The Atlas Turntable and Track Polarity

When wiring an Atlas turntable for DC operation, the tracks in the turntable area must be wired so the plus and minus (the polarity) of the rails on the turntable will match that of the track(s) it is lined up with.  Otherwise, there will be a short circuit when the locomotive attempts to move on or off the turntable.

Your layout probably won't have all the tracks shown on the sketch, but this explanation should help you wire the ones you do have. 

An Explanation of the Sketch.
The dark gray rectangle, shown here at the top of the sketch, is the turntable drive, either the crank or the shed enclosing the optional electric motor drive. 

For reference purposes, I have numbered the tracks in the sketch as though they are the hours on a clock face, with the drive at 12 o'clock. 

The original Atlas HO 30° turntable had places for eleven tracks around the turntable.  On the sketch, these are the "hourly" tracks at the 1 o'clock through 11 o'clock positions. 

The revised Atlas 15° turntable has ten additional tracks around the turntable, at the half-hour positions from 1:30 through 10:30  For simplicity's sake, I left most of them off the sketch, but their wiring polarity (as shown by the colors on the sketch) should match the "hourly" track just counter-clockwise from them, e.g. the 5:30 track should match the 5:00 track. 

Wiring for the turntable track itself is done using two screw terminals located on the turntable wall on each side of the drive.  I have shown them as red and green dots on the sketch. 

The Internal Electrical Switching. From the point of view of a person riding on the turntable, the electrical polarity of the tracks in the 1 o'clock through 5:30 positions are all the same.  But as the end of the turntable track rotates past the 5:30 position toward the 6:00 position, the polarity of the turntable track will change.  Because of this, the polarities of the two tracks on each side of 5:45 are different from each other.  Continuing on, from the 6 o'clock position through the 11 o'clock position, the turntable polarity remains reversed.  At 11:45, it changes back again.  There are no 11:30, 12:00, or 12:30 tracks.

What is Significant About the 5:30 Track?
During rotation, when either end of the turntable track passes between the 5:30 and the 6:00 tracks, the Atlas Turntable performs some internal electrical switching.  Once it is properly oriented and wired, this will make the Atlas Turntable easy to use, but the point at which the switch takes place must be taken into account when planning the tracks around your turntable.  There are two X's on the sketch as a reminder of where the track connections change. 

What is the Significance of the Red and Green?
The red and green represent the rails that must be the same electricly for the locomotive to move from track to turntable and from turntable to track.

To put it simply, for a train to move from block to block (and that's what we're dealing with, blocks) all the wires to the left-hand rails must be traceable back to one of the terminals of a power pack, and all the wires to the right-hand rails must be traceable back to the other terminal.  The red and green will help you achieve this. 

How Does Polarity Information Affect Track Planning?
Depending on the complexity of the tracks you plan to have, and the available real-estate in the turntable area, you may find you need to rotate the entire turntable (drive and all) to be sure that all the tracks that connect with it will work well together. 

There are two kinds of track that will connect to your turntable: approach tracks and storage tracks, and they each have their own electrical needs to consider, both of which affect the possible orientations of the turntable. 

North is North... Many track planners find it convienient to think of their railroad as going east and west, and to call one rail of their main line "north" and the other "south."  Even if the track goes around in a loop, the rail under the right side of a west-bound train will be the "north" rail.

• Approach tracks are those which connect your turntable to the rest of your layout.  Each one must be wired so their north and south rails match the wiring of the tracks that connect them to the rest of the layout.  It doesn't matter if the north rail connects to the red or the green.

  If you have more than one approach track, the turntable needs to be oriented to allow the north rails of all of the approach tracks to have the same color. 

  Why have two approach tracks?  If your turntable is going to be busy, it is helpful to have two: one for inbound and another for outbound. 

• Storage tracks are those that are dead-ends off the turntable, to be used for storing locomotives when they aren't in use.  If you have more than one storage track in a group, try to find an orientation of the turntable that allow the left-hand rails of all of the storage tracks in each group to have the same color. 

  Example: If the turntable is oriented in such a way as to have storage tracks at the 5:30 and 6:00 position, the setting of your reverse switch to move a locomotive out of one storage track will be one way for one track, and the other way for the other track.  This could be confusing to visiting engineers. 

Where does the "available real estate" come into this?  Well, that drive has to go somewhere.  Hopefully, you have one or more orientations of the turntable that will not put the turntable drive into an area you had planned for something else. 

What Kind of Controls Will Be Needed?

If you will have storage tracks, you will need a way to turn off each storage track, so the locomotives on them will not move out of turn.  Ideally, the controls should not allow you to turn on more than one track at a time.  Here's a few ideas:

In What Order Should the Wiring Be Done?
Start by wiring your approach track(s) to your power pack or control panel so they work properly with the rest of the railroad. 

Then compare the wires you connected to the approach track with the red and green rails of the turntable sketch for the track position of the approach track.  The object is to determine which wire, north or south, should be considered the "red" rail during the rest of the wiring. 

Now that you know which wire is considered "red," connect another pair from your power pack or control panel to the turntable track terminals on each side of the drive, observing the "Red" and "Green" of the connections.

Finally, you can wire the storage tracks, each through its on-off switch, to the north and south wires, observing the "Red" and "Green" of the connections.