Mains Power Supplies for Analogue Model Railways

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Foreword

 

This topic addresses the use of mains electricity with power supplies for analogue model railways. The subject is relatively specialised, but it does come up on the forums from time to time.

 

Most modellers begin their career with a self-contained, mains-powered model railway controller. These are the controllers supplied with train sets, and those manufactured by specialised manufactures such as Hornby and (in the UK) Gaugemaster. Some people know these units colloquially as ‘transformers’.

 

Later on, many modellers will opt for a panel-mounted controller using a low voltage input (often about 16 volts AC), fed from a separate mains power unit. Such a mains power unit usually provides more than one output - perhaps two 16V supplies for controllers, and some auxiliary outputs for point motors or building lighting. Mains power supplies are available ready-made.

 

Ultimately, some modellers want to build their own mains power supplies. The reasons to do this include the need for a self-contained power unit for larger-scale models, and a larger layout needing many controllers.

 

With this in mind, and before continuing, we remember that electric model railways use bare metal rails as the conductors to send electric power to our trains, and that mains electricity can be lethal.

 

1  Model Railway Controllers

 

If possible, buy a brand-new model railway controller from a reputable manufacturer. Smaller units are supplied with train sets, and larger units can be bought from model shops and by mail order. Be extremely cautious of the second-hand equipment sold at fairs and exhibitions. Some older equipment was built to a good mechanical standard and still has an excellent industrial design, but it will fail today’s standards for the safety of mains electrical equipment. Moreover, these controllers were designed for old-fashioned motors which needed much more current than today’s designs. They will not give the controllability you expect with modern locomotive motors.

 

Put the controller on or near the layout. Keep the mains lead of the controller away from other layout wiring, and treat the controller like any other item of mains electrical equipment in your home. Follow the instructions supplied with the controller.

 

A panel-mounted controller is a popular choice with many more experienced modellers. These controllers usually need an unregulated, low voltage input of around 16V AC. The controller itself can be fitted into a control panel or into the layout itself.

 

2  Commercial Mains Power Supplies

 

Commercial mains power supplies are manufactured for use with panel-mounted controllers. If you are buying a mains power supplies, choose one designed for model railway use. A typical power supply designed for model railways gives two unregulated outputs rated at around 16V AC at 1A, and this is sufficient to drive two trains (through separate controllers) and some point motors and other circuits like building lighting as well.

 

These power supplies have poor regulation and will often be a few volts higher (say 18V instead of the rated 16V) on a small load. Panel-mounted controllers which are specified to need a 16V input may well survive this without problems, but it is worth checking with the manufacturers of the equipment before purchase.

 

The power supply will be in a plain box with no controls except sometimes an on/off switch, and it will have some screw terminals to connect the wires to the layout. The power supply will usually incorporate some overload protection, to protect itself if you put too much load on its outputs.

 

3  Siting a Mains Power Supply

 

Most modellers put the mains power supply unit on the floor or a shelf under the layout. They use a multi-way cable or a home-made cableform to take the low voltage electricity from the power supply to the layout. This arrangement keeps mains electricity physically separate from the layout and is strongly recommended.

 

It is rarely necessary to fix a mains power supply into a layout. If you are thinking about doing this, make sure that the unit will be physically protected against inadvertent drilling from the layout above, and that the mains cable is kept separate from all other cables and is easy to identify.

 

4  Specifying a Mains Power Supply

 

Historically, model railway controllers for the smaller scales trains have been designed to deliver up to 1 amp. This is ample for modern 00 and N gauge trains and many models will draw less than half an amp during normal use.

 

Some commercial power supplies have a rectified and regulated 12V DC output for accessories like signals, point motors and building lighting:

• A signal using LEDs will draw about 10mA from a 12V supply all the time.
• Stall-type point motors like the Circuitron ‘Tortoise’ draw about 15mA from a 12V supply all the time.

 

A 12V power supply with a spare capacity of 1A might therefore drive 50 colour light signals and 30 stall-type point motors. More realistically, you might use a power supply to drive one train (up to say 700mA), eight stall type point motors (120mA) and twelve colour light signals (another 120 mA) and still have some room for expansion.

 

If the power supply unit has two 16V AC outputs, these can be conveniently used to drive two analogue controllers and two linear DC voltage regulators for accessories. You can connect the voltage regulators to give a split supply (+/- 12V) and this will simplify the switching for stall-type point motors. If you drive these point motors from a split DC supply, the usual rules of diversity will let you allow only 10mA (or even just 7.5mA) per motor per supply.

 

Many digital products are now being supplied with switch mode supplies and there is EU legislation to outlaw transformer-based linear supplies on the basis of energy efficiency. This legislation is for commercial products, not home-made power supplies, but it may have affect the availability of ready-made linear power supplies in the years to come.

 

The traditional twin-solenoid type of point motor, if needed, is best driven from a capacitor-discharge unit. Such a unit is easily powered from a low-voltage AC supply and will only draw current for a brief period while it is recharging between operations. Some modellers use a 24V AC supply for these units, but 16V AC is fine for smaller layouts. A capacitor-discharge unit (or a low voltage AC supply) will minimise the effect of arcing on the switch contacts. If you use DC to operate twin-solenoid motors, you must use much more robust switches. The motors are inductive, and the action of releasing the switch places a high load on the switch contacts.

 

You should only use one mains power supply unit for a model railway layout. If you use two or more units and connect their low voltage outputs in parallel, and one power supply is unplugged from the mains, its internal transformer will generate a high and potentially dangerous voltage across its unconnected mains plug.

 

5  Designing a Mains Power Supply

 

If your layout needs more power than you can get from a commercially-available power supply unit, you may be tempted to build your own. Only do this if you are sure you have the skills and competencies to work safely with mains electricity. You must observe all of the laws and regulations which apply in your locality, and all of the points listed below.

 

• Use a transformer with a double-wound design. Such a transformer is designed so that no internal fault can allow the primary (mains) winding voltage to appear on the secondary (low voltage) winding. This is imperative for model railways. The model rails must not be able to become live at the mains voltage during any fault condition. Do not use any kind of variac or auto transformer. These units place the full mains voltage on their output terminals.
• Fix the transformer into a suitable enclosure. Suitable materials include steel, aluminium and ABS plastic. Do not use plywood or timber, these materials lose their insulating properties when wet.
• You can use a chassis to hold the transformer inside the enclosure. If you do this, use a sheet of steel, aluminium, glass fibre or paxolin. Do not use plywood or styrene sheet.
• The transformer is the most efficient of all electronic devices, typically better than 95%, and it will not dissipate significant heat during normal operation or foreseeable fault conditions. Enclosure ventilation for a transformer is unnecessary and can introduce extra hazards, like holes for children to push things into.
• Linear power supplies are much less efficient than a transformer and their output semiconductors get hot during normal operation. If you decide to put a voltage regulator into the enclosure with the transformer, you will need to arrange suitable cooling. This might use the enclosure itself as a heat sink, or an external heatsink, or ventilation holes. It is usually better to fix voltage regulators to the layout itself.
• Consider including a fuse in the supply to the transformer primary, to protect the transformer. In the UK, the fuse in the 13A plug protects the cable, not the appliance.
• Decide how you will protect the low voltage wiring. You might rely on the overload protection in the connected controllers and other devices, or look to add a fuse or thermal cut-out on the secondary.
• Think carefully about whether you want to build a Class I or Class II power supply. A Class I design must have an earth connection. A Class II power supply must not have an earth connection. You cannot mix the two and make the earth connection optional.
• Provide mechanical support for all wires, not just the mains cable. Do not rely on the electrical connection to the conductor hold a wire in place.
• Think through all foreseeable fault conditions during the design development process. If a soldered wire breaks this usually happens at the ‘knuckle’ where capillary action stopped drawing solder up the cores. Consider crimp connections instead. Restrain wires, for example in cableforms, so a broken wire cannot touch any other electrical conductor. You must make sure that each mains wire cannot touch a low voltage wire under every foreseeable fault condition.

When you have finished building the power supply and tested it yourself, add a marking plate to the enclosure. Then get the finished unit examined and tested by a professional electrician or other qualified person before you put it into service.