# Auto-Reverse question



## BMK (Sep 13, 2011)

I am running a Digitrax operation with an AR1and Kato engines. The auto-reverse was installed per the instructions and works great. My question is what is taking place when the engines move across the gaps. Is their a polarity reversal on the auto-reverse section of the track or is the decoder in the engine changing or what???? I understood with DCC the AC voltage is constant throughout the layout, is there also DC on the track that reverses?

Brian K


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## waltr (Aug 15, 2011)

The polarity of the track power is being reversed by the AR1 when it senses a short across the rail gap. Note that the polarity of the track power is totally independent of the loco's direction. The track power is in reality an AC signal (consisting of different width pulses) that the decoder rectifies to to DC to power the motor in the Loco. For a DCC only there is no DC on the track (the only exception is the Zero Pulse stretching for running a DC loco).
On the two rails if a pulse is positive on one rail the same pulse is negative on the other rail.
Now think of the track power pulses on only one rail. A pulse is Positive on the main track and negative in the reverse section. When the loco's wheel cross the gap the positive pulse is a short to the negative pulse. The AR1 detects this and switches which rail is connected to the rail with the positive pulse. Now there is not short as the rail on both the main and reversing section track have a positive pulse. Same with the other rail but with just the opposite polarity of pulse.


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