# Two transformers and two lines on three wires?



## Brandon (Jul 6, 2011)

I've got to be wrong so I'm looking for validation. Say you have two tracks and two transformers. It's not possible to run two trains in different directions using three wires is it? In other words, you can't use a third wire as a common ground then + or - volts to control direction independently on each track, right?


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## SD90WLMT (Feb 16, 2010)

You could run trains in different directions..
Until you wanted one to change direction..

Maybe try a pair of reversing switches or relays at the end of the wire run ..just before the track..you would be feeding a constant polarity supplied by your feeders of a heavier gauge wire..to the track ..then control track polarity nearby individually ...

Depends on where your headed tho I spect...

If using relays you need a control source to operate the relays..albeit a lighter gauge wire can be used for a relay ...controlling a higher current feeder supply..just more wires..

Where are ya going?? !!

Dirk


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## toddalin (Jan 4, 2008)

Brandon said:


> I've got to be wrong so I'm looking for validation. Say you have two tracks and two transformers. It's not possible to run two trains in different directions using three wires is it? In other words, you can't use a third wire as a common ground then + or - volts to control direction independently on each track, right?


 
Yes you can. It is called a common ground and is done all the time. I do this to run 7 trains, all my lights, my rigiduo, and my automation circuits and they all share 1 wire. Trains run in both directions simultantiously.


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## Brandon (Jul 6, 2011)

So no problem with the common ground having noise or voltage drop? 

... Or does voltage drop not add up/apply on common grounds? Ie, if each of your 7 circuits pulled 1 amp each, does the common ground have 7amps being pulled through it and suffer voltage drop of 7 amps, or does the common ground only have "1 amp of voltage drop"?


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## Dan Pierce (Jan 2, 2008)

Brandon, the answer depends on whether the current is flowing in different directions or all in one direction, therefore one would assume as low as 1 amp or as high as 7 amps is what a current meter will see.
Always plan on worst case for wiring to limit the voltage drop for maximum current drawn.


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## toddalin (Jan 4, 2008)

I connect my common ground to the rails, so it is a heck of a fat wire. I do this in about half a dozen places over ~600 of track. From my junction box to my control panel, I've tripled the number of wires used for the ground.


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## Dan Pierce (Jan 2, 2008)

One thing I have against common wiring when using 2 power sources is an engine can cross between the plus and minus connections and have the total voltage of both packs. This can occur during a derailment at parallel tracks.
For decoders this does create that real black smoke some want Steam engines to produce.


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## toddalin (Jan 4, 2008)

Dan Pierce said:


> One thing I have against common wiring when using 2 power sources is an engine can cross between the plus and minus connections and have the total voltage of both packs. This can occur during a derailment at parallel tracks.
> For decoders this does create that real black smoke some want Steam engines to produce.


 
This doesn't seem to hold true in practice. If I cross a block from one pack to another, the train goes at the speed of the faster pack, not the combined packs. And if I cross a block and the packs are set to opposite polarities, the train just goes back and forth over that section of track where the wheels make contact. I try not to do this as a practice though.


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## Greg Elmassian (Jan 3, 2008)

What Dan says is possible, with 2 adjacent blocks set to opposite polarity.

If you have a short loco, I see how what Todd has seen can happen, given a high enough track voltage to move the train completely into one block or the other.

But have a long loco and have 2 trucks running opposite polarity, yes a recipe for burned out wiring in the loco, and double voltage.

Main reason common rail wiring lost favor in HO, when people realized that the extra cost of wiring more than offset melting down a couple of locos.

Greg


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## toddalin (Jan 4, 2008)

Greg Elmassian said:


> What Dan says is possible, with 2 adjacent blocks set to opposite polarity.
> 
> If you have a short loco, I see how what Todd has seen can happen, given a high enough track voltage to move the train completely into one block or the other.
> 
> ...


 
If the track is "double gapped" (both rails insulated), and a engine crosses over with one set of trucks on either side and both power packs set in opposite directions, how is this really any different once these differing polarities reach the motor?


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## toddalin (Jan 4, 2008)

Greg Elmassian said:


> What Dan says is possible, with 2 adjacent blocks set to opposite polarity.
> 
> If you have a short loco, I see how what Todd has seen can happen, given a high enough track voltage to move the train completely into one block or the other.
> 
> ...


 
Actually you're wrong, and I just did an experiment to prove it. 

*WHAT ACTUALLY HAPPENS* (and you can easily prove it to yourselfs) is that if one power pack is set to +4 volts and the other is set to -6 volts and you use a common ground and bring the two hot wires together on a voltmeter, the meter reads -2 volts.

This was done using a pair of MRC 6200s and they did not even trip their breakers.

This is why my trains run back and forth when crossing the gap with opposite polarities. The two voltages in essence cancel each other out or if one is higher, the train sees this difference and backs out only to move forward again.

Please stop spreading false rumors that propagate old wives tales.


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## Greg Elmassian (Jan 3, 2008)

content deleted due to everlasting conflict with Toddalin


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## Dan Pierce (Jan 2, 2008)

Murphy's law exists even for trains, someone somewhere will have the issue I described.
If it can not happen, it will!!!!!
A train derailing can not be predicted and an engine can derail at parallel tracks with common rail wiring and have the total output of both supplies applied to it.

Do not forget that only a few of the people running electric trains have the ability to fully troubleshoot/understand electricity.


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## toddalin (Jan 4, 2008)

Greg Elmassian said:


> What Dan says is possible, with 2 adjacent blocks set to opposite polarity.
> 
> If you have a short loco, I see how what Todd has seen can happen, given a high enough track voltage to move the train completely into one block or the other.
> 
> ...


OK,

Initially, you inferred there would be a "meltdown" as soon as they cross the gap, assuming the other axle didn't instantly clear the block or some circuit protection didn't kick in.

So I set the experiment up as you said, with a common ground but used a 12 volt bulb instead of the motor. I set the MRCs at ~ 12 volts + and - and "combined the other two "axles together. Again, no magic smoke, though the MRCs do "grumble."

I then brought this "together wire" to the bulb (other leg on the common ground) and depending on the voltage differential, the bulb lit (like my engine moves). Nothing smoked. Eventually, the MRCs tripped. I turned them off and back on and they sustained no damage.

I have several engines that have had to have their traces replaced by wires (some a few times) from shorts caused by derailing within a block. (I now put poly switches in one set of trucks them if I open them.) I've NEVER had an engine cross a block and smoke its wiring nor derail right at the block such that it ends up straddling the block to smoke its wiring.

But even so, these would also be saved by the polyswitch if what you say were to occur.


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## Dan Pierce (Jan 2, 2008)

2 25 amp supplies at full throttle with #10 wire to the rails and an engine crosses the gap of 48 volts. Somewhere someone will do this and it most likely be a showoff at an open house.

I want to be there and see if the small engine wires melt like a fuse or the supplies shut down. I do not think the #10 wire will care either way.


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## Greg Elmassian (Jan 3, 2008)

content deleted due to everlasting conflict with Toddalin


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## toddalin (Jan 4, 2008)

Greg Elmassian said:


> What Dan says is possible, with 2 adjacent blocks set to opposite polarity.
> 
> If you have a short loco, I see how what Todd has seen can happen, given a high enough track voltage to move the train completely into one block or the other.
> 
> ...


 
Greg,

You insist that double gapping (I assume that you mean a gap in both rails at the same point) is the solution and will prevent this possible occurance.

I asked you how this is any different once both sets of trucks have spanned their gaps.

_If the track is "double gapped" (both rails insulated), and a engine crosses over with one set of trucks on either side and both power packs set in opposite directions, how is this really any different once these differing polarities reach the motor? _


_*You still haven't answered this*_.

As you say "draw it out". Once one/either side spans the gap, you've created the "common ground" because the wires come into contact anyway.

So, now instead of 45 volts just going through one side of the engine, and possibly melting one side of the wiring, it goes though both sides of the engine melting its wiring on both sides.

_So now you have twice as much wire to buy for no benefit except the possibility of twice the damage to the engine._

But you certainly give up a lot.

I could not even build my control panel without a common rail. I use sp3t switches. There are no dp3t toggle switches. Rotary and slide switches can't take the current, so to do this same thing with double gaps. I would have to invest in exotic switches and/or no fewer than 42 relays to carry the current to accomplish the same goal.

I've put my equipment in jeopardy to support my stand. All you've done is puff your chest.

Now you prove to us using your equipment how double gapping the rails is any better at reducing meltdowns.

Otherwise, You Lose and Accept it!


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## Greg Elmassian (Jan 3, 2008)

content deleted due to everlasting conflict with Toddalin


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## toddalin (Jan 4, 2008)

Greg Elmassian said:


> Todd, everything that needs to be explained has been explained.
> 
> Every person that wants to understand does.
> 
> ...


 
No, you've still not explained how it is any different with a double gap versus a single gap with a common rail when both trucks cross over the gaps and power flows freely through both sides of the trucks to the other power pack so could potentially do twice the damage with no benefit.

Inquiring minds want to know.


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## chuck n (Jan 2, 2008)

Guys, please CHILL.

You are not going to agree. I don't know who is right or wrong. 

The only thing you are doing is hurting your future credibility and I have benefited from both of your suggestions in the past.

Chuck


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## Greg Elmassian (Jan 3, 2008)

I said I give up and you win Todd, I'll not argue any longer as I stated before.

Greg


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## toddalin (Jan 4, 2008)

It's not about winning or losing.

There are a lot of people on the 'net who take your word as "gospel" and when you tell them that they need to do so much more work and spend so much more $$$ (priced wire and switches lately?) for something that really has no tangible merit in today's day and age (using even 20 year old equipment), at least that you could logically point too, that gets my goat.


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## Greg Elmassian (Jan 3, 2008)

content deleted due to everlasting conflict with Toddalin


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## toddalin (Jan 4, 2008)

Greg Elmassian said:


> The original point was whether common rail could produce double the voltage at some point to a loco spanning two adjacent blocks.
> 
> You win... don't listen to me at all...
> 
> Greg


 
No, once again, you've twisted this to your own ends.

And once again, while this was not the original topic, until you twisted it, you fail to yield that this same situation would occur with a double gap in exactly the same spot, possibly with worse consequences. (Actually, we know that you recognize this but are too headstrong to fess up.)

_The original point was (as in Post #1):_

*Two transformers and two lines on three wires?* 

I've got to be wrong so I'm looking for validation. Say you have two tracks and two transformers. It's not possible to run two trains in different directions using three wires is it? In other words, you can't use a third wire as a common ground then + or - volts to control direction independently on each track, right?


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## Greg Elmassian (Jan 3, 2008)

you win Todd, I've deleted all of my posts except the first one.

As my signature states... anyone that wants my opinion or explanation please email me privately.

Greg


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## noelw (Jan 2, 2008)

We been trying to fig. out what or how you layout a common wire on a three wire sys on DC Voltage. Not saying either one is right or wrong, but can you draw out a diagram of what you are trying to show us? I keep healing dbl. gap. Do you mean the rails are not powered for a short distance on one or both track? Using two transformer, yes it can have one common track, but doing a reverse. direction with same or one Eng. only can cause a short as one set ( Like a motor block with two axle or more) wheels are still on the first rail crossing over on the sec. rail on both sides but with not dead rails. It will not show up on either breaker due to only one transformer will not see the other transformer current as a shot.. but the Eng. wiring or board will. That has happen to us for many yr's back when started out in Ho in the 50. We even tried this kind of stuff with pulse power or half wave on the same type of sys. with three wires.

Maybe this will blow your mind.. LED's for traffic lights. Two power supplys.. One A/C and one D/C.for traffic signals can work on just two wires to oper three colors. So there is ways of making cir work but not used right can make shorts cir.and depends on how you use them. so still like to see a diragram of what you are saying. 

Anyway just trying to fig. out what you guys are trying to show. No hard feeling of anyone ...but just like to see something that is new and may work or not work...


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## toddalin (Jan 4, 2008)

noelw said:


> We been trying to fig. out what or how you layout a common wire on a three wire sys on DC Voltage. Not saying either one is right or wrong, but can you draw out a diagram of what you are trying to show us? I keep healing dbl. gap. Do you mean the rails are not powered for a short distance on one or both track? Using two transformer, yes it can have one common track, but doing a reverse. direction with same or one Eng. only can cause a short as one set ( Like a motor block with two axle or more) wheels are still on the first rail crossing over on the sec. rail on both sides but with not dead rails. It will not show up on either breaker due to only one transformer will not see the other transformer current as a shot.. but the Eng. wiring or board will. That has happen to us for many yr's back when started out in Ho in the 50. We even tried this kind of stuff with pulse power or half wave on the same type of sys. with three wires.
> 
> Maybe this will blow your mind.. LED's for traffic lights. Two power supplys.. One A/C and one D/C.for traffic signals can work on just two wires to oper three colors. So there is ways of making cir work but not used right can make shorts cir.and depends on how you use them. so still like to see a diragram of what you are saying.
> 
> Anyway just trying to fig. out what you guys are trying to show. No hard feeling of anyone ...but just like to see something that is new and may work or not work...


Noel, I'm not the one who brought up "double gaps" and the best I can discern, and as you note, there could be two ways to do this.

One way is where both gaps are at the same spot/joint along the rails where you could in fact have power flow from one power pack to another. While this may have been a problem in the past (you said 50 years ago?), most modern systems have breakers to avoid this "overcurrent" just as the breakers on the 20+ year old MRCs eventually tripped when their current capacity was exceeded. Furthermore, _to my way of thinking_, double gapping in this manner would still allow for the overvoltage situation because the power still makes its way to the other pack through the other side of the trucks (unless someone wants to step up and show how this is really any different).

The overvoltage/overcurrent doesn't actually go though the motors, so they don't burn out. It does go through the wiring in the trucks. But the wiring in G-scale tends to be more "robust" than in H0, N, Z etc., so can more easily take this brief overvoltage situation for a moment before the breakers trip without meltdown.

As I alluded, you have a far greater chance of a melt down caused by a simple derailment so should take steps to avoid this, if possible (e.g., two truck diesels).

The other method of "double gap" that I conceive is to have the same side rail gapped, then gapped again a bit further down the line leaving a "dead spot."

This dead spot would have to be long enough to keep the front and back wheels (involved with power pick-up) from touching the live rails simultaneously. This would alleviate the short, but will leave a massive power gap (much longer than a turn-out frog) that would probably stop shorter engines entirely. But, this method would still allow for common rail wiring as you only double gap one side.

If you are asking how to wire the "common rail" with just three wires: Tie a "black" wire between the "left terminal" on each throttle output and to the common rail. Tie a "red" wire to the right-side throttle's "right terminal" and to the right side of two, or three, or... spdt, center off toggle switches (one switch for each block on the railroad). Tie a "white" wire to the left-side throttle's "right terminal" and to the left side of the multitude of spdt center off toggle switches that you intend to use. Wire the center pole of each toggle switch to each respective block on the railroad.


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## kormsen (Oct 27, 2009)

toddalin said:


> ...The other method of "double gap" that I conceive is to have the same side rail gapped, then gapped again a bit further down the line leaving a "dead spot."
> 
> This dead spot would have to be long enough to keep the front and back wheels (involved with power pick-up) from touching the live rails simultaneously. This would alleviate the short, but will leave a massive power gap (much longer than a turn-out frog) that would probably stop shorter engines entirely. But, this method would still allow for common rail wiring as you only double gap one side...


i don't like the idea of "double gapping"
if i double gap in the sense of gapping both rails at the same spot, i loose the main reason for common wiring = a wire for all aplications, that is uninterrupted.
if i double gap on the same rail, i would artificially create the "dead spot" problem, some turnouts have.

i think, operators, who beleive themselves inteligent enough, to play around with common wiring, should be inteligent enough, to conciently switch polarities before their trains reach a gap between blocks.
(or to place these separations that they don't interfere with shunting...)

even if it is/would be possible, to evade shorts, the gears of the locos would be thankfull, if they were not put in a situation, where they are sent forwards and backwards in rapid succession.




toddalin said:


> ...If you are asking how to wire the "common rail" with just three wires: Tie a "black" wire between the "left terminal" on each throttle output and to the common rail. Tie a "red" wire to the right-side throttle's "right terminal" and to the right side of two, or three, or... spdt, center off toggle switches (one switch for each block on the railroad). Tie a "white" wire to the left-side throttle's "right terminal" and to the left side of the multitude of spdt center off toggle switches that you intend to use. Wire the center pole of each toggle switch to each respective block on the railroad.


BRRRRR! that sounds complicated. 
i prefer to use smaller, but more powerpacks. one for each block.
the blue (LGB) for all together on the right rail, and the red from each pack directly to one block on the left side.


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## toddalin (Jan 4, 2008)

kormsen said:


> i don't like the idea of "double gapping"
> if i double gap in the sense of gapping both rails at the same spot, i loose the main reason for common wiring = a wire for all aplications, that is uninterrupted.
> if i double gap on the same rail, i would artificially create the "dead spot" problem, some turnouts have.


 
Totally agree Korm.

But there are those who seem to have a stigma about this. So, rather than grouse about it, I've come up with a relatively cheap and easy solution, and (hint hint) it does involve two gaps at the joiner, but not in the way that anyone here is thinking about. 

I'll put it in another thread so it doesn't get lost here.


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