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position double pole double throw (DPDT) switch will do the job as shown below.
Although DCC is a complex system. an in-depth understanding is not required in order to use it. An understanding of the basic concepts does help when problems arise.
An AC voltage is applied to the rails which includes a code. Each locomotive has a decoder which looks at the codes being received but only responds to those instructions (or commands) which are intended for it. It ignores messages for other locos.
The message carries information like “go forwards”, “go backwards” and how fast. The decoder converts the AC power on the rails to DC and applies it with the correct polarity and voltage to cause the motor to rotate at the speed requested. In addition there can be other instructions such as turn on a particular light or make a particular sound.
Because of all the calculations the decoder has to do whilst processing the incoming messages, it takes some power to do this. Sound adds to this power. Then there is the motor. This takes the same power as a DC operated motor. The result is that a loco working on DCC takes more power than the equivalent on DC when travelling at the same speed, and pulling the same load. A DCC loco which is not moving, has no lights or sound still takes power because it is “listening” to all instructions on the track to see if any are for it.
So what difference does this make to layout wiring?
With DCC, all track can be wired together with the only gaps at points as already described. This is fine on a small layout but if it is larger or has complex trackwork, remember that if there is a short. you will have to be able to find it. The easiest way of finding a short is to progressively isolate sections of the track until it disappears. A few gaps are thus useful even if they separate sections which are normally connected together.
With DC you need additional gaps to allow you to isolate sections where you may wish to stand locomotives.
If a loco or vehicle derails, it can short-circuit the track. With DC, this usually only affect that train. With DCC, as the same power supply is connected to all track, then it will stop all trains. For this reason it is usual to divide a DCC layout into a number of sections, separated from each other by gaps, each fed from its own booster. If a short circuit occurs, it will stop all trains fed from that booster but others on different boosters will be unaffected. Alternatively, the layout can be protected by providing sections with electronic circuit breakers. These will cut off power to a section that has a short
On a DC layout, ancillary equipment such as point motors and signals are fed by individual circuits. Using DCC technology, as well as individually operating many locomotives over the same pair of wires, it is also possible to fit point motors, signals, etc with decoders so that they too are operated over the same pair of conductors.
One feature of DCC is that not only the locos can be controlled but also other fixed layout equipment such as point motors and lineside lighting. Now if a short circuit occurs because a loco has encountered a point set the wrong way, then with this arrangement the feed to the point motor is also shorted so it can’t be operated to clear the short. The easy way round this is to have ancillary equipment fed from its own booster so that it is unaffected by shorts that occur on the track.
The NMRA has announced Layout Command Control (LCC) which is DCC for lineside equipment. To be ready for this when it becomes widely available, lineside equipment decoders should be wired on their own circuit separate from the track connections. Even if these are connected together, back at the booster, you will be LCC ready without much rewiring.
Next time, we will look at how to wire up a layout for d.c. and DCC and where gaps will be needed.
Point Motors
It is often convenient to operate points remotely rather than by operating a lever or equivalent adjacent to the point.
There are 3 types of point motor generally used on layout.
The type that has been around the longest is the two coil type.
Energising one coil causes a soft iron rod to move in one direction. Energising the other coil moves it back again. This is connected to the points to move it. Leaving the coils energised for more than a few seconds tends to generate smoke and a point motor that no longer works. Some point motors of this type incorporate switches that cut off the power once it has moved. Alternatively a control circuit such as capacitor discharge should be used to protect the point motor. The point motor may also have additional contacts that can be used to switch frogs, indicate the lie of the points, control signals, etc. The clue that this type of point motor is being used is the CLUNK sound from under the benchwork.
Another type is the Tortoise-type motor. This contains a small motor and a gear train. To move the points, the motor is energised causing it to rotate. When it reaches the stop, the current continues to flow but is sufficiently small that it does not overheat the motor so the motor remains happily stalled. To move the point back, the power is reversed and the motor drives the point back.
The third type which has only recently become popular is the servo. These motors rotate to a particular position according to the electrical signal they receive. It needs a special circuit to generate the signal that it needs to operate but these are now commercially available. (e.g. Peco Smart Switch)
Differences between DC and DCC Layouts
For a DC layout, there is generally one power supply for each operating loco. The motor in the loco is connected directly to the track through the wheels. The greater the track voltage, the faster the motor goes. If the track polarity is reversed, the motor will rotate in the opposite direction.
As a consequence of the way that DC works, there is a need to provide more gaps in the rails than are required for DCC. Each piece of track where you need to stand a loco whilst operating others needs to be separated by a gap unless it can be dealt with by turning the points against it.