Turnout Control

Wiring a number of switch machines to be activated by several control combinations causes a problem - current can travel back through a path, resulting in shorts.  Consider the fairly simple yard ladder I am wiring in this area:  (4) tracks, with (3) turnouts to direct a train to each.  See the top diagram:

I've labeled the tracks 1-4, and the turnouts A-C.  In addition, each turnout can have (2) conditions - Normal (N), or Reversed (R).

• To align the turnouts for track 1, turnout A has to be in the R position, and we don't care about the position of B or C.
• For track 2, A is N, B is N, and we don't care about C.
• For track 3, A is N, B is R, and C is R.
• For track 4, A is N, B is R, and C is N.

If we were to wire each turnout to the various positions on the rotary switch, we would have some problems with the current traveling backwards from turnout to turnout, with unwanted results (shorts).  The solution comes from using diodes in the circuit.  Diodes allow current to pass in only one direction.

To determine which wiring paths need a diode, we draw a "diode matrix" - this is the chart at the bottom of the diagram above.  Each route is charted at the left, and each turnout position is across the top.  We draw a diagonal line for each route's turnout position, representing a wire connection.  Then, where there is more than (1) wire connection to a turnout side, such as AN having (3) connections, and BR having (2), we draw a star.  This represents where a diode must be inserted in each wire path.  Any turnout side that has only (1) wire going to (1) rotary position (AR, BN, CN and CR) will operate fine without a diode.

Below is the wiring diagram developed from the matrix above:

We have power coming in through a capacitive discharge unit (CDU), then (1) leg to the momentary contact pushbutton, the other to the common terminal of each turnout coil (switch machine).  From each turnout coil, the N and R wires go to the 1 thru 4 positions of the rotary position switch, and note that where any wire has to go to more than (1) position on the rotary, a diode is inserted to keep power from traveling back to the other coils.

So, we position the rotary switch to route 1, 2, 3 or 4, then press the pushbutton, and the circuit is completed - the CDU sends a jolt of power sufficient to throw all (3) turnouts as necessary to align the route.

My thanks to the the Kalmbach book, Basic Model Railroad Wiring by Andy Sperandeo, and numerous internet sites, for the information needed to apply this to my railroad.