Common Rail Issues

Common Rail wiring can clearly be made to work with DCC boosters as long as you follow some simple rules.

1) Single Power Supply per Booster.  Maintains "One Common" rule.

2) Opto-isolated control inputs for each boosters.  Maintains "One Common" rule.  Failure to do this can result in your control bus wiring carrying booster current which it was not designed to carry.

3) Do not install a Booster Common.  Maintains "One Common" rule.

4) Never use the Autoreversing feature of a booster.  Failure to follow this rule can create "Voltage Doubling" effect of the DCC track voltage from a booster point of view!

WHY?

To demonstrate the voltage doubling problem, lets take a reverse loop section on the left power by Booster #1.  It is in the same phase as Booster #2 which is powering the track that leads into the reversing section.  

Notice the voltages between the rail gaps on each rail are the same potential.  That means there is no difference in voltage and a short across the top gap will NOT result in any current flowing or 0A.

When a locomotive with offset wheel pickup rolls through the double rail gaps (Double insulated Rail Joiners), the current path is complete with Booster #1 powering the locomotive at this point.  If there was no common rail connection between the two boosters, the engine would have stalled across the gap in the exact same position shown


OK so far so good.  But let see what happens when you have Booster #1 flipped to the opposite polarity.


It may look the same as the above diagram but look very carefully at Booster #1.  The output voltage has flipped in polarity between it's two terminals.  Since the positive side of Booster #1 is now connected to the layouts common rail, system wise that redefines that output terminal as 0V.  If that terminal is now 0V, then the other terminal of Booster #1 must be below 0V or a -12V.   

Notice the two voltages across the top rail.  They are equal but opposite polarity.  That make the difference in voltage 24V.   When any wheel crosses the top gap for the first time, the 24V is going to be shorted out.  Shorting 24V out is a lot tougher on BOTH boosters than if it was just 12V.  The PEAK current, processed by BOTH boosters will simplistically be 2 times higher!  Only when Booster #1 flips its polarity back to the "In-phase" state will the short circuit go away.

From a decoders point of view, in theory this condition will not be harmful to it.  At no time does the decoder itself see the 24V from a stead state condition.  But there is nothing steady about a "rolling" short circuit until the short is corrected which will take Booster #1 some time to do.  In the mean time, there is plenty of opportunity for voltage spikes to appear anywhere in the wiring involved since some of the wiring is shared between the short circuit current and the decoder current.    This event will all happen so fast you cannot see it happen unless you use an electronic monitoring device called an oscilloscope.  What you should take away from this is that Murphy's law says the decoder will get hammered at some level.

Bottom line: This is an operating condition that is potentially destructive and does not need to exist.  It should be simply avoided at all cost and fortunately is easy to solve.  Both your decoder, booster and wallet will thank you. 

What do you do if you cannot use the Autoreverser function of a booster?

Use less expensive external autoreversing product.  I recommend the PSX-AR from DCC specialties. Electrically wise, the Autoreverser is installed AFTER the common rail wire.   Stated another way, one of the inputs to the autoreverser is connected to the common rail wire and not directly to any given booster.

Note: In fact NCE has dropped the autoreverse function from its boosters since it no longer make any economic sense and potentially could create more problems than it solves.