Power Supply

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Summary: A power supply is a device containing a transformer which steps down the high voltage alternating current supplied by the utility to a lower voltage that is usable by the trains and layout accessories.


Basic Concepts

Safety First: Unless you are qualified to work on electrical and electronic devices, do not attempt to build, modify, or repair devices such as power supplies.

A power supply is a device containing a transformer which steps down the high voltage alternating current supplied by the utility to a lower voltage that is usable by your command station and/or booster. The power supply is usually not included with most DCC starter sets. When adding boosters, you will need to buy additional power supplies for the boosters.

A Power Supply can supply low voltage AC or DC. To supply DC, the power supply includes diodes to rectify the AC current supplied by the transformer. The quality of the DC output is related to the complexity of the power supply's design and construction. A regulated DC power supply will try to supply the same voltage regardless of the load. A better power supply will always be reflected in the price.

When purchasing a power supply, ensure that the device meets UL, ULC, CSA or other agencies' requirements for safe operation.[1] If building a power supply, purchase components that are UL, ULC, CSA, etc., approved, as required, and be sure to include overcurrent protection in the form of a fuse or circuit breaker. If you are not sure about what you are doing, ask someone to help you, particularly when dealing with high voltages.

Keep in mind that many power supplies are double insulated. To maintain that classification, do not connect the low voltage side to earth ground. The ground is meant as a reference point for the low voltage side, and it is not to be connected to the ground on the high voltage side.

Types of Power Supplies

Alternating Current Power Supplies

Alternating Current power supplies are quite simple in construction. The simplest design is a transformer with a high voltage primary and the low voltage secondary winding. The transformer ratio determines the secondary voltage. Design choices determine the current available on the secondary. They can be small, in the form factor of a wall wart, or large and heavy.

Some AC power supplies may also include inductors and capacitors to condition the output by removing high frequency components and harmonics found on the primary side.

Direct Current Power Supplies

Basic AC to DC power supply schematic.
AC Adapter or "wall wart".

There are two types of DC supplies in common usage today. Many years ago, a battery might have been used to supply Direct Current to the layout, but advances in electronics have almost eliminated that option. Many power supplies are the simpler and cheaper linear power supplies, which use a transformer, rectifier and filters to produce a DC output. There are two types of linear power supplies: Regulated and unregulated. The cheap wall wart is a simple unregulated power supply. If you overload one, it burns out. These are not worth repairing.

The other type is the more expensive SMPS or Switch Mode Power Supply, which is a lot more efficient than the linear power supply, and is smaller (usually). They are commonly employed in computers. They may also be called switchers.

Switching Power Supply as found in a desktop computer.

Linear Versus Switching Power Supplies

Why the difference?

  1. Switchers can be up to 80% smaller than an equivalent linear supply
    1. Linear supplies can be much larger and bulkier
  2. Efficient: Switchers can be up to 65% efficient compared to the typical linear supply at 25%.
    1. Switchers convert the AC directly without the need for large, heavy transformers
    2. Switchers require smaller, lower power electronics to perform voltage regulation using feedback instead of large voltage regulators
  3. Linear power supplies do not create Radio Frequency Interference and are immune to Electromagnetic Interference
  4. Linear power supplies are much simpler in construction

SELV Power Supplies

The majority of power supplies sold to consumers are SELV, or Safety Extra Low Voltage power supplies[2]. The "S" may also mean Separated.[3]

They are designed with safety in mind, with strict requirements on their construction to protect the user from exposure to dangerous voltages and currents.

SELV power supplies are the preferred type for a model railroad.


Protected Extra Low Voltage power supplies (PELV) are often used with laptops and other devices which may require a connection to the earth ground. These are not recommended for use with a model railroad. When using a laptop or desktop computer with the model railroad ensure the interface has isolation between the throttle network or command station and the computer. Devices such as the LocoBuffer and Digitrax's PR4 are isolated.

Choosing a Power Supply

When choosing a power supply, always consult the manufacturer's documentation.


A power supply's capacity needs to closely match your booster's demand, that is, if your booster can handle 5 or 8 amps, your power supply must be able to supply that amount at the very minimum. It can supply more, any extra current will go unused, but not less.

For example, a typical five-amp booster requires a minimum of five amps. An eight-amp booster set for O/G scales will need eight or more. It should be noted that having more current capacity than needed will not improve booster or train operation. But having less will hamper both. Having more current available will not cause overheating, but having more voltage will.

If you only plan to run two or three locomotives in total, a smaller power supply will suffice. Be sure to include extra capacity for reliable operation of the over current protection and the possibility of foreign power (other trains) operating on the layout temporarily.

Generally, the booster will only draw the current needed to supply itself and the load. If it only needs one amp, that is all it will draw. Attempting to draw more current than the power supply can handle can overheat and damage the power supply or booster, or trigger their protection circuitry.

With larger current draws, run heavier gauge wire for the power bus. The light gauge wiring typical of operations with classic analog throttles causes significant voltage drops over long runs of wire, resulting in an increase in current draw. See the track wiring page for details.

Potential Issues

Here is a list of problems than can occur from using the wrong voltage or current:

  • Low current can impair operational performance
    • Trains start running slower
    • Lights not at correct intensity
    • Smoke/sound units may not function fully
  • Low current can interfere with over-current protection, preventing the circuit breaker from opening during a short
  • Low voltage can cause poor train/locomotive performance.
  • Low voltage can damage electronics by drawing too much current, overheating components in the process
  • High voltage can lead to boosters overheating
    • Can lead to system shutdown during operation
    • Can lead to premature failure of the booster

It's important that the power supply output match the booster's requirements. The goal of this article is to guide you in selecting the proper power supply for your DCC system.

Remember, the booster demands only the current it needs. If the power supply is rated at 6 amps, that would be fine for a 5A booster. Connected to an 8A booster, the output would be limited to less than 6A, as some current is consumed by the booster. A higher current power supply would be needed to get the maximum output from the booster.

Power Supply Requirements

Boosters have short-circuit protection built-in. For maximum reliability, the power supply needs as much current capability as the booster is rated for, as well as the correct voltage. For example, for a three-amp booster requires at a minimum a three-amp power supply. If the current supplied is too low, the circuit protection may not work.

Traditional analog power packs are designed to power one train, not the multiple trains that DCC can handle. As you can see, we need more current. Most power packs don't state the output current, because it is usually low, and they want you to think you're getting more than you really are. If you were to check the specifications on your power pack, it's likely to say something like 30VA. This means 30 Volt-Amps. The VA quantity is determined by multiplying the voltage by the current. It is important to determine if the label means the load (energy consumed by the device) or the output of the power supply.

Watts are calculated by multiplying the Voltage by the Current, which is VA or Volt-Amps, then multiplying the result by the Power Factor. Power Factor is only found in AC calculations, in DC, watts equals VA. In AC, the wattage is always smaller than the Volt-Amps value.

Example: A power pack with a 30VA output: If you take the 30 Volt-Amp rating and divide by the voltage, you get the amps. If the voltage is 12 volts, you would have 2.5 amps of current.

Recommended DCC Track Voltages

Suggested DCC Track Voltages [4] [5]
Z N HO S O G / Large Scale
10 − 12 12 − 14 14 − 16 22 − 24 [6]

Source: Digital Command Control

To allow the booster to supply those voltages, the power supply must provide a higher voltage. The booster will regulate the output, and for that to work reliably, a higher input is needed. Check your manuals for the recommended type of power supply and the minimum and maximum voltages required. Some devices may work with AC or DC power.

If you use lower input voltages, you will notice performance issues with your trains. If track voltage falls below ~7 volts, the DCC signal on the rails becomes unreliable. Also, the booster's short-circuit protection will not work without sufficient current. However, if you were to use higher voltages, the booster will run hotter than necessary, possibly shortening its life in the process. The excess voltage will be lost as heat, heat kills electronics.

Track voltages higher than those shown will drastically shorten the life of any lighting effects (functions) installed. As you have probably already figured out, the higher the voltage, the more heat is generated – the higher heat can lead to premature booster or decoder failure, and/or thermal shutdown.

Lowering the track voltage provides no benefit. Many believe it is better, yet it only results in more current draw. Which places additional demands on the booster and the decoder. The booster has the advantage of free air for cooling where a decoder does not. If a slower running locomotive is the goal, use the speed table.

Converting from Analog DC to DCC Operation

Most DCC manufacturers, and vendors, suggest using your existing DC "power pack" to power your new DCC system when getting started. Although this is true, it is highly recommended that you get a power supply intended for use with a DCC system. Using your existing power supply could cause problems, such as train operation and short-circuit protection being unreliable, at best. This is due to insufficient voltage and/or current being unable to fully power the booster. Pulse power circuits also introduce problems.

If you are converting an existing Direct Current powered layout to DCC, check the wiring as well. Many analog control systems did not experience the current draws found in DCC as typically one locomotive was powered in one block by a small power supply. Multiple blocks spread the power requirements out over a number of small power supplies, instead of the one central booster/power supply combination found in a typical DCC setup.As a result, analog controls did not have, nor required, the heavy gauge wiring typical of DCC.

Further Reading

  • Track wiring - Details about wiring
  • Booster - Details about boosters which receive power from power supplies.
  • Command Station - The unit that creates the digital signals. Many systems now combine the booster and command station.
  • Voltage Drop - Why DCC wiring is different from analog/DC wiring
  • Grounds - Discussion on types of grounds.

External Links

  1. Many safety regulations are country or region specific. Always purchase electrical equipment that meets local requirements.
  2. Extra Low Voltage is defined as less than 50VAC or 120VDC
  3. The output is completely isolated from the input, and there is no connection to earth ground on the low voltage side.
  4. These voltages are based on which scale the booster is set for, not on which scale you're actually running. The booster has no way of knowing what gauge track it's connected to! The user must select the appropriate voltage for their application.
  5. Source: Digital Command Control
  6. The full NMRA specification of 24V is recommended for anything but slower narrow gauge steam locomotives. Large scale diesels such as those from Aristo-Craft cannot achieve prototype mainline speed at 22 volts, they require 24 volts to overcome their low gearing. Speed matching requirements for consisting may require some "headroom" in voltage.