DCC explained

I was brought up in the era of electro-mechanics. You could actually see what was happening in those days and it made sense. Electronics are wonderful but my general understanding is about that of ancient tribes observing an eclipse. Circuit boards look to me like a mad mixture of tiny Liquorice Allsorts and Dolly Mixtures and the sight of folks making up boards to their own design fills me with admiration.

Conventional model railway controls are based on the well-established principle of varying the voltage to the track and thus to the motor of the locomotive. This is usually achieved by a turning a ‘knob’, which actually controls a potentiometer (some systems use a slider but the principle is the same). This varies the output voltage to the track and thus to the motor. To run fast the voltage is increased, to run slowly the voltage is decreased, to stop the voltage is reduced to zero (or near zero). The voltage range for OO/HO is usually 0-12 volts DC (direct current). This is a direct descendant from the battery powered train sets of the 1950s (and earlier).

This system is sometimes known as analogue control.

To control a single train or loco is simple, connect up and turn the knob. Running more than one loco on the track however is more difficult but can, of course, be done by introducing track sections and switches so that each loco can be switched (isolated) when it is not to move.

Digital control has been around in one form or another for about 30 years. Older UK modellers will recall the ‘Hornby Zero One’ system from the late 1970s. Modern systems are much more flexible and adaptable but use a similar principle.

A constant voltage is applied to the track irrespective of the number of locomotives on the track or where they are. Each locomotive or power unit is fitted with a ‘decoder’ which acts as the onboard ‘section switch’ or isolator. The locomotive only moves when its onboard decoder receives the signal from the controller instructing it to switch on. The decoder also ‘decodes’ the signals from the controller instructing it as to which direction it is to move in and at what speed. Most decoders also have the facility to switch on/off loco lights, this can be done whether the loco is stopped or moving and because the voltage is constant the lights are at constant brightness once selected.

How does it control the speed when we have constant voltage? The controller sends instructions to the selected decoder to allow pulses of the full voltage to pass to the motor. The longer the length of the pulse (known as its pulse width) and the greater the frequency of the pulse results in the greater the speed of the motor. This is exactly what happens with modern diesel and electric locomotives in the prototype world. At full speed the voltage applied to the motor is in effect full voltage all of the time.

It is because each decoder only responds to its own instructions (known as its address) that enables multiple locomotives to work on the same track and even run in the opposite direction (beware of collisions!).

Decoders for use in locomotives are often described as ‘mobile’ decoders as decoders are also available for use with point motors and signals; these are known as ‘stationary’ decoders and these function slightly differently in that when selected by the controller they switch the full voltage to the accessory because speed control is not a requirement in these cases.

Advantages of DCC


Smoother slow running

A badly running locomotive will still be a badly running locomotive, miracles do not happen by using DCC however improvements in slow running are definitely achieved by using DCC. This is because of the difference in the operating principles explained above. Conventional DC control permits a low voltage at a small current to pass to the motor when we want to run slowly. In the event there is any dirt or the motor is slightly out of balance (most are not balanced) and the voltage applied is inadequate resulting in the loco stalling.

A DCC equipped loco has the full voltage applied at all times pulsed to the motor, this results in smoother slow running. In my experience this is in itself the biggest single advantage of DCC as even flywheel equipped drives will run even slower under DCC than is possible with conventional DC control resulting in much more realistic shunting moves.

Clean track and clean loco wheels are essential whatever system is used.

Simpler Layout wiring

It is claimed that with a DCC system you only need two wires and in principle this is true, just as with the simplest DC system you only need two wires. Where DCC results in real savings in the complexity of the layout wiring is where track layouts are at their most complicated AND where you want to run more than one loco at a time.

Take for example a passenger terminal with adjacent goods yard accessed from the same main line track with multiple point work, crossovers etc. In a conventional DC wired layout you would need to install ‘cab control’, switches, and isolated sections in order to work a passenger train and shunt at the same time. With a DCC wired layout there are two wires taken to each feed position, no switches, cab control or isolated sections. Any loco can run at any time to any point on the layout.

Easier Multiple Controller Control

With Conventional DC control you can wire in two controllers using cab control and section switches but it gets very complicated when three or more controllers are required.

DCC controllers allow you to just plug in another handset, which has all of the facilities of the original. It may be necessary, with some systems, to purchase ‘extension plates’ but all but the basic units will allow at least two control handsets to be plugged in. Using ‘extension plates’ does however have the advantage of enabling the additional handsets to be mounted away from the base control unit but retaining all control features, an especial advantage on larger layouts or where there is a shunting area remotely located.

Walk around control

Except for the very basic DCC control systems all will allow the operator to unplug the handset and move to another plug in point and at the same time the locos are still running unaffected. This can be a very useful feature on larger layouts and even on smaller ones when two operators have managed to get their cables twisted!  This is a feature our American cousins use extensively on their (usually) large (by UK terms huge) layouts allowing train operators to actually follow their train all around the layout. There are wireless DCC controllers now readily available.

Correct operation of lights

DCC decoders are quoted as being one function, two function or more. This can be confusing as what constitutes a function? In the case of a locomotive that has running lights fitted that change when direction is changed a two function decoder is required to make the lights work properly. This is the one instance where conventional DC beats DCC!

In a DC loco there is a rectifier (diode) circuit that prevents the wrong lights coming on in the wrong direction of running, in DCC this needs a signal for forward and one for reverse. However, decoders can be programmed not only to switch the correct lights for each direction but can also dim the lights (even flash) where appropriate – try that with DC control! A one-function decoder can only switch on/off the lights irrespective of direction. A loco parked in the platform waiting to depart can have the correct headlights at the appropriate brightness even when stationary. This can be done with DC but with additional complications, it is standard with DCC.


Not everyone’s cup of tea but excites some folks. Personally sound is growing on me provided that it is not turned full on. Sound has a place. This results in the small speakers not being ‘over amplified’ resulting in distortion. Factory fitted sound systems are despatched set at full volume (that’s how they are tested) and most purchasers do not realise that they can adjust them – read the manual! It is easier to adjust sound, use it and enjoy it with DCC because the sound system can be adjusted simply by altering a CV (it's easier to do than explain how to do it ! Just follow the manual.) DC versions are now coming onto the market but these suffer because they are not easily adjusted (if at all).

Sound in the model railway world is an interesting feature but we must recognise that the laws of physics are still valid. Sound quality from a speaker depends on the size of the speaker and the quality of the signal. In N gauge and OO/HO we have to accept that speakers have to be tiny and therefore cannot have the Bass – Treble qualities of our domestic sound systems; accept that and you will not be disappointed BUT hear it before you buy it whenever you can.

chipping the locos

Locomotives running on DC are about as clever as a bag of hammers. Chipped, locos are more intelligent than George Bush and enormously more so than Sarah Palin!

The new chips are really very small and we were advised to choose the latest Digitrax offering. The new Digitrax DZ 125 chip is similar in size to the Lenz silver mini. This chip, despite the instructions that come with it, is enabled for back EMF (BEMF) (called 'scalable speed stabilisation' by the Yanks).

DZ 125

This offers better slow speed running and also compensates for grades (a bit like cruise control).

At long last, the chips arrived and the exacting job of fitting them to our locomotives began. It was necessary to virtually rebuild some of the older engines, and installation into the L&B units was far from easy. Great care is needed to ensure that there are no shorts in the system.

Chips can actually control other functions too such as directional lighting. We have installed this on a few units and it works very well indeed. The large amount of wiring needed to do this raises challenges when working with such tiny rolling stock. I do intend to fit sound to one unit, just for the hell of it!

The generic manual for these Digitrax chips can be found here and the specific manual for the DZ125, here

chip fitted to the chassis of 'Taw'  - click on image to enlarge

 The motor unit of the railcar converted. The part of the chassis which contacted the brushes had to be milled away and the wires then attached to the brushes - click on image to enlarge

Once the Digitrax Zephyr outfit was delivered, it was installed into the viaduct section of the model. The main control box was fitted into a recess cut into the front of the baseboard. This box is held in place with Velcro. In order to programme locos, it is necessary to have a programming track...two leads come from the control box to do this. We felt that this was another complication, so the harbour branch of the viaduct section is fed through a double pole, double throw switch to provide either track power or programming.

The leads to the UP5 remote sockets daisy chain from the main controller. It is transferred from one panel to the next using 8 pin DIN plugs/sockets. These also carry the track power.

the main control box fitted. Below is the switch to track for programming or power. The DIN socket will feed to the harbour section

The box was 'lit up' and I have to say we quite quickly got the hang of it. All the locos now had their numbers and can be programmed to perform as required up to a point.

the downside

DCC promises to offer all the characteristics that I required and there is no doubt you can programme each engine to do most things.

The programming of a loco depends on altering a vast number of parameters. These are called CVs and there are well over one hundred of them. These all have to fiddled with on a programming track while your supper goes cold. In my opinion, a lot more has to be devised to make the system user friendly.

I can honestly say that I have not managed to get back EMF to work and I wonder if I shall ever be able to properly programme these little engines.

DCC manuals seem to have been written by electronic nerds for electronic nerds (or USA speak). I have to say that Lenz scores slightly better in this respect, but Digitrax and NCE just make my eyes roll. Some of the information is actually wrong and there is no logic (understandable to British and French) to the way the manual is compiled.

There are a number of forums available where help is at hand and this does ease the situation.

Thanks to some good friends who are 'in to all of this' we hope to be able to publish here the generalised programming necessary to get narrow gauge locos to run as they should.

Bitter experience has shown us that it is a very bad idea to use Peco point motors or others requiring a CDU. In 20/20 hindsight, I would have fitted Tortoise throughout.

Example:       Circuit symbol:   


LEDs emit light when an electric current passes through them.

Connecting and soldering

LEDs must be connected the correct way round, the diagram may be labelled a or + for anode and k or - for cathode (yes, it really is k, not c, for cathode!). The cathode is the short lead and there may be a slight flat on the body of round LEDs. If you can see inside the LED the cathode is the larger electrode (but this is not an official identification method).

LEDs can be damaged by heat when soldering, but the risk is small unless you are very slow. No special precautions are needed for soldering most LEDs.

Testing an LED

Never connect an LED directly to a battery or power supply!
It will be destroyed almost instantly because too much current will pass through and burn it out.

LEDs must have a resistor in series to limit the current to a safe value, for quick testing purposes a 1k resistor is suitable for most LEDs if your supply voltage is 12V or less. Remember to connect the LED the correct way round!

connecting to the chip

As a general rule, the anode, (the longer prong) is commoned and connected to the blue wire of the chip and the white and yellow wires go to the cathode.