Decoder Installation
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Decoder Installation |
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Making Your Own DCC Components
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General ProceduresThere are a small number of important rules which must be followed when installing a decoder. The decoder manufacturer will have provided notes to this effect. However, at risk of repeating familiar ground:
Stall CurrentThe stall current is easy to determine, and is a key parameter in selecting a decoder. Essentially this defines a current value which the decoder must be able to provide safely. If the locomotive motor should become stalled, it should not cause damage to the decoder. To measure the stall current, connect a 12V supply to a test track with an ammeter in series. Hold the motor over the track so the motor runs; gradually increase pressure onto the track to "clamp" the wheels down to the track until they stop turning. The current in that condition is the stall current. Incidentally, don't take very long over this: the motor will overheat very quickly (I would suggest 5-10 seconds maximum). Siting the DecoderDecoder siting is, in my opinion, the most difficult problem to solve. The decoder needs to be located in a place meeting several criteria:
Finding room for a decoder is -unsurprisingly - easier on larger models. It was a surprise to me how little room has been available in the smaller "OO" gauge models I've encountered. I've made use of decoders aimed at the "N" gauge market to make this easier. The larger steam and diesel locomotives should present fewer problems. Ideally, the decoder should be located in the same part of the loco as the motor and pickups. It is possible, however, to locate decoders in the tender of steam locomotives if needed. I've even heard of locating a decoder in a piece of rolling stock and permanently coupling it to the loco. This would be a last resort! If a sufficiently large area cannot be located, it may well be possible to create space by removing some of the material from the body or chassis. If the installation appears marginal, try assembling the model with plasticine or blu-tack in the space being considered to find out how big it is with all the parts in place. "N" gauge models are considerably more difficult, but there are some small decoders around now. It is more likely that material will need to be removed, though. For some US "N" gauge models, replacement chassis parts are available with material milled away to make room. Selecting a DecoderThe decoder needs to be selected once the stall current is known, and once the available space in the locomotive has been determined. The first port of call has to be the manufacturers' own information: there is a lot of information about decoders, sizes etc. on web pages. See the links to manufacturers' sites for further information. Many decoders are made to go into a specific locomotive (particularly for "N" gauge). This is great - as long as you want to model a US railway. I don't know how long it will take for similar specific decoders to appear in the UK. I'm working on it though! The decoder mandatory requirements are that it must have an adequate current rating (ie can supply the stall current continuously) and that it will be small enough. There are however other considerations:
Isolate the Motor's BrushesThe golden rule when wiring a decoder is to ensure that the motor's brushes are isloated from the track & wheels. The pickups must connect to the decoder only; the motor must connect to the decoder only. This may be easy or very difficult; and you can't easily tell without taking the model apart. The comforting thought is that there is always a way! The easiest models are those where the pickups are electrically isolated from the chassis, and two wires run from them to the motor. For these models, simply connect the decoder into these wires. A common arrangement is for one of the pickups to be connected to the chassis, but for the motor to have two wires running to it: one of these will connect to a tag onto the chassis somewhere. These are again easy: simply break the wires to the motor and make sure the decoder is isolated from the chassis. The "difficult" models are those where the chassis forms part of the electrical connection from the pickup to the motor. An example is the "split frame" arrangement: in these models the chassis is formed in two halves insulated from each other. The two halves occupy opposite sides of the model and are connected directly to the pickup from that side's wheels. The connection to the motor is typically via springs touching onto the motor's terminals, or at worst the springs directly hold the brushes. The entire Graham Farish range of "N" gauge models for the UK fits into this category, as far as I'm aware. It should be noted that there is another option. ZTC sells a decoder designed for this eventuality, in which the motor cannot be isolated from a pickup. I have no experience of this unit but it may be worth enquiring about if in difficulty. Complete the InstallationThe home straight! The remaining things to consider are making sure that all connections are made correctly, and that the decoder is fully insulated from the chassis. The decoders I've encountered come encapsulated in "heat shrink" sleeving. However some extra pieces of heat shring will not come amiss (and don't forget to thread them over the wires before soldering the wires in place!). Heat shrink sleeving is available from Halfords and Tandy; it can be shrunk using the kind of hot air gun sold for stripping paint.
It is a good idea to test the installation before running it. This can be done by programming a few CVs on the programming track, which should be a current-limited environment. If that works, try running the model. |
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