At my present level of knowledge, I can’t comprehend how a DCC auto-reversing circuit works on a wye, turntable, or reverse loop. MRs July 2006 DCC Corner does not enlighten!
If the reversing module reverses polarity on the mainline, why wouldn’t all the trains already operating on the mainline also reverse directions (a most undesirable effect, at least to me)?
If the reversing module reverses polarity of the “reversal block” (reverse loop, wye tail, or turntable), why wouldn’t the locomotive in the reversing block reverse direction away from the mainline unless there was a simultaneous instruction to the locomotive or locomoties on the reversal block to reverse its or their “polarity” direction?
Could someone expain how this device works in layman terms? Thanks.
DCC reversing works because the DCC voltage is a modulated square wave, kind of like an AC signal with information coded on it. The locomotive decoder includes a rectifier circuit, which takes the AC and turns it into DC. Note that the DC will always have the same polarity. So, if you flip the polarity of the AC, you do NOT flip the polarity of the DC. The decoder also takes information from the square wave’s modulation (that’s the pattern it uses when it changes) and uses that, not polarity, to control the engine’s direction. Also, it takes speed information from the modulation, not from the voltage itself, and controls speed with that.
The reverser simply switches the polarity of the AC signal in the reversing section. Since the decoder is still getting the same signal, nothing changes on the output of the decoders, so all the trains keep right on going.
If I may append MisterBeasley’s excellent explanation, in many situations the section of track serviced by the auto-reverser is isolated from the rest of the layout by gaps or insulated joiners, so the reversing is local, not layout-wide.
If this helps explain the concept at all, think about any DCC locomotive that has, for example, the short hood programmed to be “forward.”
In DC (no programming, of course), forward is determined by polarity. Point the short hood in the forward direction and it moves forward. Pick it up and turn it so that the long hood faces that direction without changing the polarity, and the locomotive moves in the direction of the long hood. It is polarity dependent.
In DCC, the decoder is programmed to have one side or the other be the front. So, assuming that is the short hood, put it on the track and it moves in the direction of the short hood. Pick it up and turn it the opposite direction, and it still moves in the direction of the short hood. The decoder understands “forward” and will move in that direction regardless of the polarity, at least in a DCC environment.
remember that in DCC the rectifiers are in the chip in the loco, so changing the orientation of the current before it gets to the loco doesn’t affect it. The reversing module is just there to prevent short circuits.
I may be a little slow on the uptake as this has been a curiosity of mine also.
So I get that the train will proceed through the reversing loop with the new signal from the loop but when it returns to the wye it will aquire the original signal that is telling it to go the other way? So when does it change back? IF the signal is the same and the engine will always move in the set direction of travel then why do we need reverse loops not just gaps?? I think I am missing something relatively simple but I can’t think of it…
Edit
I was just on the Digitrax site and I think it is more of a controlled short. When the loco trips the reverser it gives it the new signal for it to run the oposite direction. It’s a controlled short. When the Loco shorts the circuit the reverser switches the signal in an instant and the train keeps moving with no effect…
Think what would happen if a loco with multi-wheel pickup crossed the boundary where polarity is reversed. One wheel on rail A connected through the loco to rail B. The short would begin when the first wheel crossed the gap and end when the LAST wheel crossed the gap. Very bad for power supply of any types. Even the best will temporarily shut down output to protect themselves.
So … the reversing unit sees the initial short and very quickly swaps polarity to one pair of rails, ending the short while the loco still spans the gap. Momentum, flywheels, and stored electrical energy in the decoder keep the train moving for the instant of the short so hesitation is not noticeable.