Bell/whistle controller follow up

[johnandjulie13]

Hello All:

A couple of weeks ago, there was a discussion about using a diode string to create a bell/whistle controller for transformers that did not have that function. I went out and purchased a Lionel button controller. After taking it apart, I have some questions on how this works. Here is the controller:





Given my limited electronics ability, I have attempted to draw a wiring diagram of this unit:




The wiring is set up for use as a bell activator. My questions are as follows:

Is my wiring diagram correct?
The bell is activated by opening the switch (pressing the button), how does this change the voltage to activate the bell?
When the button isn't pressed, what is happening with the AC going through this circuit?

Again, thanks in advance for everyone's feedback.

Regards,

John

[lionelsoni]

When the button isn't pressed, the track is connected directly to the transformer's output and it is as though the controller weren't there.

When the button is pressed, the diodes take turns conducting. On the positive half-cycles, there is only one diode in series between the transformer and the track, so the voltage is reduced only a little. On the negative half-cycles there are six diodes in series, so the negative voltage is reduced a lot. This unbalanced voltage reduction makes the average voltage a few volts positive. The average voltage is the DC component of the voltage. So the locomotive responds to that DC voltage by blowing the whistle.

[johnandjulie13]

Hello Bob:

Thanks for the response. So, when the button isn't pressed, the current just bypasses all of the diodes? I would have thought there would have been some voltage drop irregardless. Is that because it is a parallel circuit and voltgage is preserved? Does this result in the train slowing down when the button is pressed?

I now see that I drew the diagram in error, the one diode is facing the opposite direction of the other six. It appeared at first that they were all in the same direction, which compounded my confusion.

Thanks again,

John

[lionelsoni]

That's right, it bypasses them. The voltage is the same on both terminals of a closed switch.

Another way to do this is to use two long strings of, say, six diodes, then short-circuit all but one of one string to get the imbalance. This has the disadvantage that the voltage is lowered normally and the advantage that it rises when you press the button. It is also easier to make a DC offset of either polarity, just by doing the shorting in one string or the other.

I didn't notice your diode-direction error either!