On large layouts there are times when more than one transformer is used on the same loop of track.  You might block an uphill grade and provide additional voltage to that section to maintain speed.  Conversely, you might block a downhill grade and provide less voltage so the locomotive won’t speed up out of control.  Or maybe you want to provide constant voltage to a series of O-22 switches from a separate transformer.  No matter what reason you have to use more than one transformer on a layout, they must be in phase to function properly as a team.

What, you may ask, does it mean when transformers are in phase?  I’ll explain it as I understand it and I’ll use terms that should make it perfectly clear (I hope).  Layouts using older transformers have what is called a floating ground.  That means it’s not connected to your house earth or safety ground.  In fact, you don’t really have a “hot” and “ground” system for your model trains, you have a source and return (common).  All older Lionel transformers have a primary side (120 volts AC input and a secondary side 0-20 volts AC output).  The output is alternating current (AC) and that means it alternates from a plus voltage to a minus voltage 60 times a second in the United States.  When the output of  two transformers are connected together, they are in phase when their voltages alternate together.  They are out of phase when they’re not together.  When in phase, the relative voltage differential between the source terminals of the two transformers is very close to zero.  When out of phase, the voltage differential between the source terminals could be near 40 volts.

Now Lionel used a fairly consistent method of labeling the output (secondary) side of their transformers where common was “U” and the variable or fixed voltages had letters like “A”, “B” and so on.  The ZW and KW transformers are typical.  Then Lionel threw in the clinker and made transformers like the 1033 and the LW where the common terminal was labeled “A”.  If anyone knows why this happened, please tell me!

So, when I describe connecting the common terminals of two transformers together, it’s important you know which binding posts are truly the common.  The following table lists most of the Postwar transformers and which post is nominally common.

Transformer      Common Post      Variable Post

1033                             A                           U

1044                             A                           U

KW                               U                           A,B

LW                               A                           U

RW                               A                           U

SW                               U                           A,D

TW                               A                           U

V                                  U                     A,B,C,D

VW                               U                     A,B,C,D

Z                                  U                      A,B,C,D

ZW                               U                      A,B,C,D

Here’s the procedure to ensure two (or more) transformers are in phase.

1.  Bring the two transformers close enough together so test leads can be easily connected to the binding posts.  Plug both transformers into a properly wired 120 VAC house circuit.  Properly wired means the white (neutral) wire of the house circuit is connected to the left blade of the outlet, the black (hot) wire is connected to the right blade of the outlet, and the ground (bare) wire is connected to the round hole underneath the two blades in the center.  If you don’t know if the circuit is wired properly, you can purchase a circuit tester at your local hardware store for under $4.00.

2.  Connect a wire between the common binding posts of the two transformers.   (See the diagram below)

3.  Adjust the variable output voltage of each transformer to the same level (12 volts is a nice number).

4.  Attach one end of a wire to the variable binding post of one transformer and touch the other end of the wire to the variable binding post of the second transformer.  Little or no spark means the transformers are in phase.  A healthy spark means they are out of phase.

5.  Reverse the power plug of either transformer at its outlet and do the same touch test.  You want to mark the plug of both transformers when you get little or no spark so you will always plug it in the correct way.  You can use colored dots on the outlet and the plugs or fingernail polish, or a marker pen.  No matter which method you use, you must be consistent.

6.  You can now use one of the transformers (with its marked plug) to test another transformer.  Once those two transformers are in phase, the third transformer is in phase with the first transformer.  Just continue this method until all transformers used in your layout are connected properly to the house circuit.

Note: those of you who are leery of the spark test can use a voltmeter set on the AC 0-50 volt scale.  Connect one lead of the voltmeter to the variable binding post of one transformer and the other test lead to the variable binding post of the second transformer.  (Both transformers are set to 12 volts.)  When the two test transformers are out of phase, you should read about 24 volts differential between the two variable binding posts.  When they are in phase, you should read near zero volts.