A decoder that's typically installed in locomotives, but can also be installed into cars to control lights, animation, sound or even uncoupling devices.
Mobile decoders are typically installed in locomotives, but can also be installed into freight cars. Mobile decoders receive instructions and in turn control movement, speed, direction, lights, animation, sound or even uncoupling devices.
How They Work
A mobile decoder receives power and control information via the rails and converts this into motor power, motor control and function(s) control. An explanation of how a decoder converts DCC power to DC power, and delivers it to the model's motor is discussed in the DCC Power article.
A decoder is actually a small computer system. It takes power from the track, in the form of a digital packet. This provides power to operate the decoder, as well as data. When a digital packet addressed to it is received it processes it, then acts on the instructions. Speed and direction information are applied to the motor controller, which supplies voltage to the motor, causing the locomotive to move in the direction desired. The microprocessor can also turn on lights, activate them based on direction, as well as allow operation as part of a consist. More advanced decoders have more features, including sound.
Selecting a Decoder
Please see the Mobile Decoders List for a comparison of mobile decoders.
The decoder size will be dictated by the amount of available room inside the engine or tender body as well as the amount of current that is needed by the motor. There should be enough room for cooling and wiring as well. Before selecting, it's best to determine the 'stall current' of your particular locomotive. One decoder can drive multiple motors, provided the sum of the stall currents doesn't exceed the decoder's current rating.
We have also started a mobile decoder comparison guide to get you started on selecting a mobile decoder.
Mobile decoders have a number of possible features and functions that vary with the manufacturer, and even the specific decoder model. Don't be alarmed by the large list of features listed. Usually, the defaults that come with today's decoders work well. If the decoder you have has a feature you don't need or want, you simple ignore it and don't use it. We've broken the list down to a few basic areas:
Motor Control Features
Programming and addressing
- Addressing: Two and Four Digits - Decoder address, can be two or four digits (in Hex, not Decimal)
- CV Reset - Resets all decoder settings to factory defaults
- Decoder Programming
- Decoder Programming Lock - Helps protect against decoder settings from being accidentally changed.
- Decoder User Values - User defined space - IE: Loco Owner ID. Entering values here doesn't make the decoder operate any differently, but it does make it possible to tell two identical locomotives apart.
Lighting and effects (functions)
- Directional Lighting - also usually provides individual control of headlights.
- Function Re-mapping - changes what function button a function responds to.
- FX Lighting - Special lighting modes - requires programming CVs greater than 100. Exact effects differ from one manufacturer to another. See your specific decoder specifications.
- Master Light Switch - Turn all functions off with one operation.
- Short-Circuit Protection - Protects the decoder against incorrect wiring when first turned on
- Thermal Protection - Protects the decoder against heat damage if the decoder is overloaded or overheated
- Transponding - Only Digitrax has this feature. It is not part of the NMRA DCC Standard
- Main article: Functions
Functions are solid state control circuits on decoders that can control lights, sounds, smoke, animation, uncoupling devices, etc. Many are not limited to simple on/off functions - for example, some lighting functions are capable of imitating MARS lights, Gyralights, Rule 17 dimming, single or double flashing strobes, ditch lights, and other effects. Many decoder lighting outputs are 12 volts DC. Other decoders are specific to certain models, and still others have built-in voltage regulators or resistors for LEDs. Always check your decoder's documentation to verify output levels before you install lighting.
There are currently 29 functions, F0-F28. Most DCC systems now support 29 functions, while some may still only support 13, F0-12.
- Main article: Decoder installation
All mobile decoder installations include the following steps:
- Select a decoder that fits inside the body shell and has space for wiring
- Has the required current rating for the motor
- Isolate the motor from the track pickups
- Install the decoder along with wiring for any extra effects
- Test the completed installation.
There are decoder testers available from several companies.