How to Convert Used PC Power Supplies for Your Model Railroad

The genesis of what follows was an article that I submitted to a magazine which was rejected. I reworked the article, but rather than resubmit it, I got the idea to build an e-book around it. So this will become chapter 3 of an upcoming e-book tentatively entitled "Electronic Projects for Model Railroads Cookbook". Other than an outline, the details of what the e-book will look like and it's release date are still to be determined. In the meantime enjoy this preview.

How to Convert Used PC Power Supplies for Your Model Railroad

Text and Photos © 2010-2011 by Terry Terrance

Would you like to get upwards of 20 amps of clean 12 VDC to power your model railroad for little or no money? The power supplies from discarded IBM-compatible Personal Computers (PC) can be readily converted to supply pure, filtered, regulated DC to your model railroad. Once you have that power available, you can use it to run trains, throw switch machines, light structures, etc. Anyone with average mechanical and electrical skills can do this conversion – if you know how to use a multi-meter, you can complete this project!

The power supplies found in PCs are known by an array of designations: XT, AT, ATX, WTX, and so on; and come in a variety of sizes (see Photo 1). While all of this is standard fare for your neighborhood computer geek, there are only two characteristics that we will need to know to start our conversion.

Photo 1

The first is how much DC power can we extract for our railroad? PC power supplies are rated by the total number of watts that they can deliver; anything from 65 watts to over 1000 watts. However, all of that power is not available as any single voltage, it’s distributed among all of the voltages produced by the supply such as +3.3V, +5V and +12V. Why this is important will become clear shortly. Photo 2 shows the labels from the four units in Photo 1; with ratings between 90 watts and 400 watts, their output ranges from 4.2 amps to as much as 30 amps of the three main voltage outputs.

Photo 2

Notice that the label in the lower left of Photo 2 also contains the wire color code for the various voltages; keep this handy to refer to.

If you need +12V to run a conventional DC layout, then the +12V output will be of the most interest to you. However, +5 VDC can be used to power servo motor switch machines and +3.3 VDC can be used for layout lighting with LEDs. 12 VDC (or +12 VDC & -12VDC) can be used to run Tortoise slow-motion switch machines.

However, most DCC systems will not run on 12V but require more, with 15V being the usual minimum. Unfortunately, you cannot connect the +3V or +5V in series with the +12V to get a higher voltage, the design of these power supplies is such that all of the grounds are common to every voltage and connecting any + to any – will result in a dead short.

Nor can you connect the outputs of two separate supplies in series to get a higher voltage. Most of these switching supplies are designed with the output ground common with the 115V mains ground, creating a sneak path through the wall socket that creates a short if two supplies are connected in series.

What else do we need to know before we hunt down some used PCs for their ‘ivory’ (i.e. their power supplies)?

You’ll need to know what generation of PC supply that you’ve salvaged. This is important to getting the supply to turn on. You can tell the generation by the type of connector that attaches the power supply to the main board (motherboard) of the donor computer. Photo 3 shows the connectors used in the first few generations of PCs. A power supply with these connectors will look like the large supply in the center of Photo 1.

Photo 3

These units are also characterized by a large on-off switch on the side. Early supplies like this are turned on with this switch, unlike subsequent generations of supplies, as we will discuss later.

Photo 4 shows the motherboard connector used in the next generation of PC supplies.

Photo 4

These use a 20 pin motherboard connector. These supplies are smaller, like the gray unit on the upper right in Photo 1, and, generally, produce more power.

Photo 5 shows the latest generation of connector. It has four additional pins, including an additional 12V circuit; otherwise it’s like the one in Photo 4.

Photo 5

Let’s begin the conversion.

Step 1: Find a used PC. You may already have an unwanted PC or two; but if you don’t, many municipal refuse dumps now have a section where PCs and other electronics are ‘recycled’ to prevent the lead in their solder from winding up in the dump. If your town lets you pick over the pile, you’ll have your choice. A tip here is that name-brand PCs (e.g. Dell, IBM, HP, and Compaq) will have quieter fans; but these manufacturers are so cost conscious that they often have power supplies tailored to the exact model of computer they will be built into, supplying only the minimal power needed to run those machines. And they may also have proprietary connectors that will not follow the procedure that we will use to rewire the supply (Compaq was notorious for this).

You may also want to avoid the smaller, ‘small form factor’ PCs – small machines typically have underpowered power supplies.

Another source of used PCs is garage sales. You can even wait until the sale is over, it’s very likely that an old machine will not sell and will be discarded; nothing is less desirable than a four year old computer. Goodwill and the Salvation Army sometimes have used computers for sale. Do not pay more than $10-15 for a used PC as you can often buy new PC power supplies on sale for $15.

Good hunting.

Step 2: Identify and extract the power supply. PC cases come in too many configurations to describe here, but “desktop” and “tower” cases are common variants. Most will have some hex-headed, Phillips screws around the periphery of the back that, once withdrawn, will allow you to remove the top. Some ‘tower’ cases have a single removable side. Remove the top (or side), and the power supply will be against the back wall of the case (Photo 6).

Photo 6

Look at the label on the supply if it’s visible. If the capacity of the supply is low (say, less than 5 amps on 12V or 5V) and you obtained the PC for nothing, you may want to consider throwing this ‘small fish’ back and trying again.

Next, extract the power supply. The supply will be held in the case by, typically, 4 Phillips screws on the back of the case. These screws will be arranged in an irregular pattern on the back of the supply. The circled screws in Photo 7 show the most common arrangement.

Photo 7

Do not remove the four screws closest to, and arranged in a square, around the fan. Removal of these screws will cause the fan to come loose inside the power supply (Photo 8).

Photo 8

If you do withdraw all four of these screws and the fan does come loose, DO NOT go into the power supply case to reattach it. These power supplies generate lethal voltages in excess of the line voltage that can linger long after the supply is turned off. Do not open the case of one of these for any reason. You’ll see this same warning posted on the label of the supply too.

Step 3: What power supply do you have? Start detaching all of the power cables that come out of the supply that are attached to the various units within the computer like disk drives, etc. However, save the big connector attached to the large circuit board (motherboard) for last. Photo 9 illustrates a typical installation.

Photo 9

Pull this connector and compare it to Photos 3-5. Remember which configuration that you have as it will guide you through the next steps. If your main connector(s) do not match one of the configurations above, including the colors of the wires, then you may have a proprietary power supply; you may want to try another donor computer.

Step 4: Power up and test.

4a) If your motherboard connectors match Photo 3, plug a computer power cord (i.e. an IEC cord) into the matching receptacle on the back of the supply, plug it into the wall and flip the big switch on the side of the supply. The fan should come on when the switch is turned on. Use a multi-meter to measure the DC voltage between any black (gnd) wire and any red (+5V) wire; record the results. Next measure the voltage between any yellow (+12V) wire and any black wire; record the results. You may want to clip and strip the ends of the black, red and yellow wires to read the voltages. If the black-red voltage is between approximately 4.75 and 5.25 volts and the black-yellow voltage is between approximately 11.4 and 12.6, congratulations! You’re almost done, go to step 6. If the fan does not come on, or the voltages are outside of these ranges then go to step 5.

4b) If your motherboard connectors match either Photo 4 or Photo 5, you need to take an additional step to get your power supply to turn on. Take a paper clip, break out the small ‘U’ shaped piece in the center and insert it into the connector to short out the green wire and the adjacent black wire (see Photo 10); you may have to shorten one or both legs of the ‘U’ to get it to fit down inside. What you are doing is mimicking the ‘power on’ signal from the motherboard. Repeat the steps above, insert the power cord, etc., but use the small rocker switch on the back to turn on the supply. If the fan does not come on, check the shorting jumper between green and black. If the black-red voltage is between approximately 4.75 and 5.25 volts and the black-yellow voltage is between approximately 11.4 and 12.6, go to step 6; otherwise go to step 5.

Photo 10

Step 5: What gives? The fan does not turn on; or I get no voltage; or the voltages are out of range.

5a) If your power supply connector matches Photo 3 and the fan does not turn on, or you get no voltage, recheck all of your connections; make sure that the wall outlet has power; check your multi-meter against a known voltage source (is your meter set to read DC?); try different black, red and yellow wires; clip and strip the black, red and yellow wires to guarantee good connections to your meter. If you still get no output, it’s very likely that the power supply is dead.

5b) If your power supply connector matches Photo 4 or 5, then do all of the steps above plus recheck the green-black shorting connection. Try shorting the green to any other black wire. Clip and strip the green and black wires and twist them together. If you still get no output, suspect a dead or non-standard supply.

5c) If the fan is on but the voltages are significantly out of range, or there is no voltage output at all, it may be because the supply needs an electrical load to function properly. Without going into a lot of theory (and making your head hurt), unlike the linear power supplies that we usually deal with containing a transformer, rectifier, etc., these ‘switching’ power supplies use the load as part of the regulation circuit and may not produce voltage if there is no load. I would not consider a voltage out of range by a couple of tenths of a volt either high or low as a serious issue. This would be an issue for a computer, but not for a model railroad. But if your voltage is way out of tolerance, you have a couple of choices.

Do nothing; your voltage may be a little low (or high), but it may make no difference. This is an option if you have a truly conventional DC operation with rheostats, etc.; after all, how often do you run at maximum speed? Nor will an over/under voltage condition effect the operation of lights, servos or Tortoise switch machines, so long as the voltage is not grossly out of range.

The other alternative is to provide a load in the form of a power resistor. You’ll have to go this route if the supply is producing no voltage whatsoever. A power resistor is not the little dog-bone shaped component that you find in most electronics, but a large, robust, wire wound, ceramic or metal encapsulated device (Photo 11). The power resistor will provide a constant load the moment the supply is turned on and the output voltage should always be present and within tolerance.

Photo 11

I have had good results with a 2 ohm, 15 watt or more, resistor (e.g. Allied Electronics stock #895-1334 or #296-4380) soldered between a red and a black wire. Clip a red wire (+5V) and a black wire from one of the disk drive connectors (Photo 12) and solder the resistor between them; insulate the connections. We’re doing this on the +5V line because there’s always more +5V available to waste. Be careful where you mount this resistor as it will get HOT. By putting this in a disk drive line, you’ll have enough wire free to mount the resistor in the airflow of the fan to keep it cool.

Photo 12

By the way, the voltage on these power supplies will wander a bit under load, but should stay within or near the voltage ranges given in step 4.

Step 6: Getting ready to power the layout. If you’ve gotten here, you have your PC supply working – good job!

If you used the paper clip to get your supply to turn on, you have a few choices. Leave the clip in place, secure it and insulate it with electrical tape; or cut the green and black wires and solder together permanently; or attach a SPST switch to the green and black wires. Doing the latter will allow you to mount the switch remotely and use it to turn the power supply on and off.

If you are going to be drawing a heavy load on any of the +12V, +5V or +3.3V power. Collect a number of the yellow wires (+12V), red wires (+5V) or orange wires (+3.3V) together from the main connector and the disk drive connectors on the supply so that no single wire is trying to supply the whole load. If you have a 24-pin main connector (Photo 5), be sure to use all of the yellow wires from the main connector to get the maximum capacity of +12V. Tie together several black wires to handle the return current.

If you want a voltage other than +3.3, +5 and +12 there are ways to coax a few more voltages out of these supplies. For example, if you want +7VDC connect some wires between the +5V and the +12V terminals (you read that right, positive 5V and positive 12V), the resultant voltage will be +7 VDC. All of the useful combinations are given in the following table:

Wire 1	Wire 2	Resulting Voltage	Notes
+3.3 V (orange)	+5 V (red)	1.7 V	Use lower voltage as ground
+3.3 V (orange)	+12 V (yellow)	8.7 V	Use lower voltage as ground
+5 V (red)	+12 V (yellow)	7 V	Use lower voltage as ground
-12 V (blue)	+12 V (yellow)	+24 V	Use negative voltage as ground
-12 V (blue)	+5 V (red)	+17 V	Use negative voltage as ground
-12 V (blue)	+3.3 V (orange)	+15.3 V	Use negative voltage as ground
-5 V (white)	+12 V (yellow)	+17 V	Use negative voltage as ground
-5 V (white)	+5 V (red)	+10 V	Use negative voltage as ground
-5 V (white)	+3.3 V (orange)	+8.3 V	Use negative voltage as ground

The voltage shown in the third column is the nominal voltage that will result. Since the voltages in columns 1 & 2 have a +/- 5% tolerance, it is very unlikely that the combination of the voltages will produce the exact voltage shown in column 3.

In each case the wire connected to the lower voltage will act as negative (i.e. lower potential). The current that you can draw from this type of set-up is limited to the lower of the two currents for the voltages involved. For instance, if 5 V will supply 30 amps and 12 V will supply 14 amps, then the 7 V circuit between 5 V and 12 V will only source 14 amps. In the case of any combination using the negative voltages (-5V and/or -12V) the current will be limited to 0.5 to 0.8 amps depending on your power supply rating; not enough to run a DCC system.

Which brings us to an important safety issue:

These supplies are capable of providing very heavy currents on +12V +3V and +5V, therefore all circuits attached to these supplies must be fused.

25 amps at 12V (300 Watts) surging into a short circuit is enough to weld rail, melt wires, weld wheels, melt ties – and start fires. These power supplies are supposed to be manufactured with built-in current limiters. However, these current limiters will allow more than 18 amps to flow before they shut down. This is set too high for model railroad use. Break up your power system into the smallest circuits feasible and fuse them for only as much current as you will normally use. For instance, if you will be running two throttles and each can draw 10 amps, fuse each throttle for 10 amps rather than fusing the 12V for 20 amps. Fuse each positive voltage separately. Do not put the fuse in the common ground return line as the sum of the currents may cause blown fuses. Fuse blocks for AGC glass fuses and in-line fuse holders for automotive-style blade fuses are available at auto parts stores and Radio Shack –

Do not fail to use them

Similarly, use wire of sufficient capacity for the expected currents. American Wire Gauge tables found on-line rate the current carrying capacity of 16 ga. wire as 3.69 amps and 12 ga. as only 9.33 amps! If you are going to be switching heavy loads, you can find 35 and 50 amp toggle switches at your local auto parts store.

Finally, if you are using a power resistor to load the supply, mount it to the case, or in the airflow of the fan, or to its own heat sink.

Enjoy the luxury of having more power than you will likely need.

Make a Latex Rock Mold

Making your own molds for rock casting opens up possibilities beyond those available using commercial rock molds or castings. Once you have your own mold, you can cast as many rocks as needed, cutting them and rearranging into varied patterns to disguise their origins. Making a latex mold is simple and inexpensive, and the materials are readily available. Although a latex mold will not last as long as a silicone or urethane mold nor make as many castings, this is usually not an issue for an individual modeler.

For this project I used some rock castings from "Ultimate, Make-A-Mold Master Castings" rock castings by Ultimate Scenery Center (www.ultimatescenery.com). These are highly detailed, urethane foam castings that the manufacturer actually intends to be used as masters for the purchaser to use to make his own molds.

I started with #123 "Pennsylvania Shale", a large (13" x 9 ") casting exhibiting both stratified and 'blocky' rock in the same casting. Here's a photo:



Here's a detail picture:



Click on any photo to get a full-size view.

Here's a photo just before starting the process. I used 'Mold Builder' latex mold compound that I bought at Michael's.



The surface that I am building this mold upon is a 18" x 18" ceramic floor tile obtained from Home Depot. I chose the smoothest 18" x 18" tile that I could find, although this was a floor tile and not a glazed wall tile like I build smaller molds on. To make the tile's matte surface smoother, I coated it with clear gloss spray from a spray can. A smooth surface will prevent the latex from adhering and facilitate removal of the finished mold.

The latex is brushed on. The first coat of latex must be applied thin and without bubbles - unless you want bubbles to be a permanent part of the mold and of each subsequent casting; so watch your brush strokes. I extended the latex beyond the sides of the master casting to form a lip on the finished mold about 3/8" wide.

And here's the first coat of latex applied.



Allow each coat to dry before applying subsequent coats. If you are impatient, the cure of the latex can be accelerated with hot air from a hair dryer. Additional coats are applied in the same way. Resist the temptation to apply the latex in thick coats. Thick coats tend to pull away from the master and they tend to dry with voids in the rubber.

Here's a pic of the mold after a couple of coats of latex have been applied and dried.



Apply four coats of latex before the next step.

Apply a coat of latex and wait for it to get 'tacky' then layer the entire thing with gauze. Forced the gauge into all of the cracks and crevices.

After the gauze is applied, add another coat of latex. Get the gauze saturated with this topcoat of latex. Here's a picture of the mold after adding this overcoat of latex:



After the latex/gauze/latex coat as dried; at least four additional coats are added before the mold is done; more coats can be added for additional strength.

Here's what the finished mold looks like ready to be peeled from the master casting:



And here's a video of removing the mold from the master casting:

Installing a DCC Decoder in a Brass Steamer Made Easy

These videos take you step-by step through the process of installing a DCC decoder in a brass steam locomotive - a Sunset/3rd rail B&O Q4b in O Scale to be precise.

There's no reason to be hesitant to disassemble and modify a brass locomotive; and the Sunset locos seem to be well soldered and sturdy. Hopefully these videos will give you the information and confidence that you need to tackle the job yourself. In fact, getting the installation right took me a couple of attempts to find the right combination of miniature connectors and flexible wire to make the installation work - but that is all explained in the film. Be sure to view the videos in order.

There is one omission that I must mention. In hooking up the backup light I forgot to film (or mention) the necessity to include a resistor in the line going to the backup light. This resistor is necessary once the backup light is hooked up directly to the decoder. Use a value of 500 to 1000 ohms and put the resistor in either line of the LED backup light.

In viewing the videos, do not click on the "HD" unless you have high speed internet and a fast computer (dual core processor and good video card); and you still may want to allow the video to buffer before playing.

Part 1

Part 2

Fast Tracks Tips & Techniques

Like many others, I have a set of Fast Tracks turnout construction tools. The videos on the Fast Tracks website provide a good overview on how to use each of these tools and I watched them before buying.

However, the O Scale fixtures are a little different then the ones for the other scales, and are not covered by Fast Track's videos. In addition I developed some techniques for using Fast Tracks that may be helpful. So I made this video to demonstrate all that I learned.





One more tip that came up after I finished the video. I installed a bunch of these turnouts in a yard ladder and as part of the installation I checked for shorts. While I did not find any dead shorts, I found some leakage between the left and right rails. This drove me nuts for a while until I isolated it to melted flux bridging the gaps in the PC board ties. Since the Oatley flux is a tinning flux, it's mildly conductive. So if you use the Oatley tinning flux, be sure to clean it out of all of the gaps (or use less flux than I did). I found that "Goo Gone" is a good cleaner to use for this purpose.

Some Simple Switch Machine Solutions

Mounting Tortoise Switch Machines

The problem with attaching the Tortoise machines is that the built-in mounting ears are too close to the machine to reach easily with a screwdriver. Plus the machine is small and it's hard to position it critically and drive the screws without pushing the machine out of place. Some vendors sell mounting plates for the Tortoise, however there is an easier and less expensive solution. To alleviate these problems attach each Tortoise machine to a scrap of plywood. The size of the plywood is not important so long as it extends well past the machine proper. The plywood is, in turn, mounted to the underside of the layout. The plywood forms a 'handle' that makes it easy to position and screw the machine in place without movement.

These photos should be self-explanatory.











You'll notice in the pictures above that the actuation mechanism of the Tortoise machine simply extends past one edge of the plywood mounting plate. In this way it is not necessary to drill a hole for the actuation wire through the mounting plate. Also note that I have pre-drilled and countersunk holes for the mounting screws. Four holes are not really necessary, but it provides some flexibility in where the mounting screws will be driven. Depending on your track construction the combined thickness of the mounting plate, subroadbed (plywood) and roadbed can exceed 1". This would be too thick if you use the music wire actuator that comes with the Tortoise. If this is the case cut another wire actuator and make it longer; the Tortoise instructions include a template for the actuator.

Mounting Servos as Switch Machines

Mounting radio-control servos for use as switch machines presents a problem. Some manufacturers sell mounting plate for servos but, as far as I have been able to determine, only for the miniature/micro servos. If you want to use the full-size servos or you need a lot of these mounting plates they are not a solution. There is a simpler and less expensive solution, glue the servo to a right angle bracket sold in home centers for building decks.





The bracket is Simpson 'Strong Tie' A21Z, note: this is the right angle bracket with one arm longer than the other, that's the arm that the servo is glued to. I used Gorilla Glue to attach the servo. I roughed-up the bracket and servo with sandpaper before I dampened the bracket with water and applied a THIN coat of the glue to the servo, then clamped the two together. Use a thin coat, otherwise the glue will foam up all over the work.

Here's the servo switch machine installed:



The machine can be installed above the table as well which is handy in areas of hidden trackage as the machine is easy to access for adjustment or replacement.



The servo switch machine is mounted on a scrap of 1x2 lumber (actual 3/4" x 1 1/2") to provide some height and thereby additional throw for the actuator wire. Notice that the PC Tie throw bar has been extended with another length of PC tie soldered on so that the switch machine can be mounted far enough to the side to clear rolling stock. The extended throw bar is supported by a wooden tie to prevent the throw bar from sagging.

For fitting the actuator arm, made of music wire, to the servo crank I came up with what I think is a novel solution which you can see here:



I bent a length of wire into the shape above. The length of the straight section to the right should be enough to reach up through the subroadbed, roadbed and through the throw bar on your turnout. If in doubt, make it longer, any excess can be cut off after installation. The form of the wire on the other end is shown in the detail close up here:



The distance between the bends should match the hole spacing on two opposite arms of the servo crank so that it fits like this:





In case it's not obvious how to get the bent wire into the servo crank, here's a short video on how it's done:



The self-retention feature that you see in the video works because I am using relatively stiff 0.039 wire as a actuation arm. That allows the right angle bend at the end to exert enough outward pressure to hold the arm in place. If you use significantly thinner wire this might not be the case.

Weathering Track

When I was installing a bridge, I weathered the rails with Badger Rail Brown and was not happy with the results. Badger Rail Brown produces a very orange-looking, fresh rust color; not exactly the color of rails as seen in nature or in period photographs.

Aha! let me try some of this craft store color called "Red Oxide", looks like rust to me. Well red oxide was too red, and because it was craft store paint, it would not stick to the rails.

While at one of the train shows held locally, I spied this color called Joe's Rusty Rail Painter Dark Brown paint. No doubt, this would do the trick! Applied to the rail, this color was too brown.

Not only were the colors not right to my eye, but each paint was just that - a monochromatic paint that was too even and showed no variation in color or tone.

While pondering the situation after the failure of Joe's color, and with all three colors next to the track, I got a flash of inspiration. I poured out a dab of each paint onto a paper plate, took my brush and dipped into each color in turn and applied the brush dripping with all three colors to the rails. On the rail I wiped it on and brushed the backstroke, but made no attempt to even out the color.

One was too orange, one was too red and one was too brown; but all three together were just right. The three colors blended together for a very pleasing rust color; better still, the colors did not mix completely leaving a very subtle mottled color on the rail.

[Click on the photo for a larger view]

The photo above shows the technique after I perfected it (I encourage you to click on the photo for a larger view). First the ties and rail are given a coat of Floquil Railroad Tie Brown. After this has dried thoroughly, I begin coloring the rails. I dip my brush into the red and the brown paint with every refill of paint, but the orange I use only every second or third dip; otherwise the color is too orange for my eye.

By varying the order in which you dip and how much you dip, you have a lot of control over how the final product looks. Do not brush the paint back and forth along the rail, but only just enough to cover the rail. This will produce the mottled look by not homogenizing the paint on the rail.

As I paint the rail I make no effort to confine the paint to the rail proper; in fact, as the brush runs low on paint I make an effort to bounce the brush along the spikes and tie plates to get some rust color on those parts. The next photo shows this effect.

[Click on the photo for a larger view]

I keep some Q-Tips wet with water handy and if I get a little too much paint on the tie plates, I take a wet Q-Tip and draw the extra paint out into a rust stain as you can see on the fifth tie from the left on the track in the foreground (click on the picture to enlarge to see this plainly).

This final photo is the finished product.

[Click on the photo for a larger view]

I let the track dry thoroughly before I clean the tops of the rail. I do this so that the paint has time to bond to the rail. I tried cleaning the rail soon after application and it caused the paint to crack and peel off of the rail sides. This is more work than cleaning the rail right away, however.

There is nothing special about the three colors that I chose for this technique. Try different colors in the dark reds, browns and oranges to find a combination that might work better for you.

Is this more work that spray-painting the rail, using rail color markers or one of the dedicated rail painting devices? Yes it is, but I think that the effect is worth it, I hope that you will too.