Track Polarity

[trucknightmares]

Still in layout building stage, so sorry if this has been asked already, but...

 

Does the track polarity remain constant on dcc?

 

If so,

is there any reason I can't/shouldn't run my analogue loco using overhead cantenery wires as the positive (and connected to an analogue controller) and the track as negative, thus allowing me to run alongside a dcc loco?

 

At present, looking to swap the motorised bogie for a trailing bogie allowing free movement for ease of shunting.

[kevinlms]

3 minutes ago, trucknightmares said:

Still in layout building stage, so sorry if this has been asked already, but...

 

Does the track polarity remain constant on dcc?

 

If so,

is there any reason I can't/shouldn't run my analogue loco using overhead cantenery wires as the positive (and connected to an analogue controller) and the track as negative, thus allowing me to run alongside a dcc loco?

 

At present, looking to swap the motorised bogie for a trailing bogie allowing free movement for ease of shunting.

Not a good idea at all, to mix DCC & analog DC on the same part of the layout. Either put a chip in the loco & connect the overhead (assuming you want to power the overhead) to one side of the track or run a separate loop as DC only.

[Izzy]

16 minutes ago, trucknightmares said:

Still in layout building stage, so sorry if this has been asked already, but...

 

Does the track polarity remain constant on dcc?

 

 

No, it’s AC.

 

Izzy

[trucknightmares]

Ok thanks.

 

It was just a thought as I think the loco are too old to be converted (20+ years) so was looking for alternatives.

 

Guess I'll stick to swapping the motorised bogie for non-motorised so there'll be no resistance when being shunted around.

[RobjUK]

43 minutes ago, trucknightmares said:

It was just a thought as I think the loco are too old to be converted (20+ years)

 

Some that are over 50 years old can be converted just as easily as more modern locos.

Give details and/or take some photos and I'm sure you will find someone has done the same or a similar conversion already.

 

[34theletterbetweenB&D]

3 hours ago, trucknightmares said:

...is there any reason I can't/shouldn't run my analogue loco using overhead cantenery wires as the positive (and connected to an analogue controller) and the track as negative, thus allowing me to run alongside a dcc loco?...

You would only be able to use one rail as negative. If the analogue loco is currently fitted with live axles (usual way to arrange return via both rails) then the wheelsets would have to be changed to insulated. The loco would only be able to go one way around on the track, unless fitted with a switch to select one side or the other as rail connection.

 

[trucknightmares]

Well, I'm confused, but laughing.

 

I've got 3 locos, class 08, 47 and an 86 (with working pantograph).

 

I'll think about conversion, but as I'm looking at a dcc fitted class 68, I'll keep these as static for the maintenance shed I'm planning to add.

[trucknightmares]

3 minutes ago, 34theletterbetweenB&D said:

You would only be able to use one rail as negative. If the analogue loco is currently fitted with live axles (usual way to arrange return via both rails) then the wheelsets would have to be changed to insulated. The loco would only be able to go one way around on the track, unless fitted with a switch to select one side or the other as rail connection.

 

 

That's what I was initially trying to work out - but didn't know if DCC used one rail as positive and the other as negative or of they changed depending on loco direction. 

 

I've not used dcc before so still trying to figure things out - the fact I still need to build a baseboard doesn't help either! 

[Phil S]

LIVE overhead is NOT recommended by any manufacturer, for digital model railways - because of several factors:

1/ Reliability of contact for the serial data content  ..... 'sparking' as with the real thing representing a failure to maintain contact.

2/ With 2-rail (as opposed to 3-rail/centre-stud Marklin) - then there is a 50-50 chance only of the 'live wheels' being placed on the correct rail.

3/ There is a risk of double voltage or no-voltage - if you attempt to uise 2 differing circuits (overhead or the other rail) with a common rail return.

4/ For storage and tunnelled areas of a layout, it would require continuous overhead catenery - restricting access and increasing cost

5/ partly a repeat of the contact problem - many model railways with overhead run with pantographs DOWN or below line height.

 

And on the other matter...

Because it is an alternating signal, 'phase' rather than 'polarity' would be a better term -  as in the equivalent of Live and neutral phases on 'mains' electricity (mains is not a term used in the usa for electricity though - it is reserved for water, I believe)  The 'problem' being that the colours of the wires don't alternate -[ unless youi  have created a short circuit by mistake  8-) ] - and many consider using 'red' and 'black with inferences of dc analogue wiring    (  not so many now thinking of it for ac wiring as in the past )  .... one reason I use brown and blue.

 

Perhaps a 'clearer' description to help overcome the confusion some have, might be to describe it as a 'dc-coupled- AC waveform' because, although in normal use, it has 0 average voltage  (as with an ac-coupled waveform) -  when the waveform is made assymetric with a 'loco 0' at non-zero-speed, it does then have a non-zero-average [ IF your controller permits loco zero - but the standard does, so a compatible description is useful }.   Also, it is preferable if there is no 'tilt' on the square wave - as can appear with ac-coupled connections of signals .... but this could lead to the sort of discussion found in hi-fi magazines when discussing amplifiers and how much oxygen is in the copper cable ?? 8-).   I think the 'bistable dc term was a crude attempt to depict the dc-coupled nature of the alternating (therefore ac) waveform  ... whose frequency response extends to dc.

[34theletterbetweenB&D]

4 minutes ago, trucknightmares said:

... didn't know if DCC used one rail as positive and the other as negative or of they changed depending on loco direction...

DCC puts a constant power supply on the rails. The DCC decoder in each loco decodes embedded signals in the DCC supply, and alters the DC output to the motor to change direction and vary speed. All DCC locos operate independently on the same track. (You may have head on collisions if you wish.)

 

Now you have revealed that the electric loco is a class 86 (I was wondering if it was an HO electric) a far simpler solution can be suggested. Convert this loco to DCC using rail supply. Fit unpowered catenary, and run it with the panto up. Far and away the easiest thing to do.

[Phil S]

IF you powered your dcc controlled overhead electric loco from the overhead, you would not be able to issue a dcc command to raise the pantograoph once you had lowered it 8-)

even in H0 there are commercial models which have dcc controlled pantograph. .. although it is more common in general scale ... where, in the garden, it is less likely to have any overhead contact wire ... maybe just the marts 8-)

[Pete the Elaner]

7 minutes ago, Phil S said:

LIVE overhead is NOT recommended by any manufacturer, for digital model railways - because of several factors:

1/ Reliability of contact for the serial data content  ..... 'sparking' as with the real thing representing a failure to maintain contact.

2/ With 2-rail (as opposed to 3-rail/centre-stud Marklin) - then there is a 50-50 chance only of the 'live wheels' being placed on the correct rail.

3/ There is a risk of double voltage or no-voltage - if you attempt to uise 2 differing circuits (overhead or the other rail) with a common rail return.

4/ For storage and tunnelled areas of a layout, it would require continuous overhead catenery - restricting access and increasing cost

5/ partly a repeat of the contact problem - many model railways with overhead run with pantographs DOWN or below line height.

 

And on the other matter...

Because it is an alternating signal, 'phase' rather than 'polarity' would be a better term -  as in the equivalent of Live and neutral phases on 'mains' electricity (mains is not a term used in the usa for electricity though - it is reserved for water, I believe)  The 'problem' being that the colours of the wires don't alternate -[ unless youi  have created a short circuit by mistake  8-) ] - and many consider using 'red' and 'black with inferences of dc analogue wiring    (  not so many now thinking of it for ac wiring as in the past )  .... one reason I use brown and blue.

 

Perhaps a 'clearer' description to help overcome the confusion some have, might be to describe it as a 'dc-coupled- AC waveform' because, although in normal use, it has 0 average voltage  (as with an ac-coupled waveform) -  when the waveform is made assymetric with a 'loco 0' at non-zero-speed, it does then have a non-zero-average [ IF your controller permits loco zero - but the standard does, so a compatible description is useful }.   Also, it is preferable if there is no 'tilt' on the square wave - as can appear with ac-coupled connections of signals .... but this could lead to the sort of discussion found in hi-fi magazines when discussing amplifiers and how much oxygen is in the copper cable ?? 8-).   I think the 'bistable dc term was a crude attempt to depict the dc-coupled nature of the alternating (therefore ac) waveform  ... whose frequency response extends to dc.

Some excellent points there.

I had an OLE layout once (& plan to again). Many people asked me if re-railing was an issue under the wires. I never found this a problem but cleaning the track without damaging a wire was very fiddly. If you can avoid wires away from the scenic section, then this is very desirable.

 

1 contact point on a pantograph may be ok with 25kV, but multiple wheels are much better on a 0-12v, or even DCC model.

 

When connecting 2 DC controllers for common return, these must be isolated by using separate windings (which effectively makes them a 'floating' 12v difference), then connected together at their outputs, therefore -12 - 0 - 12v. Some modellers are happy with this & some are not.

With all the electronic trickery inside a DCC system, I would not want to try connecting mine to something else in this way?

[jpendle]

To answer the OP's original question, track polarity is NOT constant on DCC.

 

Regards

[trucknightmares]

Ok guys, lots of info to process...

 

I'm running all Hornby in OO gauge

with DCC to be via Hornby RailMaster and eLink.

 

But from the comments here and my own research, I'll stick to the 86 being a display model - I've started a conversion topic just in case

Edited April 12, 2019 by trucknightmares
Removed quote

[trucknightmares]

2 minutes ago, jpendle said:

To answer the OP's original question, track polarity is NOT constant on DCC.

 

Regards

That's all I needed to know - thanks!

 

And other members' responses have been very insightful so thanks to them also.

[trucknightmares]

44 minutes ago, Phil S said:

IF you powered your dcc controlled overhead electric loco from the overhead, you would not be able to issue a dcc command to raise the pantograoph once you had lowered it 8-)

even in H0 there are commercial models which have dcc controlled pantograph. .. although it is more common in general scale ... where, in the garden, it is less likely to have any overhead contact wire ... maybe just the marts 8-)

 

The pantograph is a manual raise/lower so can easily be left in the lowered position.

[Graham Radish]

8 hours ago, Izzy said:

 

No, it’s AC.

 

Izzy

DCC current is actually bipolar DC not AC

[newbryford]

10 hours ago, Izzy said:

 

No, it’s AC.

 

Izzy

 

DCC is not your "average" AC - it is a high frequency square wave.

 

A few images here:

https://dccwiki.com/DCC_Power

 

Being pedantic, a square wave is an infinite number of sine waves....................

 

Cheers,

Mick

 

[Izzy]

10 hours ago, Graham Radish said:

DCC current is actually bipolar DC not AC

 

8 hours ago, newbryford said:

 

DCC is not your "average" AC - it is a high frequency square wave.

 

A few images here:

https://dccwiki.com/DCC_Power

 

Being pedantic, a square wave is an infinite number of sine waves....................

 

Cheers,

Mick

 

 

This may all be true, but in the context of the basic question the OP asked would it have added anything to the answer? Made any difference. I tried to keep it simple, perhaps too much.

 

Izzy

[newbryford]

13 minutes ago, Izzy said:

 

 

This may all be true, but in the context of the basic question the OP asked would it have added anything to the answer? Made any difference. I tried to keep it simple, perhaps too much.

 

Izzy

 

But "keeping it simple" perpetuates the myth that DCC is AC...…………..

[34theletterbetweenB&D]

18 hours ago, Pete the Elaner said:

...When connecting 2 DC controllers for common return, these must be isolated by using separate windings (which effectively makes them a 'floating' 12v difference), then connected together at their outputs, therefore -12 - 0 - 12v. Some modellers are happy with this & some are not.

With all the electronic trickery inside a DCC system, I would not want to try connecting mine to something else in this way?

There is something of a phobia around this. But provided that 'the rules' are followed: most critically that every connected device wired common return on the layout is powered from an independent transformer winding, then all is well. The electrons know where to go, you don't have to worry about it.

 

Over fifteen years now of long established practise in switched section common return 'cab control' wiring which typically is run all DCC, but when required any group of sections may be allocated to a DC cab - typically to test a single mechanism on DC ahead of decoder fitting - while all the other sections continue running with DCC: and never a single problem.

[Gordon H]

23 hours ago, Phil S said:

LIVE overhead is NOT recommended by any manufacturer, for digital model railways - because of several factors:

1/ Reliability of contact for the serial data content  ..... 'sparking' as with the real thing representing a failure to maintain contact.

2/ With 2-rail (as opposed to 3-rail/centre-stud Marklin) - then there is a 50-50 chance only of the 'live wheels' being placed on the correct rail.

3/ There is a risk of double voltage or no-voltage - if you attempt to uise 2 differing circuits (overhead or the other rail) with a common rail return.

4/ For storage and tunnelled areas of a layout, it would require continuous overhead catenery - restricting access and increasing cost

5/ partly a repeat of the contact problem - many model railways with overhead run with pantographs DOWN or below line height.

 

 

Manufacturers always have to assume their customers don't understand such matters in order to avoid the endless questions that might arise when something goes wrong.

Those who have contemplated working overhead wires before will already have identified most of the issues involved, not least of which is the 'live wheels' side issue. Many of these perceived issues are no different whatever the supply method.

Even the live side issue can be overcome with a bit of engineering. For the Nottingham club layout 'Carstairs' I devised a relay board which would adjust the track supply polarity on the Edinburgh line depending on the combination of point settings around the triangle.

The double voltage DCC issue could only arise when two independent boosters are being used, and happen to be out of phase when the boundary is crossed. However, this is really no different to what could happen with standard two-rail supplies in similar circumstances, especially if pick-up is only on opposite bogies for each rail.

With a properly arranged pantograph head there will be at least two points of contact, so the issue there is no worse than a two-wheel bogie pick-up. If anything the pantograph arrangement should be better because it is naturally self-cleaning due to the continuous wiping action.

[Graham Radish]

In order for great success with dcc its advisable to know how everything works, not understanding the basics ends up in a world of pain and expense.

Edited April 26, 2019 by Graham Radish

[chaz]

I would forget about using the catenary as a conductor, much safer for DCC - disconnect the pantographs and use conventional pickups. 

 

A conversation I once overheard at a show between a layout operator and a visitor might help. The latter asked "Do your locos pick up power from the wires?" The operator answered with a most telling response - "Can't you tell?". Unfortunately the visitor missed the point!

 

Chaz

Edited May 2, 2019 by chaz