Using Common Rail Wiring with DCC by Stan Ames

This report was prepared for the DCC Working group as part of the technical input for the Control Bus RP.

• Using Common Rail Wiring DCC
  Testing Configuration
  Observed Scope Traces
  Summary of Tests
  Conclusions

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  DCC Commons

There are three locations to install a common between DCC Power stations. You can install the common by connecting one of the input leads of the power station together (Type 1), you can install a common on the negative side of the rectifier inside the power station (Type 2), or you can install the common by connecting one of the track output terminals of the power station (Type 3).

Note: Only one type of common should be used on a layout and this common should be used consistently. Multiple commons can result in ground loops and unexpected shorts and thus should be avoided.

By its definition, common rail implies a connection between the track output terminals of the power station.

 Testing Configuration

Whenever you transition from one Power Station (booster) to another the DCC power stations must be in phase. (This is true no matter which form of common you decide to use. If they are not in phase a locomotive crossing the gap will cause one or both power stations to overload and shut down.

With common rail out of phase power stations pose one additional problem. The voltages between power stations out of phase in a common rail environment produce a voltage that exceeds NMRA S-9.1 limits. Therefore, when DCC is used with common rail, it must be installed correctly so this potential for a double voltage situation can be completely eliminated. 

A test of how different installation decisions can effect the phase of a DCC power station was performed to understand this issue. See the conclusions for a summary of the results.

The configuration has four double pole double throw switches which reverse the polarity of various inputs and outputs of a power station to determine the impact of connection decisions on power station polarity. A scope was used to measure the voltages at various transitions. A DC voltmeter connected to a DCC decoder was used to verify the scope results. Switch #1 represents a track reversing switch for a loop. Switch #2 is similar to the effects of an automatic reversing power station. Switch #3 represents an incorrect installation of the control bus. Switch #4 is how the power station is connected to the power station.

 Observed Scope Traces

In looking at the track voltages between two power stations three possibilities were observed at the scope.

Trace #1- (Normal DCC Track Voltage) This trace represents what a decoder should normally see on a DC track

Trace#2- (Normal transition between power stations) When a decoder transitions between power station districts there is a minor difference that can occur between power stations. Normally a decoder does not see this but in a reverse loop situation it is possible that if you over run the block, the decoder can see this signal. The voltages at this level are save for DCC decoders.

Trace #3 (Double DC voltage condition) Note the amplitude of this signal. It is twice the size of Trace #1. This represents a condition that can damage decoders in offset pickup locomotives if they straddle the gap so that the pickups on the decoder see exactly this condition. Even though it is difficult to have situations where this condition occurs, you should always wire your layout to completely avoid this case.

The results of these tests can be verified by using a DC volt meter and a rectifier. The AC side of the rectifier should be connected to the track and the DC voltmeter should be connected to the DC side of the rectifier.

Following is a table that shows the scope traces for all possible combinations of track voltage that can result in a common rail environment. Any configuration that has a Trace#3 should be avoided.

 Summary of Tests

1. Switch #4 (the AC input) has no effect on the phase output of any of the DCC Power Stations tested.
2. Switch #1 (the reverse loop track switch) has no effect on the phase output of any of the DCC Power Stations tested.
3. Both Switch #2 and Switch #3 effect the phase of the output of all of the DCC power Stations Tested. If either is hooked up in such way that the phase is inverted you can have a Trace#3 condition. This can be avoided by correctly hooking up both the track output common and the control bus input.

Conclusions

1. To properly install DCC on a common rail layout. A consistent track output wire from all power stations should be connected to the layout common. It does not matter which track output wire you use so long as you are consistent.
2. Common rail wiring is totally compatible with DCC and no possibility of double voltages occur so long as you do not invert either the control bus wires or the common point on the output of the power stations. (Inverting both is allowable but not recommended as this can result in confusing wiring).
3. To test out any multi-power station installation simply operate a locomotive slowly over the gap transition between power stations. If the locomotive shorts out one of the power stations the installation is incorrect. If your system can operate a non-decoder equipped locomotive, the locomotive will operate smoothly over the gap if installed properly or will reverse directions on either side of the power station boundary if the power stations are out of phase.
4. The switch for reversing the polarity of a loop should occur AFTER the Power Station common connection point to avoid any possibility of switching the polarity of the power stations. In the diagram this is represented by Switch #1.
5. An automatic reversing power station has the same effect as Switch #2 and thus should not be used with common rail. Automatic reversing is still possible so long as separate automatic reversing unit is used that acts like Switch #1. These units are available from multiple manufacturers.
6. Interconnecting different brands of Power Stations poses no problems so long as they are installed so that the power stations are in phase (see point 3 above). Note: for best performance with existing signaling systems in a common rail environment, the common on the track output of the power station should be the only common connection installed between your power stations.

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Copyright 1997-2007 by Tried & True Trains, Inc.
This page is written and maintained by: Stan Ames
For more information contact Debbie Ames,  President tttrains