A ‘RECYCLED’ LOW-COST DC POWER SUPPLY

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Peter Reynolds

Pictures by the author

WHATEVER the size of your railway, at some stage you will need a power supply for all those things that are not related to actually getting your trains to move. In the not too distant past, traditional DC traction controllers had either a 16V AC, a fixed 12V DC, or both, power outlets for building lighting, signals, and other items requiring a fixed AC or DC voltage to make them work. With the advent of DCC this has been neglected somewhat, unless you want to use whatever power your system has to do a fixed task other than make your trains move. You will probably, at some stage, need a stand-alone DC power supply, which nicely brings me to the subject of this article.

There are plenty of stand-alone DC power supplies available to buy, in a wide variety of voltage and current ratings. However, these can be rather expensive for the more useful, higher rated, articles. So what would be a more cost-effective alternative? Well, you can turn to the power supplies that are built into tower and desk-top computers, known as ‘ATX compatible’. These come in a wide range of current capabilities, but all provide 3.3V, 5V, and 12V supplies which are perfect for what the railway modeller/layout builder needs, and they are easily adaptable.

For example, I like to use 5V for any DC supply associated with LEDs. This is because, if an error is made, the often-inaccessible LED will not be totally destroyed; most LEDs have a maximum reverse voltage of 5V. These power supplies also have the added benefit of often being available as ‘surplus to requirements’ and it is also possible to keep, and use, the original computer case if so desired.

Older computer systems used power supplies that were rated at a total of 350W, or sometimes less, and, while these need not be discounted in terms of what we are going to build, I would recommend that you try to start off with a power supply rated at least 500W, if you can. A typical 500W power supply will give (all maximums) 15A @ 3.3V, 15A @ 5V, and 30A @ 12V which is very close to the maximum rating of the unit (485W in total). It is not advisable to run whatever unit you settle on this close to its maximum rating; you should look at no more than 80% in reality. This is for heat and reliability purposes. The supply I am going to build will have 3x 5A outlets for 5V, 4x 5A outlets for 12V, and I won’t be using the 3.3V supply at all, so about 315W when fully loaded.

The reason I have arranged the supplies like this is for safety, as, although the voltage is low, the possible current is not, and if 5V at 15A runs amok on a layout it can still do significant damage. It’s better just to replace a 5A fuse if something goes wrong. As added protection, most ATX computer power supplies have built-in thermal and overload protection. You should never open a power supply while it is on, or poke any foreign object into it. There are some seriously high DC voltages present in a switch-mode power supply, which could definitely spoil your day. A piece of advice here to consider; if you do not have anything readily available, and wish to buy one separately, please ensure you buy one with coloured external wiring. There are some replacement units available that use only black wires for everything; this isn’t a problem when you are plugging connectors together as these are keyed, so it is relatively hard to make an unconscious mistake. While it isn’t impossible to suitably adapt a unit with totally black wiring, it does make your life considerably harder, especially if you are not entirely confident in carrying out the task.

To get started you will need to check that the power supply you are going to use actually works. ATX power supplies will not start up with nothing else attached, so if you do not know the source of your supply, it will need to be tested somehow. The easiest way to do this is to us a special ATX power supply tester, which can be bought for about £10. If you are going to make more than one, this if definitely a recommended requirement to buy, especially if you do not know the source of your chosen unit for adaptation. Obviously, if the power supply is still in a computer, and that is working, this is a good indication of whether the unit is viable for conversion. However, if your supply is unknown, and you don’t wish to purchase a tester (or borrow one from someone who has one), you can go straight ahead provided you are happy that the unit is not damaged in any way (smell is a good way forward here – fried component smell lingers!). Any obviously damaged units should be immediately discarded.

An ATX Power Supply Tester - available for about £10, if you want to purchase one

A typical read-out from the tester, showing that the subject power supply is working correctly

To make the power supply start and stay started (try it without a load – you won’t damage it, but the fan should kick then stop) you will need to load the 5V part of the supply with at least 1A. The 5V supply is on the RED wires, and the BLACK return is common to all voltages. Using Ohm’s Law (V=IR), you can see a 1A load across the 5V supply is obtainable using a 5Ω resistor. We will need this for the finished unit, but it can be used for initial testing.

A discarded, but working, 750W power supply from number two son (Philip builds his own PC systems) - note the current potential of the 12V supply, 62A. Also note that the wiring is all black. Our subject 500W power supply

Two 10R resistors for the 5V/1A load - these particluar items are 25W rates, so have a high safety margin Load resistors mounted – you can see the benefit of using these particular power resistors

One other thing about this resistor is that you will need to be aware of the wattage requirement; 5V and 1A through a 5Ω resistor gives a 5W power dissipation. A 5W resistor in this instance would get very hot, and would probably fail quite quickly. As 5Ω resistors are not generally available, I use 2x 10Ω resistors in parallel to give the required 5Ω and you can use 5W rated components if arranged this way. I prefer 10W rated, aluminium clad power resistors from Rapid (62-8062 £3.16ea) as they are easier to mount (bolted to the chassis). Turning to your power supply, all outputs are available for use in this application, but you need to be aware of the colour coding on the connectors. There are any number of graphics on the internet describing what each pin on the large connector does; search for ‘ATX Power Supply Pin out’.

Orange is 3.3V and shares a brown wire on the large 20 or 24-way connector. These two wires must remain connected even if you don’t intend to use the 3.3V supply. The power supply will not work without this connection. Other orange wires can be cut short and the exposed ends covered with heatshrink sleeving.

Red is 5V, Yellow is 12V, and Black is 0V/Return. There are also Blue, Purple, Grey, Green, and, sometimes, White wires. Blue and Purple and White are not needed. They can each be cut short and the exposed ends separately covered with heatshrink sleeving.

The other two extra wires, Green and Grey, you certainly will need. The Green wire, when you electrically connect it to a black wire, is the means of turning on the power supply on. This is what happens when you press the computer ‘on’ button. The Grey wire carries 5V when the power supply is turned on, so, with a suitable resistor and LED, this can be used to indicate that the power supply is ‘alive’. Indeed, I usually extend these two connections remotely, with a suitable common return, so that the operators at a particular location can turn the power supply on with a simple switch, and see that it is ‘on’, without performing gymnastics under baseboards.

5V fuses are internal and rated at 5A each. XLR sockets and fuse holders for 12V mounted on the rear of the chassis. The socket on the right is a 5-pin socket for 12V to the control panel, plus provision for remote control of the PSU.

I always fuse the however many outputs there are with a 5A ‘F’ fuse on each voltage, and you can use what you have available or buy specifically to do this. Fuse holders with screw tops are generally the most convenient, or miniature thermal circuit breakers, but if you plan your power distribution carefully, you will only ever need access in the event of a mishap. For the outputs, I have used 3-pole XLR connectors in the past, but any plug/socket combination you fancy that has at least a 5A current capacity would be suitable. It is generally relatively easy to mount any of these connectors on the outside of the computer case using a suitable hole (make a plate to fill in one of the unused openings, for example). Whatever you do, also remember to make all your internal wiring connections using a gauge of wire suitable for the current it is going to carry – I use at least 24/0.2 for 5A wiring.

So far, I have made three ATX standalone power supplies for the SR7mm Group system, and there is the prospect of two more in the not-too-distant future. Very often, this provision on a layout is last minute with a variety of barely suitable arrangements provided. Hopefully, this article will show you that there is an inexpensive, readily available, alternative which is easy to package and that will form the only DC power supply that you would ever need.

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The output panel from the rear – always use the correct gauge of wiring for any extensions. Also note the green and grey wires to the connector on the left – green connected to ground turns the power supply on, and an LED (with a suitable resistor) from the grey wire to ground indicates ‘power on’.