Power supply

DCCWiki, a community DCC encyclopedia.

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

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 your command station and/or booster. The power supply may also convert the Alternating Current supplied by the Utility to Direct Current. 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.

Power Supplies

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 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.

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

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.

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.

Power 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 at the very minimum. It can supply more, any extra current will go unused, but not less.

Potential Issues

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 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 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 central booster/power supply combination found in a typical DCC setup. As a result, analog controls did not have, nor require, the heavy gauge wiring typical of DCC.

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.

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

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.

Common DCC Track Voltages by Scale
DCCWiki.com
Z N H0 S 0 Large Scale
10 − 12 12 − 14 14 − 16 20 − 22

Source: Digital Command Control

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. 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 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 will 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.

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. Remember that 2x = 4 no matter how you do it.

Current Requirements

Or Amps...

You 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.

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 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 bus. Light gauge wiring typical of operations with classic analog throttles cause a significant voltage drops over a long run of wire, 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