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DC wiring question on a new layout

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Posted by Lastspikemike on Sunday, June 13, 2021 2:18 PM

CSX Robert

 

 
Lastspikemike
The two "up down" coloured schematics seem to show the short circuit I refer to. The opposite polarities of the powerpacks connect at the Common terminal. That looks to me like a dead short. Why is it not?

 

This is confusing for many people when they first look into common rail wiring.  Take for example a single 12 volt isolated power supply.  The '+' terminal on that power supply by itself is not actually +12 volts, it is +12 volts relative to the '-' terminal of that power supply.  The voltage at any single point has to be measured relative to some reference point.  With two separate isolated power supplies there is no common reference point.  With common rail cab control wiring, you are creating a common reference point - the common rail.  The control rail can be made positive or negative relative to the common rail by switching the polarity of the cab controlling that rail.

It's kind of funny in way, many people have trouble with hooking the '+' of one power supply to the '-' of another, yet most of them have done the same thing putting batteries in a flshlight without thinking twice about it.

 

That's very helpful to me. I was working my way through the difference between potential and polarity as a way of reconciling the contradiction. 

Funnily enough the remark about electricity being magic sent me down that path. Whether you can "see" electricity is illuminating ( har, har).

Of course you can see light which is an effect of electromagnetism so is electricity in that sense. You can certainly feel potential as static electricity sets your hair on end. Lightning isn't a direct view of electricity because the visible light is an effect of the current passing through the air, not the current itself. You can see the electrons causing the light but not the electrons themselves (assuming for simplicity that lightning is a "flow" of electrons).

The effect of common rail is only "experienced" through the locomotive that closes the circuit. The puzzle is how two locomotives can run in opposite directions each connected to the same common rail. How do those electrons find their way? 

Another source explained it in layman's terms: the electrons leaving one Cab have to return to the same Cab. That makes sense as electricity is just potential until the circuit closes.

I have also confirmed that the MRC 780 is a single transformer twin throttle unit. I had deduced that from the output numbers as compared to the MRC 760 built in the same sized case. That's the reason I encountered the polarity assignmissue at the inputs to the 220.  The very useful coloured schematics posted in this thread show the points at which the blue and the red connect. When using the MRC 780 these points cross connect the single transformer outputs and the unit shorts internally. Had I understood how MRC separately protects the two separate throttles from external shorts the fact that both throttles shorted would have (should have) clued me in.  It is extremely unlikely that an external short could occur for both throttles at the same time. A single external short only cuts out the one throttle affected. The 220 "internal short" was illusory, the actual short would be inside the 780.

Alyth Yard

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Posted by ATLANTIC CENTRAL on Sunday, June 13, 2021 2:21 PM

gregc

 

 
ATLANTIC CENTRAL
I'm going by the drawing and memory, I don't have one here to test.

 

i do have one and i verified it after.

any reversing switch has just to positions, if not center-off, non-reversing or "pass thru" and reversing.     ... think about it.   why would it have a pass-thru and non-reversing position?

 

I agree, just looking at what Atlas says in both pictures and words....

Sheldon

    

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Posted by CSX Robert on Sunday, June 13, 2021 2:21 PM

gregc
...any reversing switch has just to positions, if not center-off, non-reversing or "pass thru" and reversing.     ... think about it.   why would it have a pass-thru and non-reversing position?
 

The center position of the cab reversing switches on the 220 are "pass-through", i. e., they pass the left hand cab inputs directly through to the right hand cab outputs.  Neither of the other positions is pass-through.  They both connect one leg to common and one leg to the output and they swap which one is connected to the common versus the output.  In both non-center positions of the switch, only one leg of the input is connected to the output.

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Posted by ATLANTIC CENTRAL on Sunday, June 13, 2021 2:23 PM

Lastspikemike

A DC primer would be handy.

I have a common rail system in that all my red wires are connected and three throttles are connected to all the red wires ( although some of the red wires are actually white because one of our group of three worked as an electrician years ago and to him white was the hot wire). 

So, I admit to becoming confused again. Since two of the three throttles are in a MRC 780 then clearly I'm not experiencing the internal short problem. We routinely have the powerpack reversing switches opposed.

My remark about train direction was a serious remark.  We have two loops and one connecting track. The "outside rail" of one loop connects to the "inside rail" of the other loop. We can't have a consistent East West or clockwise and counterclockwise. One loop is always the reverse of the other. The connecting track has neither an inside rail nor an outside rail. No left or right rail if you prefer. 

 

Once again, a drawing or picture would be helpful.

Sheldon

    

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Posted by ATLANTIC CENTRAL on Sunday, June 13, 2021 2:24 PM

CSX Robert

 

 
gregc
...any reversing switch has just to positions, if not center-off, non-reversing or "pass thru" and reversing.     ... think about it.   why would it have a pass-thru and non-reversing position?
 

 

 

The center position of the cab reversing switches on the 220 are "pass-through", i. e., they pass the left hand cab inputs directly through to the right hand cab outputs.  Neither of the other positions is pass-through.  They both connect one leg to common and one leg to the output and they swap which one is connected to the common versus the output.  In both non-center positions of the switch, only one leg of the input is connected to the output.

 

That's what I see on the diagram, but I don't have one in my hand....

Sheldon

    

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Posted by ATLANTIC CENTRAL on Sunday, June 13, 2021 2:40 PM

Overmod

 

 
Lastspikemike
one of our group of three worked as an electrician years ago and to him white was the hot wire). 

 

OK, I'll bite on this.  In what retarded imitation version of the NEC or any other valid national code does 'white' denote 'hot' instead of neutral?

 

In part I'd like to know because I found a "professionally" wired job in Springhill where the wires mysteriously changed colors passing through a junction box.  I attributed this at the time to guild malevolence against 'outsiders' messing with electrician prerogatives (NEC's position being no colors are 'official' and you should always check before tinkering anyway) but perhaps this is some distinctive color aberration taught to apprentices?

 

OK, wires in conduits should always be white = neutral, hot or switched hots other colors based on voltage/phase.

But, in cable assemblies like ROMEX, there is no code requirement to mark white wires uses as hot legs on switch leg circuits.

If in the time I have been away from the code book this has changed, I assure you such a requirement is seldon inforced.

Open up that three way switch in your hallway, and you will find a white (or two) on the switch being used as a hot wire. 

Depending on how the whole 3-way circuit is wired, you will also find a white wire spliced to a black wire in a box somewhere in that circuit.

Same with dead end switch legs, now prohibited by the code. A single black and white cable assembly going to a switch, the neutral stopped at the light fixture.

Recent code changes require neutrals in all junction and outlet boxes. Why? To provide a neutral for home automation devices.

Sheldon

    

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Posted by gregc on Sunday, June 13, 2021 2:48 PM

sheldon has said (PMs) that i like to know "why".

Lastspikemike
The 220 reversing switch has a center off but this position passes through the connection so isn't really "off".

clearly there needs to be a position when the paths between the screw terminals are isolated, before switching connections.   the  left screw terminals can't be connected to both screw terminals on the right at the same time otherwise there's a short.

but it appears the right 1/3 of the switch position is the reversing position and the left 2/3, including the center is the non-reversing position.   i was able to find a spot where there was no connection

because of this asymmetric construction, it makes sense, as Sheldon says the instructions indicate, to center the cab reversing switches in all but the rightmost 220 to indicate that they shouldn't be used to switch mainline polarity.

CSX Robert
The center position of the cab reversing switches on the 220 are "pass-through", i. e., they pass the left hand cab inputs directly through to the right hand cab outputs.  Neither of the other positions is pass-through.  They both connect one leg to common and one leg to the output and they swap which one is connected to the common versus the output.  In both non-center positions of the switch, only one leg of the input is connected to the ou

so there's reversing, non-reversing and pass-thru?   what's the difference between non-reversing and pass-thru

greg - Philadelphia & Reading / Reading

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Posted by CSX Robert on Sunday, June 13, 2021 3:00 PM

gregc
so there's reversing, non-reversing and pass-thru?   what's the difference between non-reversing and pass-thru

"Non-reversing" does not pass both legs through to the output terminals.  Only the center position passes both legs to the output. In both of the other positions, one leg of the input is connected to common and not connected to either output terminal.

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Posted by gregc on Sunday, June 13, 2021 3:11 PM

CSX Robert
"Non-reversing" does not pass both legs through to the output terminals.  Only the center position passes both legs to the output. In both of the other positions, one leg of the input is connected to common and not connected to either output terminal.

yes.  measured with a multimeter.  thanks

so if you center them, the last 220 doesn't see both power pack connections

greg - Philadelphia & Reading / Reading

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Posted by ATLANTIC CENTRAL on Sunday, June 13, 2021 4:32 PM

gregc

 

 
CSX Robert
"Non-reversing" does not pass both legs through to the output terminals.  Only the center position passes both legs to the output. In both of the other positions, one leg of the input is connected to common and not connected to either output terminal.

 

yes.  measured with a multimeter.  thanks

so if you center them, the last 220 doesn't see both power pack connections

 

???????

The drawing on the instructions you posted is perfectly clear. The middle position passes the power thru, bypassing the common wire connection. 

The right most Controller does see both power pack conncections but is does not pass them both thru. It creates the common wire connection and only feeds the two hot feeds (top and bottom right side terminals) from each power pack on to any #215 selectors used for block control.

And the right most Controller is used as the primary direction switch for each throttle.

I don't see what is so hard to understand here? Maybe it is just hard for people who have not built or operated a DC layout with cab control to see the end result?

Common rail or not, reverse loops need to get their power BEFORE the primary reversing switch for the rest of the non reverse loop blocks - that is the primary function of the Atlas Controller.

Second, the reverse loop block needs to be able to select which throttle will control it - the Controller does that as well.

By being able to stack them, you can have as many reverse loops as you may need.

The Selector then allows all other blocks to be connected to cab A or Cab B or turned off to isolate a loco/train.

Sheldon 

    

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Posted by gregc on Sunday, June 13, 2021 4:50 PM

ATLANTIC CENTRAL
???????

sorry, wrong wording.    the right hand screw terminal of all but the last 220 have both cab connections

greg - Philadelphia & Reading / Reading

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Posted by ATLANTIC CENTRAL on Sunday, June 13, 2021 5:21 PM

gregc

 

 
ATLANTIC CENTRAL
???????

 

sorry, wrong wording.    the right hand screw terminal of all but the last 220 have both cab connections

 

OK, yes, no worries.

To me, this is all so much easier to simply build with toggle switches, assuming this kind of system is all you need.

And in doing so I would not use common rail in building the layout or use a common wire return.

One day I will draw some more drawings and show the advantages of not using common rail.

The OP has received my PM and we are communicating about his layout plan and wiring needs.

I will repeat, the very best DC control systems, assuming you want to control mutiple trains at once, do not use toggle switches, and do not have to tie the operator to a control panel, despite all "toggle flipping" comments I have heard in the last decade.

Sheldon

  

    

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Posted by Lastspikemike on Monday, June 14, 2021 10:17 AM

"Wiring for DCC" explains the "home run" version of common rail. That's how I wired our DC layout (but the common rail is wired to bus bars and not DPDT) but since we are running DC only and we intend to add only a single DCC unit when we do run DCC we can delay the second part of home run: breaking the "return" common bus wires at the bus bar connections and, bingo, we have power districts. Each bus bar has a single connecting wire to its adjacent busbars. Pull that connection and wire it to the additional booster and we're good to go. Block control wires can go direct to any additional boosters anyway. Until then we can just select Cab A inputs as the DCC power source and disconnect the DC powerpacks completely while we run DCC.

Common Rail has been explained as working because the electrons leaving one powerpack "know" they have to go back to the same powerpack. That's how the circuit works. So it doesn't matter which powerpack polarity is connected to the common rail because the circuit within each powerpack is what matters. The 780 I was using was, electrically, one powerpack so the returning electrons just took the shortest available route, as they do. Nothing was left to power a locomotive connecting the two rails. Dead internal short but the "other end" of the short was across the 220 reversing switches at center off. The electrons never got to the locomotive.

Now I have connected the one throttle in the MRC 780 as Cab C not connected to the 220 at all, and the Cab B throttle is connected to the 220 and the MRC 760 is connected to the 220 as Cab A this internal circuit problem can no longer occur. Although I still match the polarity to my 220 inputs as before I now see why I did have to at that time but do not now. Oddly, this also straightens out a different polarity effect I experienced with my former setup. All three of my direction switches can now be set to operate East and West. Cool. I'm still not sure exactly how that works but it is dawning on me.

Alyth Yard

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Posted by ATLANTIC CENTRAL on Monday, June 14, 2021 10:44 AM

Again Mike, without seeing your track plan, it makes little sense, but it seems you might have figured it out.

Sheldon

    

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Posted by Lastspikemike on Monday, June 14, 2021 5:26 PM

ATLANTIC CENTRAL

Again Mike, without seeing your track plan, it makes little sense, but it seems you might have figured it out.

Sheldon

 

I figured it all out last year. This year I understand better why I had to do what I figured out I had to do.

The schematic of my layout is simple to describe: two ovals connected by two turnouts facing frog to frog. The connecting track exits one oval clockwise and enters the other oval counterclockwise. Hence the East rail on one oval connects to the West rail of the other oval.

Alyth Yard

Canada

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