Why the Diodes in a MRC 270w transformer affects AC output

The MRC transformer does the DC offset in a totally different way.  The power always goes through the diodes array, the two back-to-back sets of diodes are there to pass the full wave AC.  When either the whistle or bell button is pressed, five of the six diodes in one side of the diode strings are shorted so that there is about a 2.5V DC offset generated on the output power.

Each handle has the same arrangement of diodes as there are two sets of whistle/bell buttons.

There is not a boost winding in the modern transformers like the old PW transformers.

John - Is there some where on the WEB I can see a wiring diagram showing what you described?  I am trying to visualize a set of diodes passing a full sine wave.

The need for 2 strings of 6 diodes, is one string for the horn and the other for the whistle?

Thanks again,

Thanks,

Rad400

Don't make it complex, just think of two diodes back to back with polarity reversed.

They will pass a full-wave AC signal minus only the diode drop as a half-wave is passed by one diode, and when the polarity is reversed on the other half cycle, the other diode passes it. 

The six work exactly the same way, only there is about a 3-3.5 volt drop instead of one diode drop as there are six diodes in each direction.

Now, if you short five of the diodes in one direction out, the drop on one half-wave is only one diode drop, but on the other half-wave it's still six diode drops.  The difference shows up as a DC bias on the AC waveform, which is what triggers the whistle or bell.

John, 

I want to be sure I'm understanding this correctly, just out of curiosity...

In this set up, the transformer's output is always cut by (about)3.5 volts, then when the whistle/bell is pressed, that half of the wave gets (about) the full voltage while the other half remains lowered, and this creates the DC offset needed for trigger the whistle/bell on the locomotive.  Does this replicate increasing the voltage as an old style transformer would, or am I thinking wrong here?  

You pretty much have it John.  The old PW transformers actually boosted the AC by around 6 volts with the boost winding at the second step of the whistle switch.  This was to account for the current draw of the whistle motor, the PW tenders sucked up lots more power than modern electronic whistles.  The modern transformer maintains the same AC power and just adds the DC offset.  I suppose, depending on the specific locomotive electronics, that you could get a slight boost in speed from the DC offset being added.  It'd nothing like the old PW transformers, but a small effect is doubtless there.

John

Thanks for the update, I am on board with the 4 diodes explanation you gave. When MRC uses the 12 diodes combination, is that done so a different DC value can be sent to the engine to distinguish between horn & bell.

Thanks,

Rad 400

The more diodes you use, the larger the DC offset that is generated when you bypass all but one of the diodes.  In the case of many bell/whistle buttons and transformer circuits, they've determined that five diode drops generates enough of a DC offset to reliably trigger the whistle or bell.