Model Railways On-Line - Wiring for DCC
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Electrical Wiring for DCC
The Primary difference between DC and DCC layout Wiring
Wiring a layout for DCC introduces some different issues which previously under DC, were either not relevant or they could be 'got away with'.
The principal difference between wiring a layout for DCC and one for DC is that under DCC, much higher currents are involved because large numbers of locomotives and accessories can be concurrently active whereas in DC, only one loco is normally active per controller.
As a result of this, the cabling used on a DCC layout must reflect the higher currents involved.
Common Layout wiring methods
Different layouts may be wired in different ways, depending on their size. Some common wiring methods are:
No wiring at all
Main features: All current passes through rails and fishplates. Turnout switch blades are relied upon to liven different tracks
This type of 'wiring' is widely used on trainsets and small layouts of the 4 x 8 or 'shunting plank' variety.
Layouts which use this method normally only have a single pair of wires connected to the the track via a power connector. It is usually possible to swap a DC controller with a DCC Command Station using the same track connection without any further changes being necessary.
Main features: Typically a feeder wire on each side of a layout.
This type of wiring usually occurs when the small layout is extended and it is recognised that voltage drops occur over long distances due to resistances and continuity issues in fishplates. The wire is often minimally rated at around 1 Amp.
Layouts which use this method normally only have a single pair of wires connected to the the track. It is usually possible to swap a DC controller with a DCC Command Station using the same track connection without any further changes being necessary.
Main features: A bus wire parallels the rails and droppers are connected to it every few lengths of rail or every few metres. Intermediate rails rely on fishplates.
This type of wiring represents a conscious attempt to improve the power supply situation on a layout to ensure that voltage drops do not occur. It still relies on fishplates which are potentially vulnerable to painting and this can therefore lead to voltage drops on some track sections.
This method of wiring converts well to DCC provided the bus cable is rated at a higher amperage than the Command Station. If a DC layout has been wired using this method, the bus will often use 1 or 2 Amp cable (nothing higher is necessary), however, this will need to be uprated for DCC if more than half a dozen locos are to be active at any one time.
Main features: A bus wire parallels the rails and droppers are connected to it from every rail.
This is the ultimate approach to wiring a layout and ensures maximum long term reliability. It ensures that there are no voltage drops anywhere and that there is no reliance on fishplates. Fishplates are not vulnerable to painting.
This method of wiring converts well to DCC provided the bus cable is rated at a higher amperage than the Command Station ie Mains Cable. If a DC layout has been wired using this method, the bus will often use 1 or 2 Amp cable (nothing higher is necessary), however, this will need to be uprated for DCC if more than half a dozen locos are to be active at any one time.
DC layout wiring tends to take a 'star' topology where the control panel is at the centre and all wires radiate from it. Sometimes there may be a common wire which is really the first step towards a bus. While the star topology can be converted to DCC, all wires must be uprated, especially the common wire. It is simpler to use a bus and wire all isolating sections to it instead of running them back to the control panel.
We would suggest that the use of buses is equally applicable to DC and DCC layouts. If a layout is being built for DC with a view to conversion in the future, this is especially important.
See this article
for more information on buses.
Buses on Circular Layouts
On circular layouts, a bus should be run around and joined to form a ring. This will ensure equal loading throughout the bus. There is some divided opinion on whether the bus should be joined to create a ring, however, it should be remembered that the bus runs in parallel to the rails which often make a ring anyway. Sometimes it is cited that a ring causes problems, however, we have found that the ring is not normally the problem and that other wiring issues exist.
The advice not to wire rings is usually confused with Throttle networks which most definitely shouldn't be wired in rings.
We have mentioned the use of droppers above, but what are they ?
A dropper is a piece of wire attached to a rail which drops through a hole drilled in the baseboard surface to connect with the electrical system below.
See this article
for more information on Droppers.
Track Clips/Power Connectors
We do not recommend the use of Power Connectors as there are much better ways of
connecting wires to rails
here, however, if they are used, dismantle the connector to see if it contains a capacitor wired across its outputs: this must be removed because it will interfere with digital signals.
Short Circuit Protection
DCC systems are capable of delivering 4-5 Amps or more, depending on the system, and this is required in order to support multiple concurrent loco and accessory operation. This level of current is sufficient to cause substancial damage if the outputs of the command station are shorted together or a short occurs across the track. Typically, 4-5 amps can damage thin wire and loco pickups and can cause rail head pitting.
Because of the high current capability of DCC Command Stations, they have very high speed cut-outs installed which remove current from the track the instant a short is detected.
The side effect (although some see it as a problem!) of high speed cutouts is that if a short does occur, an entire layout can stop, although this depends on the wiring strategy used (more later).
DCC short-circuit protection is infinitely faster than conventional DC controllers and has to be in order to handle the much higher currents properly. It is therefore possible that a short which wasn't noticeable in DC causes a Command Station to cut-out.
One way to test the short circuit protection on a layout is to apply the 'coin test': lay a coin across the rails to deliberately cause a short. The Command Station should instantly cut out. If it does not, it will be because the resistance in the layout wiring is so high that even shorting the rails does not appear as a short to the Command Station. This situation arises when underated cable or wire is used. Such cable should be replaced with cable capable of handling at least 4-5 Amps.
Whenever a short occurs on a layout, 'best practice' says that it should be investigated properly: look for what is actually causing the problem, rather than the symptoms. For example, a cut-out doesn't mean that the Command Station is too sensitive. It means that there is a problem which should be fixed!
One way that many modellers use to isolate the impact of a short circuit cutout, is to implement 'Power Districts'. A Power District is a geographical area/region on a layout which is electrically isolated from the rest of a layout. It is supplied through its own feed from the Command Station where a splitter device is used to split the Command Station output into several outputs. Each output is electrically isolated and protected from each other such that a short in one doesn't bring all the others down.
Before the days of hi-tecj electrical splitters, one method of isolating power districts from each other was the 'Light Bulb Solution'. We cover the
Light Bulb Solution
here. We advocate against this solution and recommend that proper purpose-built devices as described in the article should be used instead.
Isolating Sections / Signal Overlaps
On DC layouts, isolating sections are used to switch sections of track on and off for the purposes of isolating locos from electrical control - this is the 'driving the track' concept inaction.
In DCC, every loco is individually controlled ('driving the train' concept) by sending digital messages to them individually to instruct them to perform an action. Consequently, instructing one loco to move has no effect on any others. Therefore, DCC has no requirement of isolating sections and there is no need to stop locos around strategically located isolating joints in order to keep them electrically separated from other locos.
When wiring a new layout for DCC, there is no need to wire any isolating sections: all that is required is a bus which runs around a layout with droppers connected to it.
When rewiring a DC layout for DCC, existing isolating sections can be left in place and the switches all placed 'on'. Alternatively, all isolating sections can be removed and the wires attached to the isolated track sections can simply be wired to the nearest point on the bus. Making this change can have a very significant impact on the amount of wiring on a layout because it removes all the wiring looms originating at the control panel and spidering out to all the isolating sections. On the author's layout, connecting isolating sections to the bus resulted in a reduction by 2/3 of the overall layout wiring.
On DC layouts, it is quite common to wire signal overlaps (the length of track beyond a signal up to the point of potential collision) through a switch or diode on the signal motor such that if a loco passes a signal at danger, it will stop. The diode will allow it to reverse, however, if no diode is used, then the loco will not move until the signal is pulled off. Remember that if drivers drive trains correctly, this is a feature on a layout which should never come into action!
In DCC, the same techniques can be used. Where a switch is used on a signal, a loco passing a signal at danger will stop in DCC just as it does in DC. Where a diode is used, operation changes: DCC decoders have the ability to recognise a DC power supply - which is what a DCC supply with a diode cutting out half of the DCC wave form is. Therefore, a DCC loco passing onto a section of track fed through a diode will in fact continue as though the diode was not present - the loco operates in 'DC Mode'. Decoders can be configured (CV's) to 'decellerate on DC' such that when a loco passes onto a section of track fed through a diode, the loco slows to a halt. Normal operation can be resumed by switching out the diode so that the full DCC wave form reaches the rails. By definition, a loco configured to 'decellerate on DC' cannot be run on a DC layout in DC mode.
Many DC layouts are wired with electrical blocks which map directly to signalling blocks. On real railways, block signalling is just that: signals located along a railway line, the distance between each being known as a 'block'. Rules are applied as to how a loco passes from one signalling block to the next. Block signalling is about advising the driver when to proceed. It has no involvement in the traction system of the loco he is driving (except in very modern locos).
DC layouts use block signalling not as a means of signalling per se, but as a means of controlling the track power supply ('driving the track' concept) and therefore, any loco which happens to be sitting on it. This is done so as to keep locos electrcially isolated from each other because a given controller can only control one loco at a time. It just so happens that power blocks and signalling blocks happen to allign with each other. The signalling system is used to actually affect the electrical power provided to the track.
In DCC, the methods used on real railways can be applied. The traction supply is independent of the signalling system. It is not necessary to use the signalling system as a means of controlling track power supply because locos are individually controlled and have no effect on each other when they move. DCC locomotives move based on the digital messages sent to them, not on the voltage or polarity of the track.
Banking is a method of operation used on real railways where an extra loco was used to push the back of a train up a steap gradient, thereby assisting the leading loco.
On DC operation, it is necessary to have two controllers and some strategically located isolating sections in order to perform the operation.
In DCC operation, both locos can be individually controlled with one or two Throttles. There is no requirment for any special track wiring or isolating sections. The banking loco can slow at any time and there is no need to perform this operation around strategically located isolating joints in order to maintain a loco on a particular controller as there is in DC.
Banking is a feature supported by DCC by definition of the fact the locos can be controlled individually. Shunting locos in a loco dept is a variation on the same concept.
Live Frog Wiring
Many modellers use turnouts as a means of isolating sidings on DC layouts. While this method can be used in DCC, it is generally not recommended because under DCC, all track should be powered at all times if features such as lighting and sound are to operate correctly. Similarly, it is not necessary in DCC to electrically isolate locos from each other as in DC because DCC locos are individually addressed and controlled - making one run won't make another one 'spring to life'.
Live frog turnots are probably the single most common source of electrical shorts on both DC and DCC layouts. Whereas DC controllers 'tollerate' momentary shorts, DCC Command Stations have no tollerance for them and consequently, many people experience layout shutdowns.
The fact that a DCC Command Station shuts down indicates that there is a problem which must be fixed. The problems existed in DC anyway and were probably observed as sparks or momentary jolts when a train passed over the offending area. It is not the DCC Command Station which is at fault. It is the layout wiring at fault and this must be fixed.
Please see this article on
Live Frog Wiring
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