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The first thing the controller must do is reduce the ‘mains’ voltage (usually 240 volts AC) to a  safe and usable lower voltage of 16 volts AC.
This lower voltage is now adjusted according to which system is being used to control the speed and direction of the locomotive.
Before we start with track wiring lets first refer to the locomotives and what supply they need to run. This section will refer to RTR (ready to run) locomotives manufactured by the major manufactured like 
 
      • Bachmann
      • Graham Farish
      • Dapol
      • Hornby
Locomotives are powered by electric motors  which require a DC (direct current) supply to the wheels via the track. The speed is controlled by an increase or decrease of voltage to the track and the direction is controlled by the polarity of the DC supply; for example the ‘+’ and ‘-‘ connections to the track are reversed. The voltage and direction of travel are controlled by controllers available from many manufacture.
Now we must move on to the wiring difference between DC and DCC controls because this will effect how you wire the layout. Although both systems have locomotives with DC motors the main difference is as below.
 
 

Analogue control  (DC)

The 16 volts supply is changed from AC to DC within the controller  together with the speed. Using the system you are controlling the track so all locomotives on the track connected to the controller will move in both speed and direction the same. (see right)
So, if you want to be able to pick and control a single locomotive you will need to devide the track into sections so sections can be switched on and off as you select the locomotive to be controlled.
See below for an example of wiring for DC control with section switches.
 

Digital Controls

This system known as DCC (Digital Command Control) has been available for several years now and is proving popular but again does not suit everybody who has already got a model railway.  The track has a 16 volt AC supply all the time and the control system address each loco via a chip fitted in each loco. It is the chip in each loco which feeds any voltage from 0 to 12 volts direct to the motor in the loco.
One major difference between analogue and digital is because DCC loco run on a constant 16volts.
THE AC IS FEED FROM THE DCC CONTROLLER
All running tracks must be wired together to ensure where ever the loco is on the layout when the DCC controller tries to ‘address’ the chips in all loco’s only the one with the number you ‘addressed’ will move. All the others will NOT move because they have a different code number. 
To enable locomotives (anything that has a motor inside) to run on a DCC layout it must have a DCC ‘chip’ fitted inside.  If it is a new or fairly new it may have a socket where the plug can plug in. If old it will have to have one ‘wired in’ as shown on the video on the left.
We wish to acknowledge and thank ‘Barnabas Junction’ for this great video. 
Above is a very good video on how to fit a DCC chip with sound (known as TTS) to a Lima class 20.
The cable harness refered to in above video is Part Code 860003 available mail order from LAISDCC and known as “8 pin NEM 652 socket harness
Once again we must  acknowledge and thank ‘Barnabas Junction’ for the very detailed instruction.
Above is a very good video on how to fit a DCC chip with sound (known as TTS) to a Lima class 20.
The cable harness refered to above video is Part Code 860003 available mail order from LAISDCC and known as “8 pin NEM 652 socket harness
Once again we must  acknowledge and thank ‘Barnabas Junction’ for the very detailed instruction.
If you plan to have a return loop where the train runs round a loop and returns back down the same track (see image on right) and you are using DCC controls this is a very useful piece of equipment made by Hornby part code R8238 seen on left.
Other makes are available so ask in your local model shop or go ‘on-line’

GENERAL  WIRING

Wiring your layout is in-fact fairly basic with most of it being low voltage and current so safe to work with.  You will find that much of the work is repedative, for example, wiring from point switches to point motors is the same circuit over and over again. When wiring point motors we recommend a larger size of wire because of  the current required to operate a point motor coil and reduce volts drop; resulting in poor operation.
 As most of the wiring is under the baseboard it is advisable to wear protective glasses when working under the board because of off cuts of wire and even solder could fall on you causing a great danger to your eyes and face. A good light source is also a good help.
The unit on the right is very usefull to confirm the supply is reaching the track.
When the supply is DC the light changes colour from red to green as the supply is reversed.
When the supply is DCC the remains the same colour but does in both cases confirm the supply is reaching the track.
If you are installing or have installed a control desk and want indicators for showing the position of point blades have a look at the diagram on the right.
A Peco twin microswitch assembly has been added to the point motor which does two things.
  1. Switches the feed to the live frog switch
  2. Changes the indicators onthe control panel showing the direction of travel on the point.
When wiring switches to control points a useful tip is if you have a crossover between your ‘up’ and ‘down’ line or a passing bay where you want two points to operate together then wire so that both point motors are feed from the same switch.
To explain on this idea you could let one switch set a route feeding several points. This would involve making a custom made switching matrix to suit the layout design as show below. This will ensure there are no cross connections between point motors leaving all point motors isolated from each other.
The one shown below I made for a specific station and yard having passing bays and siding. It ha snow been working fine since 2012 with no problems. All you need is a strip circuit board, a row of connectors and a lot of diodes components. 
When it come to the overall wiring you will need the items below
wire in different sizes & colours
Ribbon cable
Misc. tools
Meter
Connector blocks
Soldering Iron & solder
You will also need a circuit diagram of the layout and how all the points, track feed, street and building lights, etc., will be connected.

WIRING  WHEN  USING  DCC  CONTROLS

All running tracks must be wired in parallel so no matter where the locomotives are on the layout to ensure the supply reaches everywhere including sidings. It is important to ensure the wires do not get crossed over.
Another great bonus is because you are controlling the loco and not the track the only wiring is to wire all tracks back to the controller so any locomotive on any part of the track can be controlled by one controller. 
If you are using Hornby setrack the quick and easy way is to fit the Hornby ‘link clips’ part code R8232 to all the points. Each pack contains 20 clips and you require two for each point.

WIRING  WHEN  USING  DC  CONTROLS

All running tracks need to be supplied with DC supply so the track unlike DCC track has to be able to be isolated from each other therefore if you have two running tracks one will be controlled by one controller and the other running track supplied by another controller.
When sidings are connected to the mainline the point will connect the siding rails to the main line and a locomotive can now run down the sideing.
If any section need to be isolated , for example storing a locomotive at the end of a siding the end section needs an isolated fish plate fitted to one rail instead of a metal one and a switch connected between the end isolated rail and the main line rail so when a locomotive is run to the end of the siding that end section can be switched off. Now another locomotive can run into that siding.

SHOULD  I  USE  INSULATED FROG  OR  LIVE  FROG  POINTS ?

We are sure that the video above will help many of our fellow modellers you see the difference between electro and Insul frog points  and we would like to acknowledge and thank ‘Moorway South Model Railway’ for this great video..
When buying a point although you have to choose whether you want a left or right hand as well as long or short you have to choose between insulated or live frog.  This depends on the style of your layout, the locos that run on it, and how old they old. The older ones have less pick-ups than the newer one today. 
The slower the trains run the more it is to supply constant electricity feed to the wheels.
Metal Fishplate
Plastic Fishplate

LIVE  FROG  WIRING

The point straight from the box may need some modification depending on if you are switching the frog with a change-over switch is to be fitted to the point motor (with is recommended)
From the above diagram cut the two links and wire switch as shown.

INSULATED  FROG  WIRING

When using ‘insulated frog’ (known as insufrog) all fishplates should be metal and tracks become live when a point is switched to that track.  What ever is on that track will all proform the same.

TIPS AND IDEAS

In both DC and DCC it is of the most importance is supply to the track. The main problems could be loose fish plates and voltage drop.
This can be corrected by feeding the track in several places using thick cable. Also link lengths of track by having wire links under or along side the track before adding ballast and scenery..