Common Return Wiring
Let us begin by considering the different types of power supply that can be used to power a model railway. There are in fact three different types of supply. These are -
- Single supply.
- Split potential.
- Individual supply.
Single supply is where all of the trains operating on a model railway are powered from a single (one) transformer/rectifier. Each train on the layout operates in one or more sections which are connected to the controller which is being used to control the train. The input sides of all controllers are connected together and powered from the output of the same transformer. (See figure one). This is a relatively common way of powering a model railway. Common return wiring can NOT be used with this method of power supply.
Split Potential is an old method of powering a model railway which was used before the introduction of transistorized controllers. The purpose of split potential was to allow common return wiring to be used by using two transformer/rectifiers - one for the trains running in one direction and one for trains running in the other. The way that this worked was that the positive output from one transformer/rectifier was connected to the negative output of the other and this was then connected to one rail throughout the layout to form a common return. The other terminals on the two transformer/rectifiers were then connected up to the reversing switches on each controller (which in this case only needed to be single pole switches). The controllers used with this method were the big wire wound rheostats. (See figure two).
The comparatively new transistorized controllers do not appreciate having their supply current reversed. The idea of individual supply resolves this problem while still permitting the use of a common return. (see figure three). In this case the control circuits are kept entirely separate until after the controllers reversing switch each controller has its own totally independent transformer/rectifier and therefore the return wires from each of the sections can be made common and connected to the return terminal on all of the controllers.
So how does common return work and why in cases two and three above don't we get a short circuit when running trains in opposite directions? The answer is that we will not get a short circuit because the controllers powering trains in opposite directions are being powered by totally different transformers.
Let us consider this in more detail. A short circuit occurs when the positive terminal of the power supply becomes connected to the negative terminal of the same supply with an insufficient load in the circuit. The current flow will then exceed the rated capacity of the power supply.
When we try to run two trains (lets call them A and B) in opposite directions using common return we get the situation shown in figure four. If we have a single power supply we get a short circuit via the path indicated (dashed) since this path does not contain a load (train). However when using individual supply we find that this path connects the positive of one supply to the negative of the other. To determine whether we have a short circuit we must complete the path back to the negative terminal of the first power supply (shown dotted) and we find that there is a load in this part of the circuit (both trains) and that we therefore do not have a short with individual supply.
Let us analyse what happens with individual supply/common return wiring in more detail. The combination of transformer/rectifier and controller can be treated as a unit. Each of these units (each with its own separate transformer winding) can be treated as if it consisted of a variable battery. Turning the controller up or down increases or decreases the voltage supplied by the "battery" and throwing the reversing switch is the equivalent of turning the battery around. Our common return then becomes the equivalent of hooking two batteries up together and connecting the common rail up to the connection between the batteries.
Voltage is measured as a difference between two points in a circuit either across a component in the circuit or relative to some base point. In the setup that we have with our two controllers the only part of the system that is common to all circuits is the common return wire and we can therefore consider the common return to be at zero volts and measure all other voltages relative to it. Current flows through the circuit in such a way that the current flowing out of our controller by one wire will be the same as the current flowing back in by the other wire. Also wherever a junction occurs in a circuit the current flow into the junction will be equal to the current flow out of the junction.
Given this information we can now work out what happens when our two controllers are used to drive two trains. We can consider first what happens when the two trains are travelling in the same direction and then when travelling in opposite directions.
Let us assume that the locomotive on train A contains a motor which draws a current of one Amp and that the train is moving at a speed that requires nine volts. The second train (B) we will assume is hauled by a locomotive that only draws three quarters of an Amp but we have this train moving slightly faster and it is using ten volts. So if the trains are both moving in the same direction we can represent the situation by figure five. The side of the motor in each of the locomotives that is electrically connected to the common rail is at considered to be at zero volts (because of the decision that we made earlier). The other side of the locomotive is at nine volts for locomotive A and ten volts for locomotive B. Since there is no connection between these two wires without going through either the locomotive or the controller we can see that there is no short circuit.
Of course if we had chosen a different point in the circuit to call zero volts then the situation with regard to what we consider the voltages to be at various points would be different but the overall picture would be the same. For example if we consider the feed wire to train A to be at zero volts then the return wire will be at minus nine volts and the feed wire to train B will be at plus one volt. Similarly we can determine the current flow at various positions in the circuit. The current from both controllers flows through the common return and the current flowing through this wire (where it is common) is the sum of the two currents.
If we now reverse the direction of train B and have it travelling at the same speed but in the opposite direction what happens? (see figure six). The voltage and current flow through the feed wire to train A are unchanged. The voltage on the common return wire is also unchanged. Therefore the reversal of train B has had absolutely no affect on train A.
The voltage on the feed side of train B is now minus ten volts because we have reversed the train and the three quarter amp current flows in the opposite direction. In this case the currents from the two controllers are flowing in opposite directions and therefore the current flow in the common return wire is calculated by subtracting one from the other.
The other two situations (where train B continues in the same direction and train A is reversed or both trains are reversed) can also be easily worked out. These situations in fact are simply the same as the two cases that we have already considered but with the voltages and current flows reversed.
If additional trains and additional transformer/rectifier/controllers are added then we can still work things out the same way. By treating the common return as being at zero volts the voltage to each train will be calculated as positive or negative in the feed wire. The current flow through the common return can be calculated by adding together the current flows in one direction and then subtracting the total current flow in the other.
The common return system does not cause any problems provided that the trains travelling in one direction are being powered by a different transformer to trains travelling in the other. The split potential system of power supply meets this criteria by using two transformers one for each direction. The superior independent supply system also meets this criteria by giving each train its own transformer. This has the added advantage of making the current flow to each locomotive independent of one another as well.
The single supply system has a number of disadvantages. Firstly you can not use the common return system with single supply because the power to the controllers is common and with more than one common section in the circuit trying to run two trains in opposite directions will give a short circuit. Each section of track will therefore need its own separate return wire and this means more wire more connectors and double pole instead of single pole switches to switch the sections between controllers. Secondly (and this also applies to the split potential system) whenever one train draws more (or less) current due to rising (or falling) grades dirty track: or whatever this will affect the current available to the other trains as well.
Common return wiring with individual supply does work and the separate transformers that this system requires also ensures that the control of each train is completely independent of anything that happens to any of the other trains on your layout.