AUTOMATIC REVERSE LOOPS
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AUTOMATIC REVERSE LOOPS
As a train leaves a reverse loop the point needs to switch at the correct time to prevent a derailment.  An IRDOT-P is ideal for doing this automatically.
The IRDOT-P at 1 is positioned to operate the points just before the train reaches them, ensuring the rear of the train is clear of the points.
The IRDOT-P at 2 sets the points for trains approaching the reverse loop.   It is positioned more than a train length from the point.  This position is to prevent the train being derailed as it re-crosses the IRDOT-P at 2 on leaving the reverse loop.
If the IRDOT-P at 2 needs to be positioned closer than this the Direction Detector can be wired to the "I" terminal of the IRDOT-P at 2 so that the point is not switched by left to right trains.
The diagram shows 4 isolation breaks in the track on the reverse loop.  Whilst the train is travelling around the loop the feeds to the straight section need to be reversed.  There are several ways to accomplish this without stopping the train: 
  • Feed the controller straight into the reverse loop, but feed into the straight section through a double pole double throw switch wired to reverse the polarity.
    This switch needs to be thrown whilst the train is on the reverse loop so the straight track is ready for the change of direction. 
    The snags are it is confusing to work out which direction the train will travel as its direction on the straight line is affected by both the controller and the switch.  Also you need to remember to throw the switch whilst the train is on the reverse loop.
  • Feed the controller directly to the straight track, but feed the controller into the reverse loop through a diode bridge.  The diode bridge will cause the train always to travel in the same direction around the reverse loop regardless of the controllers direction setting.
    The snags are the diode bridge does not work with feed back controllers.  1.2 volts are lost through the diode bridge affecting the trains speed.  You need to remember to reverse the direction with the controller.
  • If a latching relay board is used in place of the switch the direction reversal will be automatic.
    To trigger the latching relay board the "P" terminals of the IRDOT-Ps can be used.  This is shown for two reverse loops. 
    Only two IRDOT-Ps are required for both reverse loops.
    Each IRDOT-P switches both points.  They set the correct direction for leaving the loop the train is on and they set the direction for entering the other loop.
The following diagram  shows the wiring of the latching relay board in detail.
The green lines are the connections from the controller to the track through the relay contacts.  The "P" terminals of the IRDOT-P are connected to "S" or "R" when the train is detected by the IRDOT-P. This connection makes the relays latch on or off, so setting the direction for the straight line.  This arrangement gives completely automatic operation of the reverse loops.
The two boxes labelled "PB" represent pushbuttons.  Their use is optional..  They allow the direction to be changed manually.  Pressing one push button sets the direction to the left the other to the right.  This allows the controller to be left set at the correct direction for the reverse loops.
A further refinement is to wire two LEDs to indicate the direction set by the latching relay board.  A second latching relay is used for this.  "R" of the first relay is wired to "R" of the second, and "S" of the first relay to "S" of the second.  This will cause both relays to be operated by the IRDOT-P or push button switch.
It is also possible to have automatic reverse loops wired to operate for trains to travel around them in either direction.