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Dumb auto reverse question

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Dumb auto reverse question
Posted by FlattenedQuarter on Sunday, May 31, 2020 6:50 AM

I'm new to this and a little confused. When your running two trains on ona layout and one enters an auto reversing section of track and triggers the polarity reversing, what happens to the other train?

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Posted by FlattenedQuarter on Sunday, May 31, 2020 6:58 AM
Will be DCC if it makes a difference
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Posted by York1 on Sunday, May 31, 2020 7:15 AM

FlattenedQuarter
When your running two trains on ona layout and one enters an auto reversing section of track and triggers the polarity reversing, what happens to the other train?

 

Not a dumb question at all.  Even after using DCC for two years, I still have questions.

The other train will not be affected.  The decoder tells the locomotive which direction to go, regardless of the polarity of the track.

 

York1 John       

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Posted by richhotrain on Sunday, May 31, 2020 7:55 AM

York1
 
FlattenedQuarter
When your running two trains on ona layout and one enters an auto reversing section of track and triggers the polarity reversing, what happens to the other train? 

Not a dumb question at all.  Even after using DCC for two years, I still have questions.

The other train will not be affected.  The decoder tells the locomotive which direction to go, regardless of the polarity of the track. 

If the other train remains outside of the reversing section, it is unaffected. However, if the other train is entering or exiting the reversing section, it may trigger the auto-reverser depending upon whether or not the polarities are matched or mismatched.

Rich

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Posted by gregc on Sunday, May 31, 2020 8:08 AM

FlattenedQuarter
I'm new to this and a little confused. When your running two trains on ona layout and one enters an auto reversing section of track and triggers the polarity reversing, what happens to the other train?

you probably know that DCC track voltage alternates (AC).  The decoder rectifies it and then controls the polarity of the rectified voltage to the motor to control it's direction

but just because the track voltage alternates doesn't mean the that the polarity of adjacent rails can be different because a short will occur when a metal wheel bridges the gap.

an auto reverser controls the polarity of the rails on one side of the gaps to a reversing section so that when there is an inevitable short at one end of the reversing section or the other, the reverser very quickly reverses the connections between the rails in the reversing section so that there is no short.

frog juicers do a similar think, changing the polarity of an all metal frog if there is a short across the gap to the diverging rail.

greg - Philadelphia & Reading / Reading

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Posted by dehusman on Sunday, May 31, 2020 9:20 AM

"Auto-reversers" do not reverse the direction of the train.  They do not reverse the direction on the engine's DCC decoder. They reverse the polarity of the track in the reversing section.  

Dave H. Painted side goes up. My website : wnbranch.com

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Posted by peahrens on Sunday, May 31, 2020 9:59 AM

FlattenedQuarter
Layout Will be DCC if it makes a difference
 

Just for info, note that you can edit your posts as needed.  You will see an edit button on your own posts that others do not see. You can even edit the title.

But sometimes a subsequent reply is a better update.  Just fyi.

Paul

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Posted by MisterBeasley on Sunday, May 31, 2020 1:12 PM

Some DCC systems allow you to run a single DC locomotive along with the DCC fleet.  It will run poorly, but it will run.

But, it will NOT work properly across a reversing section, because the DC engine will reverse when the polarity flips, unlike a DCC engine.  It could actually ping-pong back and forth, but will probably eventually just short out with an unresolvable polarity conflict.

It takes an iron man to play with a toy iron horse. 

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Posted by gregc on Sunday, May 31, 2020 2:08 PM

MisterBeasley
But, it will NOT work properly across a reversing section, because the DC engine will reverse when the polarity flips, unlike a DCC engine. It could actually ping-pong back and forth, but will probably eventually just short out with an unresolvable polarity conflict.

bear in mind, the track voltage/polarity is still DCC.  As soon as metal wheels bridge the gaps of a reversing section, the auto-reverser flips the polarity on the rails of the reversing section to "match" the mainline rail polarity.

if the polarity of the rails and average DC is now the same on either side of the gaps, why would the DC loco change direction?

greg - Philadelphia & Reading / Reading

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Posted by rrinker on Sunday, May 31, 2020 3:44 PM

 The average DC isn't the same - that's how zero stretching works, it makes the average DC different instead of net 0, so the DC motor can turn one way or the other.

                                 --Randy


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Posted by gregc on Sunday, May 31, 2020 4:15 PM

MisterBeasley
But, it will NOT work properly across a reversing section, because the DC engine will reverse when the polarity flips, unlike a DCC engine. 

i take back what I said earlier,   i think you may be right

so the average DC on the mainline has a polarity, let's say positive is forward and negative is reverse.

on the mainline, the loco travels from left to right , ">", toward a reversing section.  It crosses into the reversing section gaps and continues to travel ">" until it loops around and is now traveling right to left, "<", ready to come out of the reversing section.

as it crosses the reversing section gaps at the opposite end the auto-reverser swaps polarity to match the mainline, reversing the polaity of the DC average and the loco reverses direction.  It is now traveling ">" again. 

The operator now needs to reverse the direction of the loco so it travels "<".  Changing the direction if the DCC control reverses the polarity of the DC average.   the loco crosses the same reversing section gaps again and continues on the mainline traveling "<"

on a DC layout, doesn't the operator reverse the direction on the mainline block when leaving the reversing section?

thanks MisterBeasley

greg - Philadelphia & Reading / Reading

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Posted by FlattenedQuarter on Sunday, May 31, 2020 4:58 PM

Yeah, I tried the edit button and it acted like it didn't go through. Hence the duplication 

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Posted by FlattenedQuarter on Sunday, May 31, 2020 5:14 PM

Let me see if I got this right...

  1. Loco enters auto reversing zone  thinking it's s going forward.
  2. Auto reverse switches track polarity.
  3. Decoder tells loco motor it is now running backward.
  4. Loco continues on thinking backwards is forwards.

?

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Posted by BigDaddy on Sunday, May 31, 2020 5:33 PM

FlattenedQuarter
3.  Decoder tells loco motor it is now running backward.
4. Loco continues on thinking backwards is forwards.

No.  The digression to DC operation on a DCC layout caused unneccessary confusion.

3 & 4 are wrong if we are still talking about DCC.  The decoder tells it to go forwards.

To prove it put your loco in the track and tell it to go forward, it does.

Turn in 180 degrees and tell it to go forward, it does. Which is the real forward?

There is no absolute forward based on DCC wiring.  The reality of foward is based on the nose of the loco.  There is no logical process in the docoder that tells it backwards is forwards.

 

 

Henry

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Posted by richhotrain on Sunday, May 31, 2020 5:52 PM

How do we get ourselves into these situations? Initially, the OP merely asked how a second train reacts when the first train enters a reversing section on a DCC-powered layout. 

Rich

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Posted by BigDaddy on Sunday, May 31, 2020 6:13 PM

richhotrain
How do we get ourselves into these situations? Initially, the OP merely asked how a second train reacts when the first train enters a reversing section on a DCC-powered layout.

I believe people want to be helpful and they also what to show what they know or what might be related to the OP's question and be helpful to someone else.  I'm guilty of both.

Henry

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Posted by richhotrain on Sunday, May 31, 2020 6:22 PM

BigDaddy
 
richhotrain
How do we get ourselves into these situations? Initially, the OP merely asked how a second train reacts when the first train enters a reversing section on a DCC-powered layout. 

I believe people want to be helpful and they also what to show what they know or what might be related to the OP's question and be helpful to someone else.  I'm guilty of both. 

Henry, I wasn't directing that comment at anyone in particular. It just seemed to me that the thread has taken an unintended turn in direction away from the OP's initial inquiry.

Rich

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Posted by gregc on Sunday, May 31, 2020 6:51 PM

FlattenedQuarter
Let me see if I got this right...
  1. Loco enters auto reversing zone  thinking it's s going forward.
  2. Auto reverse switches track polarity.
  3. Decoder tells loco motor it is now running backward.
  4. Loco continues on thinking backwards is forwards.

DCC track voltage does not control the direction of a loco with a DCC decoder.   The decoder rectifies the alternating track voltage producing DC within the decoder.    DCC commands to the decoder determine the voltage polarity and direction of the motor.

when a train enters a reversing, the reverser swaps the polarity if it needs to, maintaining the polarity as the loco crosses into the reversing section.  If this happens, it happens so quickly that any momentary drop in voltage because of the short seems like dirty track to the decoder.  There is no change in polarity as the loco enters the reversing section.

at the opposite end of the reversing section, the polarity is changed to match the mainline polarity but has not affect on the decoder because it rectifies the track voltage and is unaware that polarity (phase) changed.

the loco moves in the same direction, forward, as it enters and exits the reversing section.

greg - Philadelphia & Reading / Reading

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Posted by FlattenedQuarter on Sunday, May 31, 2020 7:26 PM

Think I get it now.

Thanks everybody

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Posted by mvlandsw on Sunday, May 31, 2020 8:14 PM

Be aware that only one train at a time can cross the gaps at the ends of the reversing section. If a train is crossing the gaps at both ends at the same time the autoreverser will try to match the polarity at both ends, which is impossible, and you will get a short. This can also happen if a single train with metal wheels is longer than the reversing section.

Mark Vinski

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Posted by ATLANTIC CENTRAL on Sunday, May 31, 2020 8:21 PM

gregc

on a DC layout, doesn't the operator reverse the direction on the mainline block when leaving the reversing section?

thanks MisterBeasley

 

Yes, the traditional way to handle reverse loops on DC is to have a separate reverse switch for the main, and a separate one for each loop. 

The loop reverse switch must be set to the correct polarity for the train to enter the loop.

Once in the loop, the loop reverse switch controls the direction should you need to back up, as long as you stay in the loop.

While in the loop, the polarity of the main needs to be reversed and the turnout aligned for the train to re-enter the main in the oposite direction.

But there are other ways to handle DC reverse loops, too complex to explain without drawings of examples...........

On my new layout using the Aristo wireless throttles there will be a wye and a loop that can reverse trains. Both require the train to come to a stop and for the operator to throw a turnout and change the direction on his throttle before proceeding. 

Because the direction controls on the throttle relate to east and west movement on the layout, not forward and reverse movement of the locomotive.

Sheldon

    

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Posted by rrinker on Sunday, May 31, 2020 8:28 PM

 The decoder alwyas tells the loco which direction to move. Which rail is connected to which terminal on the DCC booster makes no differents. Assuming the motor wires in the loco are correctly connected to the decoder, forward is forward, aka, to the front of the loco, and reverse is reverse. 

 With two rails to carry current, they must always be at opposite polairy. The square wave AC nature of DCC does not change that - connect the two terminals on your house wiring, which is AC, and you get a short just as surely as connecting the two terminals of a battery, or the two DC terminals of a DC power pack causes a short.

 With DC< you typically reverse the direction of the main line while the loco is in the loop. this makes the polarity match on the exit route of the turnout, so no short, and the polarity is such that the loco continues moving forward. If ther were another loco on the main track at the same time, it would suddenly change direction.

 With DCC, the square wave AC is only carrying data addressed to each decoder which tells it which direction to go, how fast, and what functions (like the lights, horn, or bell) are on. The 'polarity' of this signal carries no direction information. So to handle a reverse loop, with DCC you typically change the polarity under the moving loco. Since this has no effect on direction, the loco keeps right on moving just as it was before the polarity flip. But now the polarity across the gaps (DCC reverse sections still need gaps in the rails, just like DC) match between the main and the exit route the loco is taking, and it can continue on as if nothing special happened.

It is perfectly possible to flip the mainline polarity with DCC, same as with DC, except that any other trains on the main with DCC would just keep right on going the same way they were, no sudden direction change. The main reason this is not usually done is that the automatic reversing units will last a lot longer switching the current of one or two locos in the reverse loop as opposed to having to switch the much greater current of every other loco out on the main.

 You can even use the same toggle switch arrangment with DCC that was used with DC. But the automatic devices mean you don't have to remember - just keep driving your train and it will continue moving.

                                --Randy


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Posted by richhotrain on Sunday, May 31, 2020 8:35 PM

rrinker

It is perfectly possible to flip the mainline polarity with DCC, same as with DC, except that any other trains on the main with DCC would just keep right on going the same way they were, no sudden direction change. The main reason this is not usually done is that the automatic reversing units will last a lot longer switching the current of one or two locos in the reverse loop as opposed to having to switch the much greater current of every other loco out on the main.

Randy, thanks for bringing up that issue. It is an interesting concept, and one that I have not really thought about, and I am sure that it is true for others as well.

Rich

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Posted by gregc on Monday, June 1, 2020 4:56 AM

rrinker
It is perfectly possible to flip the mainline polarity with DCC, same as with DC,... The main reason this is not usually done is that the automatic reversing units will last a lot longer switching the current of one or two locos in the reverse loop as opposed to having to switch the much greater current of every other loco out on the main.

the short circuit current doesn't depend on the number of locos drawing track current.   the metal wheels of rolling stock will cause just as much of a short as the metal wheels of a loco

when not switching, more current wouldl pass thru the auto reverser if there are multiple locos and lighted cars on the mainline track.   But why should any electronic circuit fail if it's operated within it's limits.  Is your 10A booster more likely to fail if it supplies 9A much of the time instead of 6A?

if you have multiple reversing sections, aren't you limited to having a single train enter/exiting each reversing section if a single auto-reverser is used for both reversing sections or just the mainline track?

if you have multiple power districts, would you need separate auto-reversers for the mainline track in each distrcit?

greg - Philadelphia & Reading / Reading

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Posted by rrinker on Monday, June 1, 2020 7:14 AM

 I'm saying the reason to switch the loop in DCC, when you can just as easily switch the main automatically, is to keep the reverser from having to switch under a high load. Relay or solid state, there's a limit to the switching current, usually much less than steady state current.

 If you flip the reverse section, you have one train in it. Whereas if you flip the main, you might have 6 trains out on the main, with one going through a reversing section. Flipping either will accomplish the same thing, but flipping the reverse section means switching a lot less current (potentially) than the main.

 Anyone with a PSX-AR want to read the part number off the switching MOSFETs on one, so we can find the data sheet and see what the switching current is?

                                   --Randy

 


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Posted by FlattenedQuarter on Monday, June 1, 2020 7:16 AM

Yikes! Above my pay grade Indifferent

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Posted by gregc on Monday, June 1, 2020 8:01 AM

rrinker
If you flip the reverse section, you have one train in it. Whereas if you flip the main, you might have 6 trains out on the main, with one going through a reversing section.

huh?

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Posted by richhotrain on Monday, June 1, 2020 8:45 AM

FlattenedQuarter

Yikes! Above my pay grade Indifferent 

FQ, you are getting a lot of technical talk thrown your way, so it is not surprising that you are confused about the issue of reverse polarity and the function of an auto-reverser.

We don't know much about your layout, such as the track plan, the types and lengths of trains you are running, metal wheels versus plastic wheels, the brand of auto-reverser, the types of turnouts, etc. More information would help to give you more specific advice.

But, let me give you an example.

Say that your track plan includes a mainline with a loop of track that folds back onto the mainline via a turnout. Let's further assume that the straight through route of the turnout heads directly into the loop and that the feeder wires inside the loop match the polarity of the feeder wires on the straight through route of the turnout. That means that when the train exits the loop via the divergent route of the turnout, there will be a short because the polarities will be mismatched.

The solution is to gap both ends of the turnout (opposite the tail end of the turnout) and route the feeder wires inside the loop to the output side of the auto-reverser. The result will be that a train entering the loop from the straight through route of the turnout will not trigger the auto-reverser since the polarities already match. But when the train exits the loop via the divergent route of the turnout, the auto-reverser will flip the polarity inside the loop to match the polarity on the divergent side of the turnout.

If you think about it that way, the next time that the train enters the loop from the straight through route of the turnout, the auto-reverser will flip the polarity inside the loop to match the polarity of the straight through route. On the other hand, if the train were to enter the loop through the divergent route of the turnout, the auto-reverser will not be triggered since the polarity inside and outside the loop match at the point of entry.

That is a pretty common occurence with reversing sections and auto-reversers. The auto-reverser has no cause to flip every time a train enters or exits the reversing section. It will only flip to match mis-matched polarities.

Rich

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Posted by rrinker on Monday, June 1, 2020 10:17 AM

gregc

 

 
rrinker
If you flip the reverse section, you have one train in it. Whereas if you flip the main, you might have 6 trains out on the main, with one going through a reversing section.

 

huh?

 

 I'm not sure where the disconnect is. 

With DC reversing - the main line polarity is changed, agreed?

With DCC reversing - you can change either the main or the reverse loop, as long as ONE of the two, and only one, gets flipped, agreed?

Now, our club layout has no reverse sections because it is basically a giant oval donut, but typically there may be 10 trains running. Say we did have a reversing section. Since we run DCC only, an autoreverser would most likely be used. If one train goes into the reverse loop, and the autoreverser flips the phase in the loop section, it's switching the load of one train. The other 9 don't see so much as a microsecond of interruption.

 If instead the autoreverser was connected to the main, it would flip the phase as the train on the reverse loop crossed the gaps, same as before. Only this time, it's switching phase with the load of the 9 trains on the main.

                                            --Randy

 


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Posted by gregc on Monday, June 1, 2020 2:56 PM

rrinker
Relay or solid state, there's a limit to the switching current, usually much less than steady state current.

The mosfets in a circuit breaker, frog juicer or auto-reverser are constantly turning on and off each time the DCC signal changes phase.

i assume even an auto-reverser using a relay can act as a circuit breaker, blocking power to the reversing section if there is a short with both polarities.   I further assume it blocks power when toggling the relay to avoid contact arcing (which avoids the problem you're suggesting)

 

the schematic below shows circuit breaker design using pairs of mosfets to conduct the alternating DCC current to each rail.  the mosfets are connected common drain.   A mosfet conducts current in a single direction.  There is an inherent diode in the mosfet that conducts when the voltage across the mosfet drain/source is reversed.  the opto provides a bias voltage to turn on one mosfet or the other as the voltage alternates.   the mosfets are constantly turning on/off with each DCC phase.

the circuit detects a short when the current thru the small 0.22 Ohm resistor is close to 0.7V, causing the bipolar transistor to conduct.

an auto reverser has another two pairs of mosfets (8 total) connecting the rails to the opposite inputs.  The auto-reverser biases one set, 4 mosfets for each polarity, or biases none to act as a circuit breaker.

when an auto-reverser "reverses" polarity to the reversing section rails, it simply biases the other set of mosfets.   there is no "switching current" that occurs when reversing polarity.  the mosfets must be capable of handling the short circuit current. 

greg - Philadelphia & Reading / Reading

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