Power supply

DCCWiki, a community DCC encyclopedia.

Summary: Supplies power to the booster and is used to power model railroad trains and layout accessories.

Basic Concepts

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 to power the boosters.

An AC power supply. A step down transformer supplies AC power to the booster. Click for a larger image.

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. 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

There are two types 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.

The other type is the more expensive 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.

Requirements

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. It can supply more, any extra current will go unused, but not less.

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

  • Low current can impair operational peformance
    • 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 shutdowns during operation
    • Can lead to premature failure of the booster

As you can tell, it's important that the power supply output match that of the booster's requirements. The goal of this article it to guide you to selecting the proper power supply for your layout and DCC system.

Remember, the booster demands only as much current as it needs. If the power supply can supply 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.

When Converting from DC

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 can also introduce problems.

If you are converting an existing Direct Current powered layout to DCC, be sure to check the wiring as well. Many analog control systems did not experience the levels of current flow found in DCC because 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 booster/power supply combination found in a typical DCC setup. As a result, analog controls did not have, or need, the heavy gauge wiring typical of DCC.

Operating

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, you will need at least a three-amp power supply.

Most power packs don't state the output current, because it's usually pretty 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 is calculated by multiplying the voltage by the current, which is VA, and then multiplying the result by the Power Factor. Power Factor is only found in AC calculations, in DC, watts is the same as VA.)

Lets break this down. If you take the 30 Volt-Amp rating and divide by the voltage, you get the amps. So, if the rated voltage is 12 volts, you would have 2.5 amps. Traditional power packs are designed to power one train, not the multple trains that DCC can handle. As you can see, we need more current.

Track Voltage

As for voltage, there's a method for figuring out the ideal value taking into consideration power regulation, voltage drops, efficiency, etc.

Without causing you great headaches, here are the maximum track voltages for the four scale settings DCC is designed for:

Scale Volts
Z ?
N 12
HO 15
O/G 20

Note: 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 scale track it's connected to! The user must select the appropriate voltage for their application.

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 power supply, and the minimum and maximum voltages required. AC voltage input is the preferred power method.

If you use lower voltages, you will notice performance issues with your trains. If track voltage falls below about 10 volts, then the DCC signal on the rails becomes unreliable. Also, the booster's short-circuit protection will not work without sufficent current. However, if you were to use higher voltages, the booster will run hotter than necessary, possibly shortening it's life in the process. The excess voltage will be lost as heat, and heat kills electronics. Track voltages higher than those shown with probably drastically shorten the life of any lighting effects you might have 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.

Current

Or Amps...

You will need to select a power supply that can deliver the amperage (current) that the booster requires. For example, a typical five-amp booster set for HO scale will need five amps or more. A typical 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.

Generally, the booster will draw the current needed to supply 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 bus. Light gauge wires can cause a significant voltage drops over a long run, resulting in an increase in current draw. See the track wiring page for details.

See Also

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

External Links