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Author Topic: Common Return  (Read 274 times)

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Offline Frenchie

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Common Return
« on: July 17, 2019, 02:44:14 PM »
I'm using a Gaugemaster Model D controller and could do with a bit of help on two points:

1. The layout will have about 20 street lights, 12v wired in pairs, and about the same number of house/station lights, probably LEDs. Can the controller handle that?

2. Can I use a common return for both track and 12v accessories, including points position LEDs controlled by Peco PL13 switches, or do I need to keep the track and the 12v uncontrolled returns separate?

Offline chrism

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Re: Common Return
« Reply #1 on: July 17, 2019, 03:19:12 PM »
2. Can I use a common return for both track and 12v accessories, including points position LEDs controlled by Peco PL13 switches, or do I need to keep the track and the 12v uncontrolled returns separate?

Don't know whether you can technically, but I'd be inclined to keep them completely separate.
At the very least it could be handy for faultfinding - I'd hate to be, for example, searching for a short in the track only to find that it was actually a bulb that had blown short circuit in the lighting system.

Offline ntpntpntp

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Re: Common Return
« Reply #2 on: July 17, 2019, 06:52:11 PM »
As I understand it,  for common return to work properly each circuit actually has to be supplied by a separate transformer secondary winding. Whether or not the accessory output of the controller is indeed a separate winding is something you'd have to try and check in the instructions or by asking Gaugemaster?   

To be honest though I strongly agree with @chrism , keep trackwork and accessory supplies separate. You're only saving one wire after all, and potentially mixing AC and DC, so the benefits of complete separate circuits when fault finding etc. far outweigh the slight saving.

You ask whether the controller can supply all the lighting you plan to use?  It really does depend whether you will use LEDs, grain of wheat, or grain of rice bulbs. The current consumption can be quite different.  Also bear in mind you can string stuff in series and share the current as long as the voltage requirements of the items in each series string add up to the output voltage of the supply.
Nick.   2016 celebrating the 20th anniversary of "Königshafen" exhibition layout!

Offline Nigel Cliffe

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Re: Common Return
« Reply #3 on: July 17, 2019, 07:30:19 PM »
I tend to agree with the post above, keep it separate. 

However, a question to Gaugemaster's technical support might answer things definitively.   
I think the answer is "can't if you mean "common return the lights in addition to cab switching between both controllers".   I believe the model D has two windings (hence can common return the two controllers), one winding supplies the 16vAC accessory output, and the second the 12vDC output.   The problem arises if you want to common the 12vDC accessory output with its controlled output - in one direction of the controller you will short-circuit that side of the controller and transformer windings.

In terms of power consumption, modern LEDs have an absolute maximum of 20-25mA (when they'll be burningly bright).   The current consumption is dictated by the series resistor, so, if using a typical 1kOhm resistor and assuming 3v forward drop at the Diode, and a 12v power source, the drop over the resistor is 12-3=9v,  using V=IR, substituting for 1k Resistor and 9v, we get I=0.009A (9mA).  Call it 10mA because the maths is quicker.  40 lamps is 40*10=400mA or 0.4A.  Allow decent headroom on power supply and you need 600mA power supply or greater.  So a 12vDC/1A powerbrick will be ideal.   A 0.5A brick is probably OK, but getting marginal.     
Or, if using the Gaugemaster DC output without common return of the accessories, then you're using about half its total power output, which will be fine running an N loco at the same time.


Offline springwood

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Re: Common Return
« Reply #4 on: July 17, 2019, 07:59:15 PM »
I am using a Model D for two of my cabs and an AGW PE144 for the other two. On the Model D I use the track 1 and 2 terminals for track feeds and have a common return from all sections of track back to the -'ve side of track 1 and 2.

I use the 16v AC outputs for all my points (in conjunction with a CDU and point indicator boards), again with common returns collected and routed into the -'ve OUT side of the CDU.

I have attached the technical spec with full details.

Offline Frenchie

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Re: Common Return
« Reply #5 on: July 17, 2019, 08:19:20 PM »
Thank you, people! Just what I needed. There is a lot of spaghetti and copper tape underneath on this layout because it’s the first time I’ve tried lighting and I’m trying to reduce it as much as I can.

I’ll go with my first instincts and keep ‘em separate!

My interest in lighting the layout started with an article (series of?) a couple of years ago but I can’t now find it. Has anyone any idea about the author? I seem to remember it was a lady but beyond that ...

Offline themadhippy

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Re: Common Return
« Reply #6 on: July 17, 2019, 10:09:08 PM »
if you do combine make the common wire bigger than the feed
Quote
has to be supplied by a separate transformer secondary winding.
if both secondarys  are seperate  there isnt  a current flow path between either so a common return wont work,connect 1 side of each secondary and all will be good

Online Bealman

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Re: Common Return
« Reply #7 on: July 18, 2019, 12:12:39 AM »
Sounds like it may have been retired mod Pengi
Vision over visibility. Bono, U2.

Offline sprogman

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Re: Common Return
« Reply #8 on: July 18, 2019, 10:08:07 AM »
if you do combine make the common wire bigger than the feed
Quote
has to be supplied by a separate transformer secondary winding.
if both secondarys  are seperate  there isnt  a current flow path between either so a common return wont work,connect 1 side of each secondary and all will be good

Surely the common is the common return at the rails, not the transformer winding?

I'm DCC so don't ever do common return, although you do have a common, or home, ground connection.

Offline ntpntpntp

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Re: Common Return
« Reply #9 on: July 18, 2019, 12:55:57 PM »
if you do combine make the common wire bigger than the feed
Quote
has to be supplied by a separate transformer secondary winding.
if both secondarys  are seperate  there isnt  a current flow path between either so a common return wont work,connect 1 side of each secondary and all will be good

Surely the common is the common return at the rails, not the transformer winding?

Yes, that's how I would treat it.  I have run common return systems in the distant past, but these days I prefer to have double isolated trackwork and wiring to allow me to mix and match any control system I want.
Nick.   2016 celebrating the 20th anniversary of "Königshafen" exhibition layout!

 

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