the +3.3 was barely within tolerance, and the +5 and +12 voltages were out of tolerance. With the resistor hooked up, all voltages were within tolerance. The source I used for the voltage tolerances was:
So it appears that even though the PS will power up without a load resistor on a +5 wire, the voltages may not be within tolerance, while with a load resistor all voltages are within tolerance. So, a resistor it is!
I have not connected any lights to the power indicator wires, so I will repeat this test with them installed and see if that changes anything. For lights I have decided to try some Model Power light bulbs since I have a whole bunch of them, brand new (well, NOS), in both green and red. But that's for tomorrow.


Well-Known Member
flyboy -

This is a great topic and a nice video. Well done!

I converted an old PC power supply into general-purpose power supply about 20 years ago and was able to package everything inside the power supply case. I've used it for an endless variety of projects over the years. It delivers +12V, -12V, +5V, and +3.3V. Very handy for all sorts of electronic experimentation!


Like you, I found that a load on the +5V line was required for good regulation of the output voltages.

The warnings about the lethal voltages inside these power supplies are extremely important. There are internal voltages on the order of a few hundred volts inside a PC power supply. And yes, the capacitors in there will store the voltage for quite a while, even after the power supply is turned off.

- Jeff


Loveably weird
Nice work on that power supply, Jeff! That would be handy for all sorts electronics projects!
I tried using some small incandescent bulbs I had on hand for the standby and power good indicator lights. Both wires had a solid 5 volts through them with the on-off switch on. The red bulb I used for the standby indicator lit up quite nicely. The green bulb on the power good circuit would not light at all! I checked the voltages with the lights connected and the switch on. The standby was right at 5 volts, the power good was at .19 volts ( point one nine)! I found the white LED headlight from my UP #4439 project so I wired it onto the power good circuit in place of the bulb. It lit up nicely, and when I checked the voltage across the wires it was now at 1.9 volts. Yesterday I made a stop at Randy's Roundhouse, my local train store, and bought some Miniatronics 3 volt LED's. There were red, green, and yellow LED's all in the same package, along with limiting resistors for use with current over 3 volts. Since these were going to be used on a 5 volt circuit, I soldered a resistor to the positive (long leg or anode) and the negative to the ground wire. I plugged in the PS, turned on the switch, and

"Captain, there be lights here!" (Sorry, Mr. Scott.) I'm sure someone more knowledgeable about ATX power supplies can tell me why a bulb wouldn't work on the power good circuit, and why the voltage seemed to drop so dramatically.
I watched a video about repairing power supplies, and the host mentioned a glass Buss type fuse that can go bad. I opened up the old power supply I have and took a look inside. I did not see any type of replaceable fuse, but this didn't look good at all! :

I'm pretty sure that's not supposed to look like that! I think that power supply is down for the count!
The next step will be to start work on getting the double crossover set up with the Tortoise switch machine, but that will be posted in my build thread
Thanks for the nice comments! This has been a fun project!


Well-Known Member
It looks like something in that old power supply got abnormally hot. Probably not worth troubleshooting!

- Jeff


Well-Known Member
I often cringe when I hear about somebody using a PC power supply for anything other than it's intended use. In this case (no pun intended) you seem to be safe in all aspects, well done!


Loveably weird
I often cringe when I hear about somebody using a PC power supply for anything other than it's intended use. In this case (no pun intended) you seem to be safe in all aspects, well done!
Thank you! Electricity is not something to play around with if you don't know what you're doing. It could give you a very interesting hairdo.


Loveably weird
This afternoon I made a little stand for the PS. The base is an 18" long 1 x 10. The front panel is a piece of 1/4" plywood. All wood-to-wood joints are glued and screwed. Nothing is permanently fastened down yet. I bought a couple of 2 x 4" pieces of Velcro I plan to use to affix the case to the board. The terminal block will be screwed down.
I need to wait for a warmer day before I can take this outside to paint it. SWMBO (She Who Must Be Obeyed) doesn't like it when I paint things in the house.



Tower Operator
That 50 watt 'load' I have concerns that all it's doing is waster power creating a larger demand unnecessary on top of the additional heat (again wasted electricity). Those voltages shown seem to be well within tolerance AFAIC.
If some resistance is really needed, why not something much smaller, just to induce some type of 'load'?


Loveably weird
OK, time to wrap this up!
I painted the base and got everything fastened down where it's going to go. I have a Dymo LetraTag label maker, and one of the borders for labels is trains. So.......

The terminal block is held in place with some #8 wood screws, I might replace those with some #10's. The power supply has a couple pieces of Velcro attached to the bottom. It's not going anywhere. The LED's are just press fit into the holes, but it's a pretty snug fit.


Now, on to one of the other 936,542,718 projects awaiting my attention!
It seems like the hurryeder I go, the behinder I get!

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