With Digital Command Control becoming ever more popular more clubs and home layouts are switching to it, but it can be quite costly to convert all your locomotives to DCC, so I’ve been looking at ways to make this cheaper.
A lot of US outline trains run with several powered units forming one consist and using one DCC decoder for the whole consist is my plan. A good example of this is the set of powered EMD E7 units below.
These models, made by Con-Cor, will always be run as a set and although the middle unit is a dummy locomotive the other two would normally both require a DCC decoder. This is because there’s no electrical connection between the three units and, despite both being good runners, the two powered units run at different speeds. This is a problem because one locomotive will always be pushing or pulling harder than the other.
Below is a video of the two powered units (without their shells) on a DC test track.
Clearly you can see that the nearest locomotive runs much faster than the other. This is because the motor in the nearest locomotive has a much lower starting voltage. If both locomotives had their own DDC decoder fitted they could be speed matched. This is done by adjusting the DCC configuration variables using a DDC controller or a system plugged into a computer. Doing this effectively alters the starting voltage of one or both of the locomotives to make them run at the same speed. The drawback of this is that you would still need two decoders, an understanding of how to do this, and, if the train is run on a DC layout, the problem would still be there as the CV settings do not have any affect under DC control.
An alternative is to increase the starting voltage of the faster locomotive motor. An easy way to do this is to add some resistance into the electrical circuit, but this can cause other problems as resisters do this by turning the unwanted power into heat.
A better way is to add a diode into one of the motor feeds. A diode works by only allowing DC power to flow in one direction whilst causing about a 1.5 volt drop. Of course this would mean the motor could only run in one direction so a second diode will also need to be used at the same location but positioned in the other direction. This meas that the power going to the motor will always pass through a diode irrelevant of the direction of the locomotive but with a reduced voltage.
Depending on the speed difference you have between the two locomotives, you may need to use more diodes to give a bigger voltage drop. For this particular pair of locomotives three diodes, giving a voltage drop of 4.5v, were required in order to make the two motors run at the same speeds. There is room between the main chassis and the shell so I constructed the diode circuit to fit in this space using 6 diodes; three for each direction.
With the diode circuit fitted into one of the motor feeds you can see the difference in this video, also on a DC test track.
Now the two power units run at the same speed I can use one DCC decoder to power both motors. It is important to select a decoder that has a maximum current capacity that is more than the combined current draw of the two motors. This can easy be checked with an amp meter. With the shell removed from your locomotive, connect a DC controller to your locomotive, passing one of the motor feeds through the amp meter. Then using your fingers, prevent the motor from turning and turn on the controller. The motor will try to turn but will stall as you have it clamped in your fingers, and the amp meter will measure the maximum current drawn by the motor. Do this with both motors and add the values together. If the combined value is less that the maximum current capacity of your chosen decoder then it will be ok.
As there’s lots of space inside the dummy unit the DCC decoder is installed in there. I used a Digitrax decoder and ran the wires through the corridor connections. This causes a small problem in that the three locomotives are now permanently fixed together by wires; should they come uncoupled it could put too much strain on the wires and cause damage. To solve this the couplings have been replaced with 3D printed permanent couplings.
These have the same ends as Rapido-style couplings and are a direct replacement using the same spring. A flexible corridor connection was also made using black foam and corrugated card.
The chassis for the dummy unit is actually a different make, Life-Like, but this is a bonus because it has power pickup on the trucks. All three units have power pickups that can now be connected together which greatly improves the performance of the combined locomotives. So this three unit EMD E7 set is now DCC converted using only one DCC decoder. Here they are pulling ‘The City Of Los Angeles’ on the GAMRG’s layout at the 2014 NMRA winter meet in Benson, UK.
Hopefully this will save you money on upgrading your locomotives to DCC, which can become incredibly costly given some size layouts and train collections. I’ve been experimenting in a similar way with a sound decoder; if I make any notable progress I’ll post it here.