DDC engine polarity question


NYC_George

Well-Known Member
I just started testing my single track helix wiring. For now I'm using a double pole / double throw switch the change the polarity. I just noticed that when you change the track polarity the loco still continues on in the same direction. This wouldn't be the case if I wasn't using DCC right? In DCC the loco moves in the forward direction no matter what the track polarity is? Is this correct?

George
 
That would be my assumption as well but why would you do this? You may confuse the decoder for a split second but it will adjust. Is the DPDT for DC running also?
 
That would be my assumption as well but why would you do this? You may confuse the decoder for a split second but it will adjust. Is the DPDT for DC running also?

I have a feeling he's doing it as a manual way to control a reversing section. If not, you're right that I can't see why you would. But to answer the original question, DCC does keep doing the same thing regardless of polarity because, among other things, the rail power is not directly applied to the motor. That's why automatic reversing sections work. It's also why a DCC locomotive goes forward if you tell it to no matter which way you put it on the track. The track power powers the decoder as a whole, and then the decoder decides what power to apply to the motor completely independent of the actual track voltage. The only reason you even need a reversing controller (or manual switch) on a DCC layout is to prevent a dead short in the actual bus wiring and the rails of the tracks. The decoder couldn't care less if you switch polarity, and in fact when I had a reversing controller mis-adjusted too sensitive, I had a loco run just perfectly across that reversing section while the automatic reverse module was busily switching polarity about four times per second. Needless to say, I did adjust it so that it quit doing that, though. Kinda tough on relays.
 
In DCC the loco moves in the forward direction no matter what the track polarity is? Is this correct?

a DCC locomotive moves in the direction it is commanded. The decoder rectifies track voltage and applies the appropriate voltage polarity to the motor so that it moves in the commanded direction.

DCC track voltage is constantly alternating. The time between changes in polarity represents bits being communicated.

i use a DPDT switch on a turntable to match the polarity on the turntable to the track it leads to.
 



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