Wiring a return loop for DCC

[Otis44]

Can a return loop be simply wired for DCC?  Do we just use insulators before the return switch?  Does anything bad happen then, when the loco hits the reverse polarity?  I have no interest in trying to make it dual-compatible with DC.  Thanks for your help.

[selector]

Yes, at one end of the serving turnout.  If you don't have a way of matching polarity to what the engine is leaving as it bridges the gap with the first set of powered axles, with what that first axle encounters in the way of polarity, you'll get a short, and the system's shorts detection circuitry will immediately shut off power to the rails.  If it doesn't, and your system can deliver more than two amps, you will almost certainly let the very faint and sweet smelling magic smoke out of the decoder.

You gap both rails at both of the main exits to the turnout serving the return loop.  Some put styrene fillers in the gaps to prevent sliding rails or warm rails from encroaching on the gaps, but I just cut gaps and leave it.  But, gap you must.  So, if you look at the loop from the single entrance, the points end, of the serving turnout, and then you get two possible routes...at the end of the turnout is a simple place to put both pairs of gaps.

Then you must wire it so that you can control the polarity.  Many of us use automatic reversers....they work well.  Some are costly, others less so.  Some use a double-pole double-throw electrical toggle switch, or simply a DPDT.   There are diagrammes on the web on how to wire these for reversal of polarity.  You feed the switch at the central poles from your power supply/bus, and the end poles provide the reversal feeding the loop rails.

-Crandell

[Vail and Southwestern RR]

The nice thing in DCC is that you can change the polarity right under the train, which makes things even simpler than DC.

[Otis44]

Thank you!  If I am soaking this up correctly, then 1) Allowing the loco to cross over to reverse polarity is undesirable, as it would seem; 2) I should isolate the complete loop from both ends at the turn out; 3) And, it's all right to reverse polarity on the isolated loop (with a DPDT switch or whatever) while the locomotive is on it?  This last part is the amazing part.  I wonder if excess amperages would be a concern at this point as well.  Thanks again.  If I have this right, then I'm gonna love DCC.

Otie

[cacole]

To answer the third part of your question, yes, you can reverse the polarity of the track (actually you're reversing the phase of the DCC signal, not polarity) BUT the locomotive should be stopped when you do this.  Reversing the phase while the engine is running will cause the engine to stop momentarily and then start up again.  This can damage some decoders.

A DCC automatic reverse module would be more advisable if you run only DCC.

[selector]

 

Otis44

And, it's all right to reverse polarity on the isolated loop (with a DPDT switch or whatever) while the locomotive is on it?  This last part is the amazing part.  I wonder if excess amperages would be a concern at this point as well.

Being a freakishly clever digital device, the decoder doesn't care what power orientation it gets as long as the base station is happy there is no conflict.  The decoder will instantly swap polarity of the current going to the motor, while the engine is in motion, and you and the motor will never know.  That is how it works on my system with the PSX-AR...my train enters the gapped loop and leaves it without a hitch.  I never know at which end the AR had to alter the current.

With the DPDT, if you do it quickly enough, the decoder's stay alive circuitry, if it has it, will keep it in motion.  Just be sure to do this when the axles on the train are fully within the loop...and that is the sticky part.  It really is safer, if your train is long, to just pause it and reach for the DPDT.

-Crandell

[Vail and Southwestern RR]

cacole

To answer the third part of your question, yes, you can reverse the polarity of the track (actually you're reversing the phase of the DCC signal, not polarity) BUT the locomotive should be stopped when you do this.  Reversing the phase while the engine is running will cause the engine to stop momentarily and then start up again.  This can damage some decoders.

A DCC automatic reverse module would be more advisable if you run only DCC.

What?!!!!! Changing the phase under the loco while it is moving is exactly what happens with an autoreverser! There is absolutely no reason not to reverse the phase under the loco. In fact, when you get down to it, it's happening all the time, for practical purposes that's all the DCC signal is. The only way you could have any problem would be if you somehow had a switch that made the new connection before breaking the old one, and even then it would just be a short.

[locoworks]

cacole

To answer the third part of your question, yes, you can reverse the polarity of the track (actually you're reversing the phase of the DCC signal, not polarity) BUT the locomotive should be stopped when you do this.  Reversing the phase while the engine is running will cause the engine to stop momentarily and then start up again.  This can damage some decoders.

A DCC automatic reverse module would be more advisable if you run only DCC.

 reversing the phase is what an auto reverser does anyway,  and they don't stop the loco first before they reverse things.   if this could damage a decoder i doubt the manufacturers of AR units would still be in business?  some DCC manufacturers produce their own AR units ( digitrax i know for sure ) so if there was an issue i can't see them doing so??   with modern loco's with big flywheels giving some momentum the quick disruption in phase reversal is barely noticeable, if at all??   in light of your post i would expect there to be a list of complaints from DCC sound users complaining about the sound resetting/going off in a reversing section, i haven't noticed any threads on the subject?   though quite a few sound decoders use a capacitor to keep things alive over very short power interuptions, maybe that was the cure for the problem?? 

 

edit;   beat me V&S RR  

[Vail and Southwestern RR]

The worst thing that will happen if your timing is off is that you'll create a short through the loco pickups (or a lighted car, for that matter), and the command station will shut down until you throw the switch back. If you change the phase under the loco, the motor isn't going to know anything has changed, it's not going to hesitate, and life will be good.

[Otis44]

Thanks, all.  An automatic reverser is sounding better all the time.  It seems like you shouldn't fall asleep while this is going on, no matter what the equipment can do.  I think with just a DPDT switch, if some skeptics are correct, it could only take one first trip around the loop to cause a problem. 

[trainnut1250]

I have two reverse loops/sections controlled with operator thrown DPDT's. no problems unless the operator forgets to throw them!!  In reverse loops, you are usually paying attention anyway because you often need to re-line the points while the train is in the loop for it to exit. 

I also control turntable bridge polarity with DPDTs.  Same possible operator issue there but no other problems.  All DPDTs will eventually be replaced with auto reversers when funds permit.

 Guy

[rrinker]

 If it's a 'typical' simple reverse loop (one turnout, the track loops to connect the straight and diverging routes together), you can use a DPDT relay controlled by switch machine contacts to alter the phase of the loop section. Whichever wya the turnout is lines, the phase will match for the train to enter/exit the loop via that leg. As it proceeds around the loop, you need to line the turnout to the exit route (or else derail) and that would automatically flip the phase to match the main. No forgetting like if using a standalone toggle switch, and simpler than a DCC autoreverser. However, for a more complex arrangment that includes a reversing section, the autoreverser would probably be easier to hook up.

                                 --Randy