A U transformer terminals

[Barryf]

At what point did Lionel switch the transformer terminal designation?  Now A is U (common ground) and U is A (inside rail connection. On the old post war transformers you hook up A to outside and U to inside.  Now it is the opposite.  What was the point in switching it?

Thanks

[Michael6268]

To my knowlege U has always been common, and A was hot.

[cwburfle]

Several Postwar transformers use the "A" terminal as the common. The use of the "U" terminal as common was not consistent. It's best to check the instructions for each transformer.

[lionelsoni]

The postwar scheme seems to have been that a ("Multi-Control") transformer for  multiple trains (like the KW, Z, or ZW) was meant to have U connected to the outside rails and the independently controlled outputs, A, B, etc., connected to the center rails of the tracks.

But a transformer (like the 1033) for controlling a single train was meant to have U connected to the center rail and A to the outside rails, perhaps with an option of a B terminal instead for the outside rails if you wanted a lower voltage range.

If this convention is followed, the polarity of the DC voltage inserted by the whistle controllers always makes the center rail more positive.  This of course doesn't matter to whistle relays, but does make a difference with modern locomotives that distinguish a positive center rail for whistle and negative for bell.  (Note that the postwar-transformer schematic diagrams consistently have the rectifier symbol reversed.)

[Penny Trains]

So does that mean that a modern engine, like a Williams GG-1 may not respond to the whistle control on a 1957 (I think) KW?

[rtraincollector]

Penny it may react reverse meaning it may ring the bell instead of the horn. The direction of the engine will not be affected. 

[lionelsoni]

If the KW (being a 2-controller transformer) is wired with the U to the outside rails, the whistle polarity should be positive at the center rail, which a modern locomotive would interpret as a whistle signal, which is what you would want to happen.

If the locomotive doesn't respond, that probably has to do with the amplitude of the signal, which can be quite variable since the whistle signal voltage on a postwar transformer is greatly affected by the current that the train draws.  But that is a different problem and has nothing to do with the polarity.

[Barryf]

Thanks to all who commented on the subject.  I guess to simplify things, ignore the A and U designation and which ever one is the ground attach it to the outside and the remaining one to the center track.  It's just that when Lionel switches letters it confuses things.

[lionelsoni]

None of the terminals are grounded.  Just count the number of speed controls on the transformer.  If there is only 1, connect U to the center rail.  If there are more than 1, connect U to the outside rails.

[sir james I]

Actually until modern electronics came along everything worked just fine regardless of which way the wires were connected. With one exception, using a common ground between 2 or more transformers.