Common return wire

[Dale]

Just a quick question for the electrically minded folks out there. I have read that a common return can be used for things like lighting, points and signals in order to minimise wiring under the baseboard.

 

My question is “does a common return enable you to return both DC (layout lighting) and AC (points) as AC’s polarity is constantly switching?â€

 

Obviously I am no electrical guru so offer this one to the lions…. educate me gents.

 

Dale.

[kevinlms]

Here is a good explanation of how Common Return works.

 

http://rail.felgall.com/crw.htm

 

Also the advantages over the Single power supply or Split power supply systems. Both of these belong with the Dodo, IMO and definitely unsuitable for transistorised controllers. The main reason is that the power supply won't be the item to give up, when rather than if, you have a short. The loco (or whatever) will act as a fuse for the power supply!

 

The rule you have to remember is that you MUST use separate power supplies for each controller. Plug packs are ideal.

 

You can also use AC to control points, but why would you if using solenoids? Much better to use a CDU, which puts a higher voltage DC and with current limiting if a point(s) for whatever reason is continually switched and supplied with power.

 

 

If you are thinking of going DCC in the future, the recommendation is to NOT use Common Return. You MAY get away with it on a small layout, (where you can simply turn all block switches on to the same controller & plug the DCC unit into that power source), but on a more complex layout, it is a decision likely to haunt you, requiring extensive modification to your wiring, after coming to grief.

 

Kevin Martin

[Dale]

Thanks for the response Kevin.

 

In all honesty I had not considered the effect of the CDU. Like many I have a Gaugemaster transformer wired to a CDU and then out to the point motors (SEEP) and focused on the AC output from the transformer. Am I to understand that the CDU provides a short, high amp DC burst of power then?

 

If that's the case then I can use the return for my points and wire in all the returns from my DC layout lighting to the same wire?

 

My DCC bus is separate and scared. Naught shall go near the DCC bus except track feeds. Blessed be the DCC Bus LOL.

 

D.

[cpman46]

Dale

 

CDU = Capacitor Discharge Unit as you surmise provides a short high power pulse to the point motor solenoid. All the returns from your points should come back to the CDU common. You don't say what you are using to power your lighting but I would not use the same common return as your point motors. Use a separate common return with a wire suitable to handle the total current for all of your lighting circuits See Brian Lamberts site http://www.brian-lambert.co.uk for a wealth of information regarding electrics, points, lighting etc.

 

I'm interested to know what your DCC bus has to be 'scared' about!

Regards

Mike

[kevinlms]

Dale

 

CDU = Capacitor Discharge Unit as you surmise provides a short high power pulse to the point motor solenoid. All the returns from your points should come back to the CDU common. You don't say what you are using to power your lighting but I would not use the same common return as your point motors. Use a separate common return with a wire suitable to handle the total current for all of your lighting circuits See Brian Lamberts site http://www.brian-lambert.co.uk for a wealth of information regarding electrics, points, lighting etc.

 

I'm interested to know what your DCC bus has to be 'scared' about!

Regards

Mike

 

I see no problem 'as long as the common return has sufficient capacity', for the points, lights and whatever, AT THE SAME TIME.

 

Brian gives some idea of the wire sizes required. If in doubt, go bigger or double up. A lot depends on the size of your layout. Automotive cable is a good source, as often comes in pairs.

 

http://www.brian-lambert.co.uk/Electrical.htm#Wire

 

 

Kevin Martin

[davefrk]

Here is a good explanation of how Common Return works.

 

http://rail.felgall.com/crw.htm

 

Also the advantages over the Single power supply or Split power supply systems. Both of these belong with the Dodo, IMO and definitely unsuitable for transistorised controllers. The main reason is that the power supply won't be the item to give up, when rather than if, you have a short. The loco (or whatever) will act as a fuse for the power supply!

 

The rule you have to remember is that you MUST use separate power supplies for each controller. Plug packs are ideal.

 

You can also use AC to control points, but why would you if using solenoids? Much better to use a CDU, which puts a higher voltage DC and with current limiting if a point(s) for whatever reason is continually switched and supplied with power.

 

 

If you are thinking of going DCC in the future, the recommendation is to NOT use Common Return. You MAY get away with it on a small layout, (where you can simply turn all block switches on to the same controller & plug the DCC unit into that power source), but on a more complex layout, it is a decision likely to haunt you, requiring extensive modification to your wiring, after coming to grief.

 

Kevin Martin

 

Hi Kevin and Dale, just studied the link to the example of common return wiring. Sorry but I think it's a particularly bad example, why, because the diagrams show all the controllers connected to the same piece of track implying that one can control two or three locos on the one track. The diagram would be much better showing separate tracks side by side as in a multi circuit layout, this would get the message over to the less understanding or Newbies in the hobby.

I'm involved with a large exhibition layout which was done with common return but in my humble opinion it's less complicated having double pole section switches to each track section, okay there's more wires but it does make fault finding much easier.

Perhaps the given diagram would work in DCC if that's what is intended but the the wiring would be much simpler than the diagram, all very confusing to the novice.

Also, I wouldn't put lighting on common return with anything else as when you switch a point or signal you may well get a significant flicker in the lighting...

Just my thoughts and experiences,

 

Dave Franks.

[Dale]

I'm interested to know what your DCC bus has to be 'scared' about!

 

That was supposed to say Sacred but my rushed typing let me down LOL.

 

My natural OCD approach to things would have all of my loops separated so each function was clearly identified and as has been mentioned, fault finding is easier. My reason to look at the common return has arisen through building the control box for my first layout, a small tail chaser aimed at the kids. Even with 7 points I have had my eyes opned by the amount of wiring and connections required to build the box, specifically the connectors from the box to the baseboards. I am not a fan of D connectors and working in industry, have searched for something more robust and locking. Settling on 7 pin DIN connectors for this job I am worried that when the time comes to build the box for my big layout the connectivity is going to be an issue.

 

Common wire return means less panel-boards connectivity requirements but in the end, if i have 20+ points, DCC bus, lighting, programming track isolation, off track automations, hidden sound etc, the wire runs and looming will be a nightmare anyway. Heaven knows what sort of connectors i will end up with then, a 50mm gland and SWA Multicore?

 

D.

[Brian]

Hi Dale

22mm copper pipe with soldered elbows for corners should make a good common return conductor! Just joking.

 

So long as all your feeds come from totally separate transformers or separate windings on dual/triple wound transformers and the common return wire is of a larger enough wire size, then the wire saved over convention wiring can be vast.

I recommend where a common return wire is used for both track feeds and point motors plus anything else (Lighting etc) then the wire size should be at least 1.5mm² solid or 32/0.2mm flexible . Remember...... A larger wire size is always better than one that's too small!

[Dale]

Remember...... A larger wire size is always better than one that's too small!

 

You sound like my wife mate... its always about the size!

 

Seriously though and whilst the thread is rumbling on... wire sizes: I am using code 75 on my main 'serious' layout and to avoid huge weld's (sorry, solders) on the track sides I was planning on 7/0.2 droppers to a solid core 2.5mm² bus wire.

 

My SEEP points on the kid's 6'x4' layout have 16/0.2 wires out across the board to choc block at the point. I have put the choc bloc in so if i have to change a point motor out its an easy job but the local wire at the point motor is again 7/0.2

 

The points on my grown up layout are Cobalt so I have not even considered what wire will be needed for these blue beasties.

 

The kid's DC bus for lighting is also the same as the points, with 16/0.2mm bus wires but with 7/0.2 feeds to the lights etc.

 

Have i made a hash of things as this wiring is still in progress and its not too late to change things.

 

D.

[Brian]

Hi Dale,

Your on the right side with your bus wires.

Droppers of 7/0.2mm are ok so long as they are kept short Approx no more than 300mm overall. Otherwise increase their size to 16/0.2mm.

Solder droppers to the rails undersides whenever possible. But where track is already laid then you'll have to solder to the outer rail web.

16/0.2mm is ok for solenoid point motors where the wire run is no more than approx 4 mtrs over this I would increase it to 24/0.2mm or 32/0.2mm.

Cobalt motor draw little power running and around 30milliamp stalled so 7/0.2mm is fine for them

[PLD]

My usual method is to use a seperate common return for each job so for example while there will be one return wire common to all point motors this will be a seperate with to the return for traction power and the return for Electro-magnets.

 

That way there can be no worries about mixing voltages or current forms, and if there is a fault it only takes out one function rather than the whole layout...

[Donw]

My usual method is to use a seperate common return for each job so for example while there will be one return wire common to all point motors this will be a seperate with to the Commo return for traction power and the return for Electro-magnets.

 

That way there can be no worries about mixing voltages or current forms, and if there is a fault it only takes out one function rather than the whole layout...

 

Echo that It makes it much easier to understand what you are doing. Also if you mix DCC with AC or DC turnouts ( i.e not via DCC decoders) there is a danger of 'noise' affecting the signals. Common return for point motors can make a lot of sense especially as usually only one is being fired at a time so the return current is low. It also makes sense for DC cab control where otherwise you double the number of switch contacts. DCC is effectively common feed as well as common return.

Don

[Baby Deltic]

The trouble with mixing AC and DC in a circuit is that the AC tends to 'sit on' the DC level.

[Ron Solly]

The trouble with mixing AC and DC in a circuit is that the AC tends to 'sit on' the DC level.

 

Things must have changed coz when Ii worked for Australian Telecom, the telephone exchanges had DC of different voltages and AC of low and high frequencies all utliizing a common return without any problems.

 

As long as each supply does have its own secondary coil output, common return is no problem.

My common return has not only the DCC but 7 other independent supplies of varying voltages of DC & AC.

 

Of course if one is not familar with multiple power supplies & the process of keeping things isolated as required, then don't do it. Electricity is not a black art, just one that takes some time to learn about it.

[Baby Deltic]

All I'm saying is that if you hook an oscilloscope up to a circuit with AC in it, the waveform will move up with the DC level when you add DC. There is not real problem with having a common ground as such. The only time I would be careful is if track is wired with a common return on straight DC control and you are using polarity reversal as a means of direction control rather than using a dual polarity push-pull output controller.

[34theletterbetweenB&D]

...My DCC bus is separate and sacred. Naught shall go near the DCC bus except track feeds. Blessed be the DCC Bus ..

The national bus has three AC phases on four wires one of which is the common return for he other three, and because the load on each phase is very closely matched there is a much smaller current in the common return wire than on the 'live' side. The electron knows what it is doing, even if we can't quite get our minds around it.

 

You can have multiple DC or AC currents (provided each derives from separate transformer windings) on a single layout DCC bus no trouble at all, provided the bus has the maximum current carrying capacity for the combined load. Makes life so simple, as one wire is the feed for any particular application and 'everything' then returns on the bus. Half the labelling required for fault tracing, etc.

[Jeffbam]

common return problems, I am progressing well with the wiring but have now got to the 7th section on the third baseboard which is near a pair of points, when one or two of the points are thrown a fault (short) is shown on the controllers (both switched independently I suspect the common return shorting to positive through the blades. The points and controllers have separate return wires, can anyone help

[Ron Solly]

Jeff ,did you wire the points/turnouts like this

http://brian-lambert.co.uk/Electrical%20page%202.html#Live

 

if wired correctly. common return wiring will not affect it

[DCB]

Sounds like you have missed an isolator at the point frog rather than common return problems