This is a attempt to explain the differences in the placement of a common, how they are wired and what the effects can be. These will all be based on the general Rules and Laws of Electronics and Physics. Rules means you follow them or you will be in trouble. Laws means this is what is going to happen. This does not mean to say any one type will work or not work. It does not mean that any one type is better or worse then another type. But it will show that there are differences, and some that you should be made aware of.

RULE #1  Always follow the directions supplied by the manufacturer, and if you are not sure ask them!

LAW #1  Murphy's Law, if it can happen it will happen.

Always follow Rule #1 and remember Law #1 this is the first step to keep yourself out of trouble.

RULE #2 The choice as to what type common you want to use on your layout is your choice to make. Make sure you understand, know how to wire, and what their different effects can be.

LAW #2 Same as LAW #1

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Additional Rules and Laws.

RULE  Any time you have multiple controls, DC Cabs, Power Stations, DCC Boosters, or combinations of any of them, you need a common to connect each one together to make them all work.

LAW  No common between controls, and a loco will either stall across the gaps, or have to coast past them.
Note: The controls do not actually have to have a physical common wire between them. The pickup of a loco can actually form the required common as it crosses the gaps. But this requires all wheel pick up, if the pickups are offset and/or dirt does not allow all wheel pickup, the above LAW still applies.

RULE This common can be before or after any reverse mechanism but never both at the same time. But we can have a common return in any given control district.

LAW  If the common is before the reverse mechanism, you can not have common rail across the control districts.

RULE If the common is after a reverse mechanism, you can switch adjacent control districts in and out of a series operations.

LAW Anytime adjacent controls are in series operation, the sum of the voltage across the ends of the controls, is the sum of both controls together. Better know as voltage adding or doubling.

RULE Always check the unloaded output of a DC Cab, if pulse is available make sure to measure with it on.

LAW The unloaded output can be as high as or greater then 30-40 volts.
Note: Max safe voltage for a DCC decoder is 27 volts.

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Operation Modes

To help explain the above Rules and Laws. Here are two very common examples of both Series Operation on the left, and Parallel Operation on the right. Both using the very common 1.5 volt flashlight batteries as examples.  These two modes of operation are the building blocks that any type of multiple power sources are made from. The can also be combined in to Series/Parallel operations also. They are also the modes for any type of supply, DC or AC. Since these operation modes are the basic building blocks of power sources, they also apply to our inputs and output of our controls on the Model Railroad, any time there is more then one power source. So if you should get lost, refer back to here. One thing to note before you move on, notice that in the Series Operation on the left, where the + of one battery meets the - of the other battery. This is shown as the Red meeting the Blue. You can relate this to anytime you see a common in the following examples as Red & Blue.

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Reverse Mechanisms, what are they and how are they used.

This is how a typical reverse mechanism works. The one on the Left "N", means Normal. You can see that the Red goes out the same side as it can in, so does the Blue. The one on the Right "R", means Reverse, and you can see that the Red and the Blue are coming out the opposite way as they come in. I am sure there are a lot of you who will recognize these as DPDT wires as reversing switches. And you are correct. But I call them reversing mechanisms, because for the purpose here, they can represent the direction switch in a DC Cab, AUX contacts on a turnout, a DPDT relay, they can be all solid state, and they can also be the Auto-Reverse section of a DCC Booster, so on. So keep this in mind as we go.  The reason is I am trying to make this as universal as possible. You will see rails, controls, and wires only in Red and Blue. This way when you look at the drawings that follow, you can assign what ever you like to the colors, such as polarity, phase, etc. When we talk control, you can look at it like a normal DC dual cab, two DCC boosters, or a DC cab and a DCC booster. The controls are nothing more then a box around our little reverse mechanism, one marked A and the other marked B. The rails are not actually connected to the controls for clarity, but remember everything is color coded, and I use the normal N rail and S rail designation. Also remember the control can be what ever you want it to be, the reverse mechanism can represent what ever is proper for what you want the control to be. And yes you can mix and match control types. The choice is yours, and use any way that you feel most comfortable with. The LAWs are the same for any of them, but the RULES will be different. I do show input volt of 12 volts for a typical DC cab and several  references to voltages here and there. If you look at the controls as anything other then DC cabs, or mix and match such as DCC boosters, you should fill in the proper input voltage yourself. Such as for DCC, Nscale = 12 v, HO scale = 14 v, and Large scale = up to 22 v.

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Typical Common Rail

The proper name here is common return, but because there is no gap in the N rail we call it Common Rail. We can and usually do cut gaps in the N rails, for any time we need to isolate the block for testing or finding shorts. But because the common return is parallel with the N rail, it is still called common rail. When we come to areas that need gaps on both the S and N rails, we keep our common return, but can not have a common rail. These places are called reverse sections, Turntable, Wye, Reverse Loop. These are defined  as any place the N rail meets the S rail.

RULE Here Any time you see the common in both Red and Blue, and one S rail is Red and the other is Blue, you have series operation.

LAW Here Any time you measure the voltage across both S rails, it will be the sum of both controls.

 

This is Common Rail in it's truest form. You will notice there is no gap in the N rail, our common rail. You will notice it also only blue. This means that the same side of our controls are tied together, and as we can see from the reverse mechanism in the controls, they are the normal position. This can represent, a typical dual cab system on a DC layout, as a train moves from one cab to another. Only thing we would add here is a very simple SPDT toggle for each block to act as a cab selector switch, so we could assign either cab to any block. But left out for clarity. Control A could be a  DC cab and Control B could be a DCC booster. Or a typical DCC layout as the train runs from one booster to another. And when a set of wheel or a loco spans the gap on the S rails, the controls are put in parallel operations, this means the same voltage. This represents no problems at all for any type system. A loco will always pass over this gap without problem.
 

 

Again this is still common rail in it's truest form. And everything from above about DC dual cab still applies, but there is some difference. We want to be able to run our locos in different directions. And at this point we will usually add one more reverse mechanism called the East/West switch. This is so our forward/reverse mechanism in the control will be just that no matter which way the loco is heading.  Again left out for clarity. As you can see the reverse mechanism in control B is in the reverse position, just so we can do that. But because we did this notice we now have the Blue of cab A connected to the Red of cab B via our N rail, common rail. As you can see the N rail is now both Blue and Red, S rail from control A is Red, and S rail from control B is Blue. Because the N rail common rail is connected to both Blue from control A and Red from control B they are wired in series operation, this means, the voltages add. With a DC dual cab system, this is not cause for concern, but would be called bad operations, because we failed to properly align our next block. But needs to be lived with if we want common rail and to be able to change direction. There is a way around this, which we will get into in a little bit. And this is to use a reverse mechanism after the common, but this defeats the whole idea of common rail. Reason is we need to also gap the N rail to do it, just like we would a reverse loop.  With DCC there is no reason for this, does not need this to change directions. A auto-reversing booster should never be used here or the feature should be turned off. There is no need for it, and if it were or a booster hooked to the rails improperly, the added voltage from both S rails is not a good thing. Even though this should cause a short as soon as a set of wheels crosses the gap. But if one of the controls is a DC cab and the other a DCC booster we still have this based on the reverse mechanism of the DC cab.  Murphy's Law, states that if it can happen it more then likely will happen. Such as a derailment, or any thing that can allow one set of pickups to contact one S rail, and the other set of pickups to contact the other S rail. This is not a short, but what ever the added voltage is, this is what the decoder will see. Contact Murphy for all the possible combinations here.
 

The Reverse Section.

Remember we already talked a little about these. Well here we are, and using a typical reverse loop as the example, but this could also be a turntable, or a wye. The only thing that makes them different is the shape. What makes them the same is the N rail will meet the S rail at one end or the other.
 

 

 

The first thing you should notice, there is a added reverse mechanism here called AUX. Next notice it is after our common. And this is possible because we no longer have a common rail, both rails are gapped [isolated] and do not share a common any more. So not only is it possible, but is the proper way to do it. The AUX reverse mechanism can be the aux contacts on the turnout if this is just simple reverse section. Or it can be the AUX or RN/RS to control the direction we enter-exit the reverse section. All very typical of a dual DC cab system. Or it can be a auto-reverse module, not a auto-reverse booster. No matter which it is, you should notice our common is Blue no matter which if it's set for normal or reverse.

 

What do we have here. The same setup, but now our common has gone Blue and Red. And no matter what position the AUX is normal or reverse, the common stays Blue and Red. Just like before even with a common rail, we want to reverse our train in the loop, and used the reverse mechanism in our control. And the end result is again the same, that is the voltage is added from S rail to S rail. Because we have put our controls in series operation again. And again with DC cab control, this is not a cause for concern, but would just be called bad operations because we failed to proper align for our next block.  And even though DCC manufacturers make auto-reversing boosters just for this, if one is used here, you still have voltage doubling across the S rails. And to make matters even worse, remember the fact that a reverse sections means that the N rail meets the S rail. This means that at one end of the section, the S rails are across from each other, and at the other end they are offset from each other. Remember the last time this happened we were looking at a derailment or what ever else Murphy could come up with. Well with offset S rails, we do not even need a derailment, all we need is a loco with offset pickups to get across the gaps. This can be by design, such as our typical steam locos or by dirt. Ever have a all wheel pickup loco stall on a none powered frog or section of rail shorter then the wheel base. This is offset pickup, because dirt on the wheels, or anywhere in the wipers inside, even on the rail. Ok we can say there should be a short. Well what happens if the loco coasts across the gaps, short removed and power is back on. If the reverse mechanism is a auto-reverse booster it should take care of it, but what if there is a second short. Or what if control A is a DCC none reversing booster, and control B is a DC cab. Again contact Murphy for all the possible combinations here. Keep in mind the word IF is the basis for Murphy's Law.

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System Common or Power Supply Common
AKA  Direct Home Wiring or Two wire system

As you should have noticed from above, the only time we can stay away from voltage adding or doubling, it to place our reversing mechanism after the common. Now wouldn't it be nice if we could put all our reversing mechanisms after the common. And at the same time never have to worry about voltage adding or doubling. At the same time with this regard, never even have to talk with Murphy about it. We can and it is not hard at all, and most of the time much easier then common rail, with out all the IFs. In fact if you remember we need that AUX reverse mechanism and can not have common rail at reverse sections right. All we are going to do is wire each control district just like it is a reverse section. And remember just like any reverse section, we can have a common return with in any control district.  We also have the added advantage of being able to use a auto reversing booster any time we want or need. And yes we can use a auto reversing module if we like.

RULE here Both rails must always be gapped across control districts, just like a normal reverse section. But we can have a separate common return within any or all districts.
NOTE: If your layout is already common rail, you only need to make sure that booth rails are gapped between control districts. And two wires are connected back to the controls. Hence the name Two Wire System.

LAW here If any thing crosses either gap where the rails are of opposite polarity/phase, will result in a short. But never series operations.
NOTE: This is not only required, but actually desired. This is what allows  auto-reversing boosters to work reliable with offset pickup locos, and/or dirt, etc.

 

As all the above these controls can be a DC Cab, Booster or combinations of such. But the common must be at the input not the output like common return. Now all internal reverse mechanisms are after the common. Also it is a good idea to use ballast lamps at the DC cab output if you plan to use both DC and DCC together. This will allow the DCC signal to over ride the DC voltage. These will absorb the short as the DCC AC wave form is of opposite polarity of the DC voltage. Thus allowing a smooth transition from one to the other. As long as the common is here at the input or before any control mechanism, there is no possible voltage adding or doubling.

The common here can be at the control or system level, such as supplied by manufacturers such as Digitrax and Easy DCC.  The advantage here is the common is internal to the system, and just about Plug and Play because the manufacturer says attach common here, or it is done when the system is connected together via supplied cables.

Or you can use a common raw power source AC or DC supply feeding the controls, a single large supply. Or the common can be connected between multiple transformers observing phasing, or at the DC end of the supply observing polarity.
NOTE: For any common here, other then the system level common, follow RULE #1 and check with the manufacturer if you are not sure.

We also have one more advantage to placing the common at one of these points. We can connect the common to a earth ground, this is not only a safe common sense method, called a common ground.  This assures there will never be any voltage on the system/s  higher then that of the control voltage or power supply. And the lowest voltage will be that of the common ground 0 volts.

Note on Detectors:
Detectors designed for common return, can and do work fine with a Two Wire System. Most of these use back to back high current diodes to detect current draw. Generally the only requirement is that the detector power supply does not bridge any gaps across control districts.  Keep in mind a common return is fine within any given control district. These will not be any problem at all as long as their output controls either opto-isolators or relays.
Or you can use opto-isolation on their input , this will allow a single power supply, that can be connected to the common ground also. This is because their input is now isolated from our rails.
A better method with DCC,  is to use the power on the rails as the power for the detectors. Such as Dick Bronson's detector or the Digitrax BD1 and/or Digi-Toy's BD8 type detector. All these detectors use a very small current from the rails as a power source.
There are also detectors designed for DCC with isolation built in already. Such as the NCE BD20 current transformer type. Here a toroidal coil is used to isolate the input from the rails. And the current built in the transformer is amplified to be used for detection. You just wrap the track feeder around the toroidal core to form the transformer.

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