Reversing loop and other logic problems

[Jono]

In order for me to be able to design / think about designs and such like, I need to be able to think them through.

Currently my biggest issue is with reversing loops. Looking at loads and loads of pics and what people have to say, and using YouTube, I see that they work. But the HOW is completely doing my head in. The gap in the rail is TINY. Locos typically have more than 1 electrical pick-up point. I see many online boasting 12. So at the points of entry and exit from the loop, some of the loco is going to be ,electrically, in the loop, and some 'out' the loop. . and yes, i know it all works, somehow. And then it all works going back the way it came... with the right hand wheels on the left hand tracks. This is doing my head in.

Then we have "power blocks" for boosters in DCC. I see LGB and others sell insulated track. where ONE tack is insulated from the previous block. OK< I understand how N works in electrical systems, and how that can just be bridged and joined together whereever etc, as long as all N goes together to all power supplies etc. But does this not cause all sorts of interesting electrical phenomenon when something goes wrong ? Stuff like stray voltage, back pressure and such like ? Or when a train has just been around a reversing loop to return to a station , where there is already a train waiting ?

Think I go pull weeds out the garden before I get my brains all over the wall.

 

[Cliff George]

Currently my biggest issue is with reversing loops. Looking at loads and loads of pics and what people have to say, and using YouTube, I see that they work. But the HOW is completely doing my head in. The gap in the rail is TINY. Locos typically have more than 1 electrical pick-up point. I see many online boasting 12. So at the points of entry and exit from the loop, some of the loco is going to be ,electrically, in the loop, and some 'out' the loop. . and yes, i know it all works, somehow. And then it all works going back the way it came... with the right hand wheels on the left hand tracks. This is doing my head in.

It is the phase/polarity of the loop that is changed so it is not true that some part of the loco is in one phase and another part another. To traverse the rail gaps the phase/polarity must be the same.

There are currently two ways of detecting that the phase/polarity in the loop is incorrect (or not) and needs changing (or not).

1) Small sensor tracks before the start of the loop. In this case the phase/polarity of the loop has been made compatible with the entry phase/polarity BEFORE the train crosses the gap in both rails.

2) A quick short detector switches the loop polarity/phase as soon as a short occurs. It does this so fast that nothing is noticed. This is not the preferred method.

 

[Cliff George]

Then we have "power blocks" for boosters in DCC. I see LGB and others sell insulated track. where ONE tack is insulated from the previous block. OK< I understand how N works in electrical systems, and how that can just be bridged and joined together whereever etc, as long as all N goes together to all power supplies etc. But does this not cause all sorts of interesting electrical phenomenon when something goes wrong ? Stuff like stray voltage, back pressure and such like ? Or when a train has just been around a reversing loop to return to a station , where there is already a train waiting ?

LGB also sell track where BOTH tracks are insulated/broken. To separate power districts you need to have a gap in both sides of the track.

Two examples of using the track where only one track is broken is where you want to isolate a train from power somewhere or when you want to detect the presence of trains.

I hope this helps.

 

[Deleted member 1986]

This is a pictorial explanation in it`s simplest uncomplicated format, for use under DCC the DPDT switch usually is replaced with an auto reverser, which takes care of the automatic&nbsp; polarity switching, the distance between the insulated joiners is deemed by the length of the longest train, taking into account that all electrical pickups for the same train must be within the area between the insulated joiners.

 

[Cliff George]

With DCC it is more usual to control the phase/polarity of the loop itself, not the main track as the diagram shows. The diagram is the usual way DC is wired.

By controlling the loop phase/polarity it is possible to use more than one auto reversing loop on the same layout and run multiple trains at the same time.

 

[playmofire]

This is a pictorial explanation in it's simplest uncomplicated format, for use under DCC the DPDT switch usually is replaced with an auto reverser, which takes care of the automatic polarity switching, the distance between the insulated joiners is deemed by the length of the longest train, taking into account that all electrical pickups for the same train must be within the area between the insulated joiners.

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So do you flip the switch to reverse the polarity once all the loco pickups are in the loop when running non-DCC?

 

[Jono]

With DCC it is more usual to control the phase/polarity of the loop itself, not the main track as the diagram shows. The diagram is the usual way DC is wired.

By controlling the loop phase/polarity it is possible to use more than one auto reversing loop on the same layout and run multiple trains at the same time.

Thanks Cliff. Getting there, I think.

So, essentially, if the loop is "correct" polarity for the train when it enters the loop, nothing "happens" and everything keeps on going as it was. As the first element (I would think almost always a loco) starts to leave the loop, once the first set of electrically active wheels cross the divide, the entire loop changes polarity to meet the new requirements. As DCC is a very clever Dc / AC hybrid, the controllers keep the train running in the correct direction, and lights and any other carriage related items doing what they should ?

I would assume this sort of loop would just cause hell for a analogue loco ?

Power districts ...

This may seem odd. On my HO trains (what 30 years ago? or there abouts) I had power districts so I could have 2 trains on my simple layout. Other than keeping some of the track powered in the event of one of the power supply modules dying, getting switched off (deliberately or otherwise) and that sort of thing, why do DCC layouts need "power blocks" - as i used to call them Sure, if you have a "mining" scene running a closed loop or something like that, simple to keep separate .. But for mainline / normal track, surely it would be better to have all one electrical block, with "hardwire" cables to various parts of the track to reduce voltage drop / bad join problems. That way all equipment powered from the tracks would always have full access to all power supplies regardless of how many items are drawing power at any point at any time ?

Thanks


Jonathan

 

[ntpntpntp]

...why do DCC layouts need [power districts]?

The main reason for our large scale layouts would be where you're running more trains than a single power booster can handle (too many amps). You split the layout into districts each with it's own booster and spread the trains around the districts. Most power boosters will have their own overload / short circuit protection allowing just that district to be cut out if there's a problem.

[edit]
Another reason to have districts is specifically to limit the amps in each district to a reasonable power capacity for the scale. If you run small scale trains with a big powerful booster you run the risk of a short circuit NOT being enough to trigger the booster's safety cut-out, so the current continues to flow and you end up with melted trains - see half-way down this page: https://sites.google.com/site/markgurries/home/technical-discussions/boosters/10amp-booster-ho
[/edit]

It's also worth putting accessories like the pointwork on it's own power district, so that if the track power it cut due to a short or derailment you still have control of the pointwork.

On my line I have the point decoders on their own DCC bus rather than connected directly to the track. This means I can run the trackwork on good old plain DC for the benefit of visiting locos and still have the points under DCC control via my track diagram.

 

[Gavin Sowry]

The diagram is the usual way DC is wired.

??? Not in this part of the world (where DC analogue reigns supreme).

I leave the 'main line' alone, letting the control always be consistant (switchable, of course, from controller) on that line. I use a DPDT switch to draw power from the main to power the loop. That way, I'm not constrained by 'one way' running. My power switch is adjacent to the points, operating rule is that the switch should point in the same direction as the turnout is thrown (yes, I know, that can all be done through LGB point motor and switching).

However, this is starting to thread drift.... so I'll leave it there.

 

[Cliff George]

I leave the 'main line' alone, letting the control always be consistant (switchable, of course, from controller) on that line. I use a DPDT switch to draw power from the main to power the loop. That way, I'm not constrained by 'one way' running. My power switch is adjacent to the points, operating rule is that the switch should point in the same direction as the turnout is thrown (yes, I know, that can all be done through LGB point motor and switching).

Yes, that works but presumably to you would need to stop the train in the loop, switch the DPDT and then drive the train out by using the controller in the opposite direction?

 

[Cliff George]

So, essentially, if the loop is "correct" polarity for the train when it enters the loop, nothing "happens" and everything keeps on going as it was. As the first element (I would think almost always a loco) starts to leave the loop, once the first set of electrically active wheels cross the divide, the entire loop changes polarity to meet the new requirements. As DCC is a very clever Dc / AC hybrid, the controllers keep the train running in the correct direction, and lights and any other carriage related items doing what they should ?

Yes, switching the phase/polarity under a DCC train and it keeps going in the same direction. Switch the polarity under an analogue train and it will immediately reverse direction.

 

[PhilP]

Yes, switching the phase/polarity under a DCC train and it keeps going in the same direction. Switch the polarity under an analogue train and it will immediately reverse direction.

And possibly strip the gears!! - Yes, I know this was for illustrative purposes, but have seen it done.. Sheer cruelty to dumb mechanicals..

 

[playmofire]

Yes, that works but presumably to you would need to stop the train in the loop, switch the DPDT and then drive the train out by using the controller in the opposite direction?

So a signal somewhere in the loop would do that, stop the train and, while the train is standing at the signal (an obvious precaution as it is going to run back on to the line it has just left when travelling in the opposite direction), the controller can be put into "reverse", the DPDT switch thrown and when the signal is changed the train continues forwards and back on to the line.