DCC in 7mm NG

Digital Command Control (DCC)

in 7mm Narrow Gauge

We are indebted to Don Mason for this article, which is referred to in the new all-colour "Getting Started" Edition 4.

Wiring DCC Decoders in 7mm Narrow Gauge Locomotives

Wiring DCC chips into your favourite locomotive is essentially extremely simple. If you can wire a conventionally controlled locomotive chassis, then you can install a DCC chip. One good thing about DCC is that all of the various decoder chip manufacturers have standardised on a wiring scheme, which makes installation relatively foolproof. For example a red wire on one decoder manufacturer’s chip will have exactly the same function as the red wire on any other decoder manufacturer’s chip….and so on. So it is difficult to get it wrong. However, please be aware that some manufacturers of ready-to-run locomotives (such as Bachmann for example) do NOT follow the standard wiring scheme but use their own. However if you buy a stand alone chip to install yourself, then all will follow the same colour scheme.

The sketch below shows a representation of a Soundtraxx™ ‘Tsunami’ decoder (not to scale), a type of decoder which also offers on-board sound; hence the additional connection for a suitable speaker. Not all decoders include the sound facility, and these will not of course have the wires for the speaker. Decoders from different manufacturers may look very different, but all are wired exactly the same. Not shown is the capacitor, which is again found on decoders offering sound, and is usually attached via 2 more wires at the rear of the decoder. You need not worry about this as nothing has to be done with it, where present, it is simply a part of the decoder chip.

Installation is a simple matter of connecting (i.e. usually soldering) the appropriate wires to the appropriate places on your locomotive, as per the following sketch:

Note that the brown wire is not always needed, unless you have extra sound or other functions on your loco which you wish to control.

There are a few rules to remember when wiring DCC decoder chips:

The first and most important is the proper isolation of BOTH motor brushes from the track so that they are driven ONLY through the decoder. In other words, the pick-ups must NOT under any circumstances be directly connected to the motor (as in conventional ‘DC’ systems). If your locomotive is already wired for ‘DC’ you MUST first completely remove the existing wiring from the pick-ups to the motor.

FAILURE TO ISOLATE WILL PERMANENTLY DAMAGE THE DECODER !!

Follow the diagram above. Grey and Orange wires go to the motor brushes. Red and Black wires go to the wheel pick-ups. If the locomotive runs the wrong way, reverse the grey and orange wires (with many DCC systems it is possible to reverse the polarity via the control handset, but for simplicity and permanence, reverse the grey and orange wires at the wiring stage). So as you will see, the power goes from the rails to the wheels/pick-ups and thence through the red and black wires to the decoder. Only then does it return via the grey and orange wires to the motor. You should have further instructions in your DCC system handbook, to cover certain specialist installations but essentially, that’s all there is to it.

A few precautionary notes

1. Decoder chips are sensitive beasts, particularly to static electricity. So it is important when handling decoders to either wear an anti-static wristband or similar, or to ‘ground’ yourself before touching a decoder (and at regular intervals during installation) by grasping a metal radiator, water pipe, or other similar ground conductor. This is particularly important if you are one of those people apt to generate static, as many are. But all should follow this advice anyway.

2. You must also make sure that no metal part of the locomotive touches any metal part of the decoder. Most decoders are covered with heat-shrink insulation tubing or similar – but play it safe. If you are putting the decoder into a metal locomotive, then cover the interior of the loco at any questionable points with insulating tape.

3. If you use metal couplers with a direct ‘metal-to-metal’ connection with the loco, then again, isolate the couplers from the loco body with tape. Failure to do so can lead to some mysterious problems when running two locomotives coupled together. Plastic coupler mounts are fine.

4. You will sometimes purchase a decoder chip which comes pre wired to a miniature 8-pin plug. This is useful for installing in certain proprietary or ready-to-run locomotives which often have a built-in ‘circuit-board’ with a matching 8-pin socket. Such locomotives are often marketed as ‘DCC-Ready’. With these, you simply plug the decoder into the socket on the loco, and "Bob’s your Uncle". But make sure you get it the right way round. There will usually be some kind of indicator on the plug and on the socket. If in doubt, seek advice. Plugging in the wrong way round could permanently damage your decoder. If you acquire a decoder with such a plug, but have no circuit board or socket on your loco, just cut the plug off, leaving the wires as long as possible (cut one wire at a time just to be sure) and then proceed as described above and following the diagram.

It really is that simple.

In reality, the main problem with wiring DCC chips is finding the necessary space inside the loco body to accommodate them (a tender is often the most logical place, if there is one). So choose which DCC chip you want to use by reference to the space available and the size of your proposed decoder chip. Most chip manufacturers’ web sites will provide this information. Note that decoders with sound will almost always be larger than those which control motion and direction only.

Finally, don’t forget: Do also thoroughly read the handbook of your DCC system and any instructions which come with the decoder, in conjunction with the above.

Don Mason.