Wiring Diagram For EZ Track DCC Crossovers and Turnouts

[jbrock27]

Question:  (and I realize it works this way) Why does the Common have to be + for this to work, as opposed to it being  - as Common normally is?

Thanks.

[Len]

Based on the labeling on the decoder board, it appears to have a DC output. With the '+' DC output for the common. If you connect that to the '-' of the external power souce for the pushbuttons, the CDU in this case also has a DC output, it creates a momentary short whenever a button is pushed. By connecting the '+' of the CDU to common, along with the decoder '+', current is always flowing in the same direction through the coils, regardless if it's the decoder or CDU providing the power. Without any shorts being created, so everything works.

I suspect things would get even more complicated if an AC external power source were used.

Len

[jbrock27]

So the short answer is because the DCC chip is involved here?  Would I be correct to say if the DCC chip was left out of the equation and just the wires to the switch machine were directly involved, then the Common would be -, as Common normally is designated and the other wires (the Gray and Brown) would be the ones to carry the momentary, + power?

Thanks.

[Len]

Jim,

If no decoder was involved it wouldn't matter if you used the '+' or '-' of the CDU as common, as long as it was consistent throughout the layout. Adding the decoder to the mix essentially adds a second power source for the switch machines if external buttons are also used.

Since there's no way to change the common output of the decoder, at least that I'm aware of, the external source has to be set up to match it. I took a quick look, and the Digitrax DS64 switch control decoder is set up the same way, '+' common, for twin coil switch machines. Again, with no way to change it.

Inductors, which is all a solenoid coil really is, are current driven devices, meaning the '+' being common makes sense from an electronics standpoint. Everyone thinks of DC voltage flowing from the '+' to the '-', but current actually flows from the '-' to the '+'. Making the '+' terminal the current sink. And the sink side of a circuit is normally what you want as the common connection.

Again, if you're just using an external power supply and push buttons, it doesn't matter if you use '+', '-', or AC power. Just so it's consistent.

Len

[jbrock27]

Thank you Len, that is what I thought.  The red herring if you will, appears to be keeping the decoder in the loop as opposed to leaving it disconnected or isolating it. 
BTW, while what you say makes sense to me, I have only ever heard of the Common in reference to the neutral or ground, not something you would connect (pun intended) with being the positive side of the circuit.  And if I have my choice of interrupting the circuit, I will choose to interrupt it (by push button for example) on the positive side of the circuit as opposed to the negative side.

[Len]

Jim,

As I said, with a single power source it doesn't really matter, as long as it's consistent throughout. When I hear 'neutral' and 'ground', I start thinking AC house wiring with hot, neutral, and ground. Which may be why a lot of 3-rail O-gaugers operating on AC refer to 'hot' and 'return' or 'ground' for the center and outside rails respectively.

Negative voltages, e.g., -48VDC, with a '+' common are pretty much the norm in the telecommunications industry. A lot of early digital and computer circuits also used a '+' common for their -5VDC and -12VDC ciruits.

Personally, I don't mix buttons and stationary decoders on the same switch. I either use ground throws, buttons, or DS64 stationary decoders. Like you, I use the '-' side of the CDU output as common for the buttons, on the DS64's it's built in for common to be '+', and the ground throws don't care.

Len

[jbrock27]

Personally, I don't mix buttons and stationary decoders on the same switch.

I am with you on that Len and would not have gone about it that way.  Too many variables entering into the equation, making any troubleshooting a little more complex.