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DCC Clinic – 2006

DCC Clinic – 2006

David Gairo

South Jersey Garden Railway Society

What is DCC?

Digital Command Control. Specification developed by the NMRA to provide independent control of many trains without separate power blocks.

Power and commands are integrated and applied to all tracks. Signal on the track is an AC square wave at the full voltage. Each powered unit has a decoder, which will recognize commands addressed to it and ignore commands sent to other addresses. The decoder rectifies the input signal and applies pulses to the motor to effect speed and direction control. The decoder provides other functions such as headlight control, horn and bell.

DCC Manufacturers

I have provided a list of DCC manufacturers and their WWW addresses. Which is the “BEST” system? The system YOU like! You should try different systems and decide what features and throttles you like. The throttle is the MAIN interface and you must be comfortable with it. Prices vary by system and by dealer. Tony’s has a very nice system comparison page.

System Components

Power Supply: usually not provided with a DCC system, except some of the newer, starter sets. You must purchase it separately or provide it yourself. 14 – 18 volt at about 5 amps.

Command Station: Connects throttles and other control components to the system.

Booster: Applies the command from the command station to the power and sends the combined signal to the layout. Boosters will handle between 2.5 and 10 amps, depending on make and model.

Throttle: Your interface to the system. Probably the most important component to the user and to system satisfaction. You need to try different systems and decide what you want!

Decoder: The brain of each powered unit. Recognizes commands sent to its address and controls the motor and other actions via function outputs. Decoder prices keep falling like computer prices. Look for a decoder that provides high frequency operation which reduces motor noise and vibration from the pulses.


- Radio or infra-red throttle

- Transponder: senses the location of decoders by their addresses. (Digitrax)

- Switch Machine decoder: allows automatic routing and / or control of switches from the throttle.

- Auto Reverse module: For two-rail layouts, provides an automatic control of polarity for reverse-loops.


Layout Wiring

In the simplest form, one wire to each rail. For a larger layout, there may be a need for additional power handling. This is accomplished by adding additional power supplies and boosters, then dividing the layout into power districts. Still only two wires to each district. The command station is linked to each booster through a network. These networks are designed by each manufacturer and vary in capability and speed. If you are looking for a large capacity system, the network may be a critical part. In my opinion, Digitrax has the best network.

Module / Layout Wiring

A small or medium sized layout can consist of one power district supplied by one booster. With an 5 or 8 amp system, that may be all you need.


Decoders are rated for their continuous and maximum current load, which includes the motor AND lights. Many are rated 1.5 amps continuous and 2 amps peak. The peak is with the motor stalled. I use HO decoders for all my O scale trolley cars. They all have HO sized motors in them! (A class C freight motor has two motors, so it has an O decoder with 2.5 amp capacity.) Decoders are made with capacities of up to 8 amps. This Heartland power block is running on an HO 1.5 amp decoder!

To measure current draw, I hold the car or locomotive and let the wheels spin at max voltage to get the maximum current draw. I don’t bother with stall current with the wheels locked. If the motor ever does stall, I may loose the decoder, but that has not happened.

Decoder Installation and Wiring

The minimum number of wires to install a decoder is 4, 2 from the wheels to the decoder and 2 from the decoder to the motor. Motors MUST be isolated from the frame! The Heartland unit had to be opened and the wheel contact wires disconnected from the motor. Then 2 new wires had to be added back to the motor. The Aristo power unit already had the 4 wires with the motor isolated.

All decoders have front and rear headlight outputs. That adds 3 more wires to the decoder, one for each light and a common return. That is a total of 7 wires and in large scale it should not be too much trouble to wire the decoder. The lights will change from forward to reverse with the motor. They can also be turned off with a function button. I always put a current limiting resistor in series with the bulbs. Lamps draw a LARGE current for the first few milliseconds. This current could damage a decoder over time. (Also the reason bulbs blow out when you first turn them on.)

Computer Interface

Not required! However, depending on what DCC system you select, there are interfaces available to connect a computer to the DCC system. I use one to program the decoders and store a record of the programming on the computer. Programs are also available to automate the operation of a layout.

DCC Internet Links        NMRA DCC Special Interest Group

DCC Manufacturers                               ATLAS                            Digitrax                                  Lenz                Model Rectifier Corp.                            North Coast Engineering                             Train Control Systems                         Wangrow System One                                         Zimo

DCC Dealers                   Lin’s Junction – Lansdale, PA                         Loy’s Toys                     Micro Mark                              Tony’s Train Exchange

DCC Internet Links      NMRA DCC Special Interest Group


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