DC to DCC

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Converting a DC model railway layout to DCC operation.

This guide is aimed at DCC beginners and will show you how to convert a DC layout to Digital Command Control. We will explain some simple principles of DC and DCC to make some of the DCC concepts easier to understand.

DCC primer: DCC is a digital signal that is added to the track power. A cab that controls a locomotive sends out a signal to that locomotive giving it instructions on what to do. The locomotive takes the instructions, decodes them thanks to a decoder that is mounted aboard and carries out the instructions. The track voltage remains constant. If the drive wants the loco to speed up, he turns the know on the cab, thus sending the signal to the loco under control by the cab. The loco receives the signal and acts upon it, speeding up. This signal is sent only to the one locomotive and other locomotives on the layout will carry on doing what they have been previously assigned to do.

Using DC on your layout

On a conventional DC model railway layout we have a mains powered transformer that feeds a variable DC voltage to the track. We are basically controlling the voltage supplied to the track and thereby controlling the voltage supplied to the locomotive. For this example there is one DC cab controller, one simple track and one simple connection to the track. With this arrangement, you can control one locomotive on your track. To switch the direction of the locomotive, the polarity of the DC current is switched on the cab.

Simple DC operation of a layout


Moving on, the desire to allow more locomotives onto the layout and also to have extra cab controllers required a little modification to the simple layout. To allow multiple cab control, you have to break the layout into sections known as blocks. These blocks are isolated from each other and are controlled by either cab depending on the setting of a double-throw switch connected to one power feeder for each block. With the set-up below, we can now have two locos running on the layout, each controlled by it's own cab. Cab A can manage say Block 1 and Block 2, and Cab B can manage Block 3 and Block 4. Blocks are set with double-throw switches T1 to T4. There is a danger that a loco may be driven out of the blocks under control of Cab A into the blocks of Cab B. This could be further complicated if the polarity is switched on the Cab B blocks.

Block control of a DC layout

Passing control of a locomotive from Cab A to Cab B involves bringing the loco to a halt in a block controlled by Cab A (say Block 2), then switching control of the block to Cab B, then powering up the loco from Cab B on the rest of the layout under the control of Cab B.


Using DCC on your layout

Adding a DCC controller is remarkably similar to using a DC controller. It simply connects to the track with it's two power feeds. This will work fine on a small layout or test track with no block control.

Simple 2-wire cab control


When converting the layout above to DCC, we can make use of the blocks that were previously created so leave the isolating rail joiners in place. More about that later.

You need to ensure that all track is connected to the DCC signal BUS. The DCC signal BUS is a pair of low resistance copper wires that carry the DCC signal around the layout maintaining a good quality signal to all parts of the layout. Usually feeder wires are connected from the DCC BUS to the track every 3 to 4 feet (90cm to 120cm). To make this practical if using flexi-track, connect every section of flexi-track to it's own set of feeders and to the DCC BUS.

DCC operation of a layout

A DCC controller is made up of a DCC power station and a Cab control unit. Depending on the brand of equipment that you have, these could be separate units or they could be combined into one device. A DCC command station supplies a fixed amount of current to the track. This amount of current depends again on the brand of equipment that you have. For a comparative look at some DCC manufacturers, look at the DCC Systems Chart on Model Rail Forum.

The DCC Cab A above now has full control of the layout and can control one or more locomotives. The locos can travel in any direction and at any speed. Yes, you can have head-on collisions! The number of locos that the DCC cab can control depends on it's output power.


DCC network BUS

Usually, to control more than one locomotive at a time, you have to cycle through locomotive numbers on the cab controller, this can be easy or tricky depending on what brand you have. To make it even easier, you can ad an extra cab controller to the layout. With this setup, your brain can relax a little and you can drive a locomotive with it's own controller. Also, you can now share the fun with someone else and let them use the extra controller.

DCC operation of a layout with 2 cab controllers

Extra cab controllers in DCC connect to the main Command Station using another cable called a DCC network BUS. There are three main DCC network BUS standards: XpressNEt, ECoSlink and Loconet. We'll be looking at XpressNet as it is more common in Europe. The cabs are plugged into the XpressNet BUS and they can even be unplugged and plugged in elsewhere if you would like to move around the layout. The loco will continue doing what you told it to before you unplugged. This is useful if you have a walk-around layout or a long exhibition layout. You can set the locomotive off on one side, unplug, go over to the other end, plug in and resume control of the loco. A wireless system would make things more easy, more about wireless later.

With XpressNet, for example, you can connect up to 32 devices to the XpressNet BUS.


Boosters

What if you need more power? You would like to run more locomotives and you find that you are stretched for power with your command station. The blocks are useful here.

DCC operation of a layout divided into 2 power districts with a booster

A DCC device has a limited power output to the track, this power is used by the locomotives and accessories connected to the track and DCC BUS. The DCC signal is combined with the constant track power. This is called the DCC signal BUS. An average modern locomotive will draw about 200mA to 300mA. This means that for a DCC device the has a 3 amp output, it can control 10 locomotives at once. 10 locomotives all running at once is quite a lot and you would have to be very quick to manage them indeed! But we are talking hypothetically here. Some large layouts and club layouts will have 10 or more locos running at once and they will need to increase the power to the layout if more locomotives are required to run. To do this we use a DCC power Booster. The DCC booster adds extra power to the layout and thus increasing the capacity of the layout. Some systems allow the booster to piggy-back the existing command station thus increasing the available power foe use with power hungry locomotives. Most boosters however are connected to their own Power District. Power districts become transparent on the layout and you can drive the locomotives from one to another without stopping. Each power district however is limited in power output to the specified rating. In the example above, if the command station produces 3 amps and the booster produces 3 amps, each of the power districts could hold a maximum of 10 locomotives.

In implementing this concept of power districts, you would normally assign a region of the layout to a power district - say a station of a goods yard and on the other end of the layout, you could assign another station or terminus to another power district.

The Booster BUS transmits the DCC signal from the command station to the booster or boosters. There is also a short circuit wire in this bus that is used will power down the whole layout if a short is detected in one power district. It would perhaps be more useful to not use this total power-down solution so as to let other operators carry on driving their locomotives whilst you solve the short circuit problem in your power district.


Point Control

DCC allows you to control points, signals and various other devices around the layout using the same cab that drives the loco. An accessory decoder is a stationary decoder that can control 4 to 6 accessories depending on the brand. It connects to the DCC signal BUS to receive the commands and also usually to another power source so as not to overload the command station. The accessory decoder is mounted under the layout at a suitable position and is connected to 4 points in the vicinity.

DCC point control using Point / Accessory Decoder

You will notice in the diagram above that the track power for the point itself is fed from the toe of the point and that the exit tracks are isolated. The tracks adjacent to the frog of the point need to be isolated to prevent short circuits as the locomotive passes through the points, but you may isolate both rails to make block occupancy detection and feedback easier. More on that later.

The power to the frog itself is from one or the other toe rails depending on which way the points are set. If the point blades get dirty, this could cause a problem, so often the frog is powered via a separate switch that is linked to the point motor and fed from the DCC signal BUS.

See "Points" section under Laying Track for more information.

Doug 09:20, 2 May 2012 (BST)

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