Conventional Transformer designs

[SPFan]

Greetings everyone. I have been lurking for a few weeks and already have learned a lot from some very knowledgable folks.
I am interested in making my own transformers using 16-18 volt fixed transformers and some variacs. I knew the whistle is acheived by sending DC down the track but have for the first time connected my 1033 to an oscilloscope to observe the waveform. It looks like all Lionel did was to recitify one half of the waveform. Is this the case for all of the Post War transformers or were some more sophisticated? What I actually see is the rectified wave for half the rotation of the whistle, then full wave but greater ampitude when moving the whistle to full on.

[lionelsoni]

The transformer has an auxiliary 5-volt winding that is put in series with the output when you blow the whistle. In the first step, this augmented voltage is half-wave rectified, as you observed. The strong DC component causes the slugged whistle relay to operate. In the second step, a resistor shunts the rectifier, allowing a considerable AC component through to keep the locomotive moving at approximately the same speed while supplying the extra current to run the whistle motor. This dilutes the DC component; but, if you look carefully, you should be able to see a slight asymmetry in the waveform, representing the DC needed to hold the relay closed. The extra 5 volts is not always appropropriate for the actual circuit impedance; so the train will sometimes slow down anyway, or speed up, in the case of a diesel, that has no whistle motor to load the transformer.

[SPFan]

Bob, thanks for the info. I tried a little experiment last night and simply inserted a full wave bridge in the transformer output. The whistle blows and there is no loss of engine speed. I sort of suspected this might happen since the loco motors appear to be universal (AC-DC) types. At least on the 2035 and 622 I tested. I am thinking the reason for the second step in the whistle where additional voltage is applied is to give the the two tone effect as well as compensate for loss of track current to the whistle motor. This will be a little trickier to implement with a standard variac but I have some ideas.
Pete