Digital Command Control

Operations with Digital Command Control
on
NTRAK Layouts

Judging from the DCC Internet Mailing Lists, Club Newsletters and the model railroad press, most model railroad clubs have adopted Digital Command Control (DCC) or are seriously evaluating doing so. Two events more than 25 years ago in 1998 appear to give the boost DCC needed. The first was the introduction of easy-to-install decoders (Plug 'n Play decoders) and the availability of some locomotives with decoders already installed. The second event was the availability of radio-controlled throttles. With an RC DCC throttle you really do control your train, and not the track!

The North Raleigh Model Railroad Club has been successfully using DCC on our layouts at Train Shows for over 25 years. Our layouts are designed with regard to placement of passing sidings so that trains could be run in opposite directions using the siding for meets. With the radio-controlled throttles it is easy to follow the trains and make smooth meets and passes.

The prime track requirement for successful DCC operation is that it not be possible for the wheels of locomotives and lighted rolling stock to bridge (or short circuit) across two rails where the rails are of opposite polarity, even when the short circuit is of very short duration. The DCC Power Booster shuts down virtually instantaneously on detection of a short, then restarts and applies power again to the track. The effect is all locomotives powered by that Booster would jerk, at best, or stop, at worst, during the period when the wheels bridge the adjacent tracks. This process will continue for each axle that bridges the rails, and could include an entire train if the cars are lighted passenger cars.

The most likely track configurations where such bridging may occur are frogs in turnouts and at crossings, both frogs and where the outer rails meet. The gap between the adjoining rails is not as wide as the wheel tread, thus allowing the short circuit as the wheel passes over.

Such short circuits between rails powered by DCC Power Boosters affect train operation, but if the short circuit is between a DCC-powered track and a DC-powered track, serious damage can be done to the DC power pack if it is not protected by a digital circuit breaker or other current limiting device, and to the decoder in the locomotive bridging the track sections. See below for more information.

DCC Friendly Crossings
The problem with N scale crossings, especially the Peco Long (8°) crossing, is that the wheelset treads on locomotives and lighted cars bridge the two rails where they join at frogs and at the point where the outside rails meet. One solution is to widen the insulating gap between the rails at these points.

The required modification is to cover the frog or point where the rails meet with a very thin layer of non-conducting material (such as paper glued with CA) or to slightly file down the rail surface and smooth over with epoxy. A short-term solution is to coat the rail with a thin layer of clear nail polish.

A better solution is to feed the section where the short circuit could occur through a DCC circuit breaker or a No. 1156 automotive light, which would limit the short only to the specific section. The use of an automotive light may not be practical on an NTRAK module which is subject to rough handling during set up, take down and transport.

DCC Friendly Turnouts
In general, Atlas and Peco insulfrog turnouts do not present a problem for DCC operation (but see comments under NTRAK Module Wiring, below). Each turnout should be tested, however, to ensure this is the case. Where short circuits occur, the turnout frog should then be modified as described above under "Crossings."

Peco insulfrog turnouts have power routing on both sets of exit rails. This places the two polarities of the power supply on each side of the frog. These are made very close together so that there are no dead spots electrically. Many of the wheels on locomotives and lighted cars will short these opposite polarities together and cause a momentary pause in the train as ut criosses this short.

Peco Electrofrog turnouts should be made more DCC friendly, as follows:

• Solder jumpers from the stock rails to the points. This ensures that dirt between the stock rails and the points cannot create a poor electrical connection.
  
  This can be done easily with any turnout, either on the bench or installed on a module/layout. Use a piece of bare 22- or 24-gauge solid telephone wire. Solder first to the point rail beyond the joiner that holds the moving point rails. Route the wire parallel to the ties under the stock rail and solder to the stock rail. If the turnout has not been installed in the module/layout, this wire can be soldered to the bottom of the rails.

• If using a turnout machine with electrical contacts to throw the turnout, also cut the factory frog gap jumpers. Use the electrical contacts on the turnout machine to correctly route power from the stock rails to the frog, with no possibility of a short circuit.

As noted below, always place insulated rail joiners on all four rails at the frog end of every turnout on the module/layout. There is no surer way of ensuring that module/layout wiring will be problem-free.

The wiring specifications of the NTRAK manual are, in general, suitable for DCC as well as DC operation. The following additional requirements will apply:

• To prevent operational problems the use of insulated-frog (insulfrog) turnouts is seriously discouraged. Existing insulated frog turnouts from any manufacturer must be modified if necessary so locomotive and lighted car wheelsets cannot bridge both rails at the frog, causing a short circuit. Use the methods described above in the section "DCC Friendly Crossings."

• Insulated rail joiners or electrical gaps are mandatory in all four (4) rails at the frog end of all turnouts. This helps ensure the electrical integrity of the module, and significantly helps in case of electrical problems. Powering of track beyond the turnout gaps can be via the contacts of a dual-coil turnout machine or by a toggle switch, but must not be by the points themselves. Terminal blocks in the wiring, as opposed to soldered leads, are mandatory for ease of troubleshooting.

• Common rail type track wiring must not be used. There must not be any electrical connection between any of the rails of the three/four tracks.

• Powering of private tracks on a module can be connected via a toggle switch to the DCC power, providing all the other requirements above are met, and these tracks do not constitute a reversing loop or wye.

Note that all these requirements, except the last, are already part of the NRMRC Club Standards and Recommended Practices.

Track Power Wiring
The NTRAK layout should be physically designed and configured according to whatever guidelines the Show Superintendent chooses to use in terms of size, shape and positioning of modules. Once this is complete then DCC electrical design for the layout can proceed. Note that this design is almost independent of the DC wiring configuration of the layout, except that one DCC power feed to the layout will probably be at the point of the main DC power feed.

The location of the DC slowing blocks needs to be known to ensure that the slowing block controls can be bypassed for the track(s) using DCC control. The location of any reversing loops or wyes should be determined so they can be included in the DCC electrical design. (The wye on the yard T module and the reversing loop in the yard are examples of two separate reversing loops.)

The DCC electrical design proceeds as follows:

• Determine the number of Power Boosters that will be available for the layout, along with the current output of each in Amperes.

• Determine the current requirements in Amperes for each track that will be operated under DCC. This would include an estimate of the current requirements of locomotives that will be running simultaneously, along with the current requirements of any lighted passenger cars.

• Assign Power Boosters to the tracks based on the total current requirement of each track.

• Determine the location of required insulated rail joiners in each track according to the individual boosters selected. Consider the following examples:

• Current requirement of 6A.
  Use 1 DB200 8A Power Booster with no insulating joiners required.
  
  Use 2 DB100 5A Power Boosters with insulating joiners at the main DCC feed point and halfway around the layout. Each Booster feeds one leg.

• Current requirement of 10A.
  Use 1 DB200 8A Power Booster and 1 DB100 5A Power Booster with insulated joiners at the main DCC feed point and 1/3 of the way around the layout, with the DB100 feeding the 1/3 leg.

Except for very large layouts, there should rarely be need for more than 8A per track. Thus a single DB200 Power Booster should be sufficient for any of the NTRAK main lines.

These rules have been revised and expanded based on experience at a number of train Shows, and moved to their own web page. Click here to go to the Rules page.

"Wiring for DCC" by Alan Gartner. A very comprehensive web-site that should answer virtually any question about wiring a layout for DCC. Also contains many valuable hints and tips. http://www.WiringForDCC.com

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Last updated: Thursday, February 12, 2026

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