How does a DCC Reverser work ?

[dipeedo]

Can one of you gents explain how a DCC reverser works ? I mean, I realize the track section has to be isolated from the main track by cutting rails. I assume cutting both rails? But are both rails cut in two paces to isolate the problem?  Then what happens when an engine goes through the section ?
Or is it just plain PFM ?  (pure functional magic)

[ACY]

First you need a DCC system. Then you isolate the reversing section or loop from the rest of the layout, but rails need to be isolated. When the loco enters the loop it switches the polarity if necessary in a few milliseconds. If two locos enter the same loop then you have a problem.

[Doneldon]

dpd-

There are two tricks to understanding how an automatic reverser works: First, you have to be aware that you will have a dead short if you simply bend two rails around in a loop, reconnecting them with a turnout (track switch). That's true whether you have DC, AC or a DCC system. You can manually prevent a problem if you electrically isolate both rails at both ends of the loop, stop your train in the isolated section, change track polarity to match the polarity your train will encounter upon exiting the loop, and "telling" the locomotive to go the opposite direction of what it was going before. Of course, that's the same direction because it will be going the opposite direction on reversed-polarity track. It's kind of like a double negative. The disadvantage here is that you must stop your train and do all of this by throwing electrical switches. You can simplify the procedure with relays and some fancy wiring but, essentially, the same things must happen.

Second, you have to remember that track polarity doesn't matter to a loco in a DCC system. The decoder inside the loco determines direction, speed, effects and so on. So what the automatic reverser does is sense the short circuit which occurs when a locomotive's wheels bridges the gap between the loop rails and the rails which exit the loop, and change track polarity so the train doesn't cause a lasting short circuit. It does this so fast that no breakers or fuses can blow. The speed and direction of the loco aren't affected because the DCC decoder inside the loco automatically compensates for the reversed polarity and lets the train proceed as if there had been no problem. Technically, there was a problem, but the electronics in the automatic reverser worked so fast that no harm came from the short and the decoder worked so fast that the train appeared unaffected.

I hope this helps.
                                                              -- D

[timhar47]

Hello- i have been sort of confused about these also. I am familiar and use basic(bachman) DCC. So I understand the little thing does it stuff, but what if you were running a two engine consist-does the switcheroo mess up the engine still milliseconds behind still on the regular section or is it indeed all plain magic? Just curious

[Nathan]

Yes, you cut both rails on both ends of the reversing section..  The revering section should be longer then your longest train.  If you do not have any freight or passenger cars that use track power for lights or anything else, you have plastic wheels on the cars, and your train does hot have 'pusher units' you can get by with a section longer the the longest lash up of locomotives you plant to use in one train.

The Auto Reverse units available from a number of manufactures take power in from the DCC system.  The output goes to the reversing section.  There is a shot circuit detector in the Auto Reverse unit.  If a short is detected the Auto Reverse unit swaps the 'polarity' of the rails in the reversing section.  Most Auto Reverse units have an adjustment that allows you to make sure the Auto Reverse unit works before the DCC system detects the short and shuts down.

[Doneldon]

tim-

No, not at all. Remember that the decoders run the locomotives, not the track. The decoder in the second loco will adjust just as fast to the changed polarity as the decoder in the first loco. And that will be true for each subsequent loco, too.

You could in theory have an isolated track section of a certain length and a train of a certain length with locos (or a caboose with a track-powered light) bridging the rail gaps at both ends of the section simultaneously. My guess is that this circumstance, which is highly unlikely, would just cause the electronics to do their thing very quickly until there was no longer a conflict. Or, perhaps this situation would cause the electronics to have a nervous breakdown. Why not try it out? I couldn't cost more than a few hundred dollars for new decoders, a new reverser and a new DCC system for a failure. Or, have a rule (even the twelve-inches-to-the-foot railroads have rules) that trains of a certain length cannot use that particular reverse loop.

The real solution, of course,  is to have an isolated section long enough that head-end engines, helpers, cabeese and lighted passenger cars of all trains are accommodated within the isolated section, as the instructions state.

                                                                     -- D

[Joe Satnik]

Dear All,

For all practical purposes, the train can't be much longer than the isolated section (2 gaps on each end) of the reversing loop,

even if plastic wheels are used on the tail end of the train. 

Why?

Because the gaps are usually just off the "main" and "divergent" ends of the turnout that forms the loop. 

If your train was a little longer then the front of the (exiting) loco would slam into the side of the (entering) caboose at the turnout. 

Hope this helps. 

Sincerely,

Joe Satnik 

[dipeedo]

Hey thanks ! That was a pretty good description - I think I understand now - it ***IS*** PFM !

My second question then was: What happens on a double-header going thru the loop  ?  But I think you already answered that.

As always, PFM saves the day.

[Nathan]

Joe,

Not all reversing sections are reversing loops, some are 'Y's with the end being a branch line, and some are very extended loops many 'scale miles' long.

[Joe Satnik]

Nathan,

I read your post too fast, you were trying to shorten the isolated section within the loop by using plastic wheels.   

I mis-read that you were trying to extend the length of the train longer than the loop. 

Plastic wheels dirty the track.  I've only heard of replacing them with metal, not the other way around. 

Are there insulating wheels that aren't as dirty?

Joe 

[Jim Banner]

tim-
Why not try it out? I couldn't cost more than a few hundred dollars for new decoders, a new reverser and a new DCC system for a failure.

Doneldon, you are being much too pessimistic.  If a fault that simple damaged any of the DCC hardware, it should be going back to the manufacturer for warranty replacement.  This situation happens on my home layout occasionally: one return loop which is mostly in a tunnel is used by both the local switcher in that town and by through freights.  Sometimes, to avoid one another, they both end up entering opposite ends of the loop at the same time.  The auto reverser chatters a few times trying to sort things out, then the power manager (or before we had that, the booster) cuts off the power.  Once the 0-5-0 pulls one of the trains back onto the main, the power manager turns the power back on, the auto reverser clicks once if it has to, and the operating session goes on.  But do let me point out that all the components are being operated within their limitations.



As to how an auto reverser works, internally it works like this:  it has a pair of input wires which are connected to the main line.  I also has a pair of output wires connected to the isolated track (return loop, wye, turntable or other.)  The input wires are connected to the output wires by a polarity changing switch, typically a cross wired DPDT relay.  With the relay pulled in, the contacts reverse the polarity of the output compared to what it is when the relay is dropped out.  The rest of the auto reverser is an electronic circuit which can detect how much current is being drawn by the output wires and can reverse the state of the relay (pulled in or dropped out) whenever that current is more than normal operating current.  When a locomotive enters a loop, it must cross a set of gaps.  The wheels connect from the main line side of the gaps to the loop side of the gaps.  If the polarities match, then there is no extra current draw and the relay stays as is.  But if the polarities do NOT match, then there is a short circuit and the current in the output wires exceeds the setting of the auto reverser.  This causes the electronic circuit to reverse the state of the relay, reversing the polarity of the loop.  Now there is no short circuit and the train proceeds.  When the locomotive reaches the end of the loop, it encounters the second set of gaps.  At these gaps, there will for sure be a polarity mismatch which will cause a short which will make the electronics reverse the relay which will make the polarities match and let the train proceed.  Remember we are talking DCC here and with DCC it is not the polarity of the track that determines the direction of the train - it is what the decoder inside the locomotive does with the power from the track that determines the direction.  And for the purists, let's also remember that DCC puts ac on the track so when we talk about "polarity" we really mean the "phase" of the voltage on the track.

Jim      

[Nathan]

Joe,

I did not mean to CHANGE to plastic wheels, if the person is happy with plastic wheels, and many are, then they could use the trick.  I myself use all metal wheels, but my Dad has been using plastic wheels on his in door large scale for many years and has not to clean the track more then once every few months.