Wiring, Power and the Layout Control Bus

The Layout Control Bus

The Layout Control Bus (LCB) supplies all the power used by LCOS LCNs, LCN peripherals, most layout devices and layout lighting. The general power requirements for the Lew’s Duino Gear Layout Control System are:

  • +12 volts DC to power LCNs and 12v layout devices/lighting
  • +5 volts DC to power accessory boards, servos and lighting
  • Master Common Ground
You can run your Layout Control Bus next to your main track power bus. This layout uses Green and Purple for DCC, Yellow +12v DC, Red +5v DC and Black common ground. This LCB includes an additional +3.3v circuit (Blue). We like barrier strips every few feet for basic connections.

The LCB should be a 3 wire bus running from end to end, using 14 or 16 ga wire. Place terminal blocks every 4 feet or so as primary connection points to the LCB.

Power Supply

Your power supplies have to be sized to accommodate the expected current draw of your layout control devices. When calculating current requirements, here are some basic values to keep in mind:

  • An LCN draws about 50 mA, with bursts up to 120 mA during radio transmission;
  • An sg90 microservo draws 1/4 amp when moving, and 1/2 amp or more when stalled,
  • A Circuitron Tortoise switch machine draws about 16 mA.
  • A standard LED draws between 20 and 30 mA.
  • A typical +5v relay will draw 90 mA when the coil is active.

To size your power supply correctly, count your devices and calculate the maximum power requirement. A typical small layout might need less than 10 amps (@ 12 volts) of power, while a large basement empire might need multiple 20 amp power supplies arranged into power districts.

For example, a simple layout with a control panel with indicators, a reversing loop and some layout lighting, might require this:

  • 3 LCNS @ 50 mA = .15 amps
  • 5 microservos @ 500mA = 2.5 amps
  • 70 LED lights @ 25 mA = 1.75 amp
  • 2 relays @ 90 mA = .180 amp

Which adds up to 4.58 amps. Since you want your power supply to have greater capacity than your expected maximum, an 8 – 10 amp power supply would be a good choice.

A typical multi-voltage power supply. This one has fairly low amperage output (8 amps at 5 v; 7 amps at 12 v) that would work for a small layout. Bigger power supplies are available.

Multi-voltage power supplies are convenient, but they tend to have lower output current because capacities are divided between the different outputs. For a higher current handing, use multiple single voltage power supplies and tie their grounds together to make a single common ground.

Large Layout Considerations

You probably know that a good way to limit wire resistance and voltage drops is to use larger gauge wire. Still there are limits to how far you can go without power loss.

Larger layouts, especially over 12 linear feet may need supplemental power supplies to compensate for voltage drops over longer distances. If you do that, we recommend creating power districts. Each district is nominally independent, with the exception of the common ground which should extend continuously around the layout.

Another strategy is to use high output +12v power supplies, like a 30 amp, 12 volt regulated supply, then add a regulated DC Converter to supply the +5 volt circuit.

This regulator can supply substantial 5 volt current from your larger 12 volt power supply.

Using Computer Power Supplies

Another way to power your LCB is with one or more computer power supplies. The beauty of the computer PS is that it produces 3.3, 5 and 12 volt outputs (usually called “rails” in the computer PS world) with substantial current handling capability; up to 1200 watts combined (10 amps @ 120 volts) on really big power supplies. Computer power supplies are regulated and produce noise-free power. Its really the best possible power source on a budget, for under $100. A used computer PS in good condition will work well too; you may even have one lying around!

A typical 300 watt power supply compatible with older Dell desktop computers. The big white connector is the main power connector. This one sells for $42.

Should you modify a computer power supply to meet your needs? No, please don’t.

I’ve done it but I don’t recommend it for most people. There are some very large capacitors inside a computer power supply that can hold a big charge for a very long time. Serious injury is possible if the power supply is not fully discharged before opening it up. And, if you miswire it, a fire hazard will result.

Thankfully, it is not necessary to modify a computer PS in order to use one. Today, there are a multitude of breakout adapters available that accept the main system board connector (20 or 24 pins) and give access to all the “rails.” Two styles of adapter are generally available.

One style consolidates all the rails into 4 power pairs: +12v/GND, +5v/GND, +3.3v/GND and -12v/GND. The limitation of this type is that the consolidated rails may be current-limited, so check specs carefully before purchase.

This type of breakout consolidates like rails into 4 outputs. This particular one has fuses limiting current to 5 amps on each output. For smaller layouts, that limitation should be acceptable.

The other style is a pure breakout, with each rail individually exposed for connection. Here the current limitations are the limits of individual rails. Combining rails (connect all like rails into one power output for each voltage type) maximizes the current available for your layout.

This power supply breakout gives access to all power supply rails.

Of the two types, the latter is probably best because it gives you the greatest flexibility and current handling capacity.