Text and Photos © 2010-2011 by Terry Terrance
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 1The 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 5Let’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 6Look 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
Photo 8Step 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 9Pull 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 10Step 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
Photo 12By 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.
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