The dumbest electrical question ever

[Soo Line fan]

This is not a dumb question at all. Actually it is a good one and dbduck and fwright have done the best job of correctly explaining it. Why they are not fighting each other is this: If the common rail circuit to both packs is set to the same polarity, lets say both packs have the common rail set to + then you have a Potential parallel circuit. It is never complete because of your toggles or Atlas selectors are interrupting the other side of the circuit. No way they can be fighting each other because the polarity is the same on the common rail from both packs. Now lets switch one pack so that you have + on one and – on the other. Why is that not a problem? Because you have a Potential series circuit at this point and as dbduck has shown with his battery analogy, you can connect + to – and you do not have a short. You simply increase the voltage as ohms law states. When I say Potential, I mean it could be one but because of the way the block toggles or atlas selectors, are wired the other side of the circuit is always interrupted so the voltage increasing potential is never realized. Either way you have no potential for a short. If a short should occur, you already have short circuit protection or should have in both your power packs. Interrupting this circuit serves no purpose at all as far as safety is concerned. Does your house wiring interrupt both sides of the circuit when you turn off a light? Does your car interrupt both sides of the myriad of circuits it contains? Of course not. I would be more concerned that the packs are plugged into a surge suppressor with a circuit breaker, which is your second level of protection, prior to the circuit breaker or fuse in the main box in your home. Therein lies the cure for “magic smoke”.

Jim

[Anonymous]

If you have never seen something suffer a truely magnificant electrical fault and let the magic smoke out, its something to behold. [:D] And if you're the one footing the bill for the destruction, you NEVER want to experience it.

Cheers.

- Tim

[fwright]

Common rail is no more of a problem waiting to happen than gapping both rails in DC block control. If the 2 power packs are each controlling adjacent blocks, and the block gaps get bridged by a conductor you get a feedback current from the power pack set to a higher voltage to the one set to a lower voltage. Since there is very little resistance in the circuit if it's metal wheels bridging the gaps, the feedback current looks very much like a short circuit.

Only difference between common rail and gapping both rails is that both rail gaps have to be bridged simultaneously in the non-common rail case for the feedback current to happen. Surprisingly easy to do since the gaps are very seldom staggered. After all, if you are staggering gaps, you are effectively using common rail over short distances. :-)

No special reversing switch set up is needed with common rail. If you have a reversing loop or wye, you must have an independent direction toggle for the main and each reversing loop, just as in non-common rail wiring.

Common rail was/is used to reduce poles and costs for block control toggle switches and the amount of wiring going to the control panel. That said, the rules for using common rail must be adhered to:

1) You cannot use DCC with common rail. Period.
2) The power packs must have independent transformer secondary windings. Either use independent power packs or the twin power pack must be wired internally for common rail. Virtually all the MRC twin power packs meet/met this requirement (other makes usually did not), so this is not generally a factor these days.
3) Common return conductor must be sized to handle the total short circuit current - again not much of a factor unless you are talking about a large layout in O. Just don't use anything less than 18 gauge wire for the common return in smaller scales on normal size layouts.
4) Common rail does not apply to reverse loops, wyes, and turntables. These must be totally isolated and powered the same whether common rail is used or not.

Just trying to dispel myths and misunderstandings.

yours in wiring
Fred W

[dragenrider]

I agree that a common rail is a problem looking for a place to happen. I gapped both rails, seperated the positive terminals through a single pole, double throw, center off switch for each block. The negative wires are all bundled together.

I'm watching for smoke! After all, I am a fireman!

[BR60103]

The only problem I've had with common rail is if something (metal wheel, stray wire, devious diesel) bridges a gap between two blocks set to different throttles.

[TomDiehl]

QUOTE: Originally posted by warhammerdriver

It is a common misconception that automotive electrical systems work using electricty. Well, this automotive electrical tech is letting the cat out of the bag--they really work on smoke. Once you let the smoke out of a circuit, it stops working.

I'm assuming that MR electrical systems work on smoke, too.[:D]

In electronics, resistors are made of smoke. If the smoke stays inside, they're OK, if you let the smoke out........

But seriously, the best advice I can give on dual cab control is, forget the common rail idea. Isolate both rails with plastic joiners, and have a DPDT for each block to select either Cab A or Cab B. This way there's no connection between the packs, and less chance for problems. Remember the KISS principle.

[warhammerdriver]

It is a common misconception that automotive electrical systems work using electricty. Well, this automotive electrical tech is letting the cat out of the bag--they really work on smoke. Once you let the smoke out of a circuit, it stops working.

I'm assuming that MR electrical systems work on smoke, too.[:D]

[Anonymous]

I thought that too that it would short out, but it only shorts if the negative and positive are coming from the same contoller. So the common wire can carry the positive of one and negative of another (don't ask me how), it must be the same type of thing as sending two messages along the same telegraph wire.

[Anonymous]

Not so dumb! I once had a guy ask me why we didn't use nylon brushes on an armature, seeing the metal ones always seemed to arc to much. Think about it.

[dbduck]

On common rail systems you are only tying together ONE of the ternimals of each power pack together to be connected to the common rail. That rail then serves as the return path for the other terminals of the two powerpacks. The remaining terminal of either powerpack should never be interconnected to the other nor the common rail.

I know it may be hard to understand ...but at times the common rail may be negative to powerpack #1 while at the same time it is the positive side of the other powerpack. Since the other terminals are not interconnected, for all intent & purpose, (electrically speaking) neither powerpack knows that the other one exists ...

think of it like this...take 2 "D" batteries ...each one is a 1.5 volt power supply in itself (the powerpacks) Now put the batteries in series with each other + - l+ - the orange line represents where the two batteries interconnect (common rail). For the battery on the left it is the - (neg) side, but for the battery on right it is the + (pos) side. If you turn the battery on the right around ( the same as throwing the reversing switch) then the common rail is now - (neg) to both circuits

It has been a while since I have done any common rail wiring...but if I am not mistaken you could do this with more than 2 powerpacks...just never allow the other terminal of any powerpack to become interconnected with a any terminal of any other powerpack nor the common rail.

[Virginian]

Nope. Many years ago someone asked me why no one liked the Watts? I asked why they thought that. They replied everyone was anlways taking about killing Watts; i.e. kilowatt. I was speechless.

[dragenrider]

QUOTE: I read many years ago why this works but can't recall exactly why except that, in effect, the polarity going back to the cab (powerpack) is always the same.

Well, you must be right. I found my backwards wire(s) and everything is running just fine with a common negative wire. I don't understand it either, [%-)] but it is working just fine. I'll keep my fingers crossed! Thanks for all the input.

[howmus]

QUOTE: Originally posted by Hudsonmike

No I guessing that you forgot that the rails between blocks have to be insulated
from one another .

That is true if you use the reversing switch on the power pack. You will get a short and possibly fry the power pack. However, you only need to isolate (insulate) one of the rails if you use a seperate reversing toggle. That is known as a common rail system. With that arrangment you can have locos going in opposite directions on other blocks as long as they are using another powerpack (2 cab system). I read many years ago why this works but can't recall exactly why except that, in effect, the polarity going back to the cab (powerpack) is always the same.

[Marty Cozad]

Just don't ask where electricity comes from. Or , can I touch it?? heheheheh

[Hudsonmike]

No I guessing that you forgot that the rails between blocks have to be insulated
from one another .

[dragenrider]

That brings up another question, though. Wiring diagrams show a single common rail or shared negatives for simple block controlled layouts. If this is so, then won't there be a problem if one train is reversing on a block while another train is in forward motion on another? They are sharing a common wire or rail, which is now in conflict, right?

[dragenrider]

Thanks, Husdonmike! That's my next move then. I've traced my wiring and isn't that. I'll flip a switch!

[Hudsonmike]

Yes the switch throws the Neg and pos to the opposit screw these are DC
current.
I don't know what block signal you are using but you can not have
a common ground off these terminals. you can on the accessory
terminals because they are AC current.
Mike

[dragenrider]

I'm really going to show my ignorance here! [:I]

If you have a DC power pack and the left screw is positive and the right screw is negative, then you flip the direction switch, does this make the right screw positive and the left screw negative?

I'm installing my block wiring using a common negative and two power packs. They are identical MRC 2400's, but something isn't right and they are fighting each other. I'm beginning to suspect one of them is wired backwards to the other and I'm wondering if flipping the direction switch is the cure. [?]