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
I'm a network admin/computer person by day, so I always wondered why more people didn't use old Power supplies. They are great for this purpose. Especially when wall warts are so stinking expensive!
Part of the reason I hadn't yet done this is that I didn't know the color codes of these wires. Also, most everything I have purchased has come with its own wall-wart. Thanks for this information! I have at least two power supplies and probably three or four sitting in dead computers.
Great info. I think the wire colors are critical and I certainly didn't know them. Any chance of being electrocuted while building this device?
Well, there are some hefty capacitors in the unit, so don't ever open the casing without having the power cord unplugged. And, even then, wait for the capacitors to discharge. I would never work on any of the wiring "live", even 3.3V. The link that I provided does have all the work being done inside the PS (i.e., clipping all the leads and just running to plugs on the case). I just clipped wires off the 24-pin Molex connector, and then attached my wires to those, and tucked the remaining wires to the back. For my common, +5V and +12V, I actually used the plugs for hard drives (4 pin Molex), and an extension cord that I had in the old PC, brought the lines out all the way to where I wanted them on the bus.
Rick, like you (and others), I use one also, and I'm glad you brought up a post about using one. :thumbs_up: I have an output wire from the 12v running around the whole layout that I can tap into whenever I need full power. And I run the 5v to all my tortoise switch machines. Makes them move nice and slow But I'm wondering about all those extra connections you are talking about. I'm guessing they would be necessary if you went out and purchased a stand alone power supply for a computer. Correct? But to the masses that have an old comp sitting around, you wont' need all those connections. Just turn it on and you're good. It doesn't matter if your old Windows wont' work or hangs up. In fact, you don't even need the hard drive installed. As long as the mobo will turn the power supply on, you're good to go. Here is a pic of my way old HP, with the wires just temporarily connected, since I'm still not sure where I'm going to mount the old comp. Mike
Mike, You're correct, if you just leave the power supply in the PC case, with the 24-pin molex attached to the motherboard (whether alive or dead), then the other connections are not required. The green wire on the Molex is connected to the power on/off switch; and the motherboard provides the necessary load. I took the ps out of the PC (and scavenged other parts, such as LEDs, jumper pins, etc. Even the heat sink!).
Here are a few discussion forum threads on the topic of converting PC power supplies for hobby use. Most of the threads come from RC Groups, a forum for radio controlled aircraft enthusiasts. http://www.rcgroups.com/forums/showthread.php?t=368639 http://www.rcgroups.com/forums/showthread.php?t=315179 http://www.rcgroups.com/forums/showthread.php?threadid=55555 http://www.rcgroups.com/forums/showthread.php?threadid=114054 http://www.rcgroups.com/forums/showthread.php?threadid=41161 http://www.flyinggiants.com/forums/fg189/12159-frankenpower-building-12v-power-supply.html In addition, here are more helpful web sites on the topic. http://web2.murraystate.edu/andy.batts/ps/DESIGN_GUIDE.HTM http://www.marcee.org/Articles/PCPowerSupply.htm http://www.dream-models.com/eco/DIY-powerSupply.html http://www.extensiontech.net/articles/howto/jb/pstu/ http://reckerclub.tripod.com/id105.html http://homepage.ntlworld.com/fast.electrics/PSU.pdf http://www.nfdc.net/home/cbdb/Teardrop 12 V Power Supply.htm Depending on the age of your PC power supply and the type of computer it came from, the conversion process may vary a little bit from unit to unit. Use common sense, read the instructions on the various sites, and respect the fact that lethal voltages may exist inside even a 12V power supply. If you can adapt the power supply without opening the case, so much the better! - Jeff
Rick and others thanks for the important lesson. I had no idea that these Power Sups could be used in this manner.:thumbs_up:
Never occurred to me, thanks! It probably didn't occur because I have all these old transformers from the early 1970s, and always ran 12 VDC around my layout. They were dirt cheap in 1970; they are no longer cheap.
That's what is amazing. Regulated 12VDC transformers used to be dirt cheap, now they are very expensive. But, get a switched multi-volt regulated power supply for a PC, and they are inexpensive!
this is the one we're using to power the system with, http://www.allelectronics.com/make-a-store/item/PS-1512/12V-12A-FAN-COOLED-POWER-SUPPLY/1.html with a fuse inline to limit the amperage to the 8amp super chief.
This is an excellent thread. Never occurred to me to do this and I have nasty old PC case sitting in the loft waiting to go to the dump. Now I'll scavenge it for parts. I like the heat sink idea too. Cool.:thumbs_up:
Seems like it would be easy to build a card with some kind of backplane connector, and one of the HDD plugs... you could put it in a running computer and plug your layout into the back. You'd only have +5 and +12, but that's plenty. So the card would mount in the backplane, but not actually connect to PCI. Instead, you'd plug a spare HDD plug into the card, and it would have some kind of external connector for a bus cable to the layout. The card itself would just provide a clean way of accessing the PSU from outside the case. The result would be neat & clean. Hmm... perhaps I should dust off my layout software! The computer with the "adaptor plug" could actually be the one controlling the layout, if you're into JMRI or whatever. I doubt a modern 300+W power supply would even notice a moderate sized layout. I SO plan to power my layout this way. Though, since I use a Mac, it'll probably be an old recycled Linux box under the bench.
That's the power of VOLUME when talking about electronics. Things get really, really cheap when you're making tens or hundreds of millions of them.
Here's one of my power supplys, work had a big cleanout, lots of old pc's (save LED's wire, switches ect.) this PS didn't need a load resistor. pics show 5 and 12 volts using 12v globe. going to use them for tortise switches. the 5v is nice and slow!
I have a power supply and have decided to give this a try. So far I have everything I need exept the binding posts. I've check the locate outlets and online with out results. Can anyone tell me were they can be found. I also have a potentiometer and a digital 20V meter and am wondering if this could be used to make a variable 0V to 12V output. The pot also has the shut of switch installed. I've also got everything for the resistor selector on page 26 of the March issue of Model Railroader. It looks like something that could be usefull.
