Sizing Your 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:
- A Layout Control Node (LCN) draws about 100 mA, with bursts up to 200 mA during radio transmission;
- An sg90 microservo draws 1/4 amp (250 mA) when moving, and 1/2 amp (500 mA) 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 @ 200 mA (max draw) = .6 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 5.03 amps. Your power supply should have a 20% or more greater rated capacity than your expected maximum current draw, an 8 – 10 amp power supply would be a good choice.
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.
Single Output Power Supplies #
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. The example below costs about $12 USD.
Wall Wart Power Supplies #
Layout Control Nodes and some other boards can be powered by a 12 volt wall wart with a 2.1 x 5.5 mm DC plug. The wall wart should have a capacity of at least 2 amps at 12 volts. See the example below.
Wall warts do not connect to earth ground. So long as the layout has a Master DC Ground to which all devices are connected, everything should work. In the event of any system instability, we recommend attaching your Master layout ground to an earth ground such as a metal water pipe.
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.