The diagram is correct because as Greg noted, the connections on the bottom of the switch are opposite to the handle position. When the handle is to the left, the center and right terminals are connected.
Common rail works because when you connect two transformers together common wire becomes just like the neutral wire in your home that gives you two different phases of 120 volts and also gives you 240 volts.
My DC layout uses 12 volt/24 volt system like that for signaling, control relay power, switch machine power and more. I control two color signals (two separate LED's or lamps) with just two wires. If the input wire is "postitve" the red lamp lights, if it is "negative" the green lamp lights. The other wire is the common for both.
Now, one or two more thoughts. Common rail dual cab control will not work with most older traditional "dual packs" that have two speed controls - because they typically contain only one transformer - you need two separate power sources for the common wire principal to work.
Next, common wire was popular in the early days of the hobby, but few advanced DC users still use it today. The list of reasons is long and a bit complex for those with little electrical background, so I will save that info in case someone actually wants to know.
But I will give a few hints. There are modern DC throttles that provide DCC level speed control - they don't work correctly tied to other throttles. There are ways to provide constant lighting, even to stopped trains, those don't work with common rail. There is a way with DC control to provide collision protection that is basically free and built in - it requires that you not use common rail.
Sheldon
chatanuga wrench567 With DC layouts, running multiple cabs, there can't be a common wire. With a common wire when you change directions the polarity switches and the whole layout would change instead of the district controlled by the switch. Pete. That's one thing I never understood about common-cab wiring. I can see it working if everything is moving in the same direction, but it just seems like if you reverse one transformer/power pack, there's going to be a short between them. Kevin
wrench567 With DC layouts, running multiple cabs, there can't be a common wire. With a common wire when you change directions the polarity switches and the whole layout would change instead of the district controlled by the switch. Pete.
With DC layouts, running multiple cabs, there can't be a common wire. With a common wire when you change directions the polarity switches and the whole layout would change instead of the district controlled by the switch.
Pete.
That's one thing I never understood about common-cab wiring. I can see it working if everything is moving in the same direction, but it just seems like if you reverse one transformer/power pack, there's going to be a short between them.
Kevin
The reason common rail works is because polarity is relative. The '+' of a power pack is not inherently '+', it's '+' relative to the '-' of that at power pack, and the same is true of the '-'. So, if you have the '-' of one power pack and the '+' of another power pack both wired to the common, then the common becomes the '-' of the first power pack and the '+' of the second.
wrench567You can run two locomotives in the same direction only.
of course they can run in opposite direcitons because current flows around a complete "circuit" requiring both (2) connections to each cab.
while both cabs share a common connection with one rail, they are isolated from one another on the other rail. both rails need to be isolated only for reverse loops.
Linn Westcott explains how to do this in an HO Railroad the Grows (1972). Atlas sells products for doing this
greg - Philadelphia & Reading / Reading
wrench567With DC layouts, running multiple cabs, there can't be a common wire.
I ran my rather extensive DC layout with common rail (i.e. one side of the circuit common to all cabs) and changed locomotive direction with no issues. I had seven cabs but not all blocks used all seven. The main lines each had two cabs but there was some shared trackage in yards and the roundhouse where I had maybe five cabs selectable on a rotary switch.
I used one side of a DPDT center off switch so effectively a SPDT cab selector switch. I see nothing wrong with the diagram Greg posted. Yes, if you have an engine going into an adjoining 'block', bridging the gap, you have to have the selector switch aligned to 'your' assigned cab. You might get a short or if the other power pack is in the same phase you might get a sudden increase in speed depending on the output the other cab is making.
In 2005 I began rewiring the layout for DCC and this necessated cutting gaps in the common rail, although on a small layout that might not be entirely necessary if you only use one command station/booster.
Good Luck, Ed
The way this is wired. You can run two locomotives in the same direction only. There will be no way of bi- directional running. Might be good for going around the Christmas tree but a lot of work and money for going around in circles. If you go with DPDT switches, you can get bi- directional travel.
wyatteeTHE YELLOW WIRING SHOULD CONNECT TO THE "A" SIDE OF EACH SPDT AND THE GREEN WIRING TO THE "B" SIDE OF EACH SPDT.
depends if the A/B labels indicate the terminal position or the switch handle position, they are are typically opposite
Interesting thread. It shows me what I don't know about DC which is just about everything.
Rich
Alton Junction
this shows common practice.
chatanugabut it just seems like if you reverse one transformer/power pack, there's going to be a short between them.
there's only a possibly short when a metal wheel bridges the gap between two blocks controlled by separate cabs
http://chatanuga.org/RailPage.html
http://chatanuga.org/WLMR.html
I'm not a DC guy nor have I read the article, but you should write to the magazine with your challenge to the wiring diagram.
I UNDERSTAND THE WHOLE CONCEPT OF BEING ABLE TO OPERATE FROM TWO DIFFERENT DC CAB CONTROLS ON THE LOOP EXAMPLE, BUT I THINK THE WIRING FROM THE TWO CABS A & B IS BACKWARDS. MEANING THE YELLOW WIRING SHOULD CONNECT TO THE "A" SIDE OF EACH SPDT AND THE GREEN WIRING TO THE "B" SIDE OF EACH SPDT. ALSO, THE ARTICLE SAYS USE "DPDT" SWITCHES BUT I DON'T SEE WHY SINCE THE BLUE WIRING FROM BOTH CAB'S IS COMMON.