Need help with common Cab Wiring

[Anonymous]

Hi, I'm wiring a dual main layout common cab control. I have (2) MRC power packs and (2) atlas controllers. I thought things were good but I have some quirks. At one half of the layout powering up one pack provides power to both trains. Also the direction of both packs must be set the same or they cancel out. I have a common wire between the two cabs that goes to common ground of the RR, and then the + DC goes to the A and B of the Atlas controlers respectively. What should I be looking at. Thanks, Dave

[mcouvillion]

Dave,

I'm not a big fan of common rail for just the reasons you are experiencing. For the problem on half the layout running off one power pack, my guess is that you have the wires crossed somewhere. Did you do a loop or balloon track?

For the other problem with the directions of both packs having to be the same or else they cancel out, try two things. 1) Unplug one power pack and rotate the plug 180 degrees and replug it in. The packs may be out of phase and this could be a problem. 2) Take the two output wires from one transformer and swap them. Imagine the common rail as "zero" voltage or potential. One transformer should supply voltage above zero, the other should supply it below zero. You may have both supplying on the same side.

Without knowing more of how you have it wired, and what you have wired, its going to be hard to help.

Mark C.

[Anonymous]

you gave me some things to try. One thing I didn't say in the first post is one of the MRC power packs is a 2500, the other a 3000GS. The 3000 GS gets it's power from the AC of the 2500. It's basically a power pack with no transformer. Also I'm using two selectors . not controllers, my mistake. Dave

[egmurphy]

Okay, stay calm, we can probably walk you through this. I have almost the identical setup, except that I have two independent MRC powerpacks (old Tech II 1500's) and 3 Atlas Selectors. While seeing a track plan or wiring plan would have been nice ( you don't have one on an electric file that you could post, do you?), we can probably start troubleshooting with what I went through starting to wire mine. If this doesn't work we'll go on to something else.

BTW, here's what my little control panel looks like. The schematic is temporary until I can generate something better. Click on the photo to enlarge it.



When I checked out my power packs, the 'variable DC out' terminals did not have a +/- marked. Nonetheless, I checked the two power packs out individually (before hooking up to a selector) by setting the direction switch to the right, connecting the right hand terminal to my + rail, and the left hand terminal to my common neg rail (using alligator clips). To my surprise, the two power packs ran the engine in different directions, I have no idea why. But in any case, after checking out both power packs and determining which lead needed to go to + to get the engine to go in the direction indicated by the direction switch, I figured out which two terminals represented my ground. Those two got connected to my common ground wire. The two + terminals got connected, one to the top terminal (A) at the end of the strip of selectors, and one to the bottom terminal (B) at the end. (btw, the plugs are polarized, so it wasn't an issue of turning the plug around).

My suggestion for step one, is to disconnect the power packs from the selectors. Temporarily connect each (one at a time) to the tracks in front of you. Put an engine on the track and verify that both power packs agree on which direction they are powering the track (i.e. right hand terminal to the top track, left hand terminal to the bottom track, direction switch to the right or left, check which way the engine goes.)

Report back after you've checked that. It could be that simple. If not we'll probably need to check for improperly gapped blocks.

Did I thoroughly confuse you?

Regards

Ed

[Texas Zepher]

QUOTE: Originally posted by davsachz
One thing I didn't say in the first post is one of the MRC power packs is a 2500, the other a 3000GS. The 3000 GS gets it's power from the AC of the 2500. It's basically a power pack with no transformer.

Ouch. That means the "common" might not be. A "common" wire assumes the power supplies are 100% isolated from each other. Similar to what another reader suggested, but on the output side, try crossing the AC leads going from the 2500 to the 3000GS.

I would be wary of that arrangment. Considering the low price of DC power supplies, I got a throttle master for $5 at a swap meet the other day, you might want to consider investing in another.

[Texas Zepher]

QUOTE: Originally posted by davsachz
At one half of the layout powering up one pack provides power to both trains.

I am surprised this is only on one half the layout. I am guessing the power is feeding out through the common of the 2500 then backward through 1/2 the 3000GS.

[Anonymous]

In the areas that are isolated, should I put an insulated joiner in both tracks ? Each isolated section has both + and - running to it seperately.


Thanks for all the replies, I'll try to post a plan. Basically it's a double main folded over itself in an "L" shape, Kato Unitrack "N" scale with a few sidings all isolated and a yard. The one thing that could be the culprit, but I don't think so is a doubel crossover switch. The Frogs are all insulated on it. I have a degree in electronics, but I haven't used it in 15 years, I switched careers to become a photographer. This will make me look at my wiring more closely, label each wire etc. I want solid wiring so I can just go to DCC someday. I'm thinking maybe I have a ground loop somewhere.

[rrinker]

That is EXACTLY your problem, you cannot use common rail if you use an MRC power pack plus an MRC cab control running off the very same power pack. Both supplies must be isolate from each other, OR just don't use common rail wiring. You mentioned going to DCC in the future - start here, and remove the common rail wiring. DCC can do common rail but it is very tricky and you have to use optoisolated booster so there is NO common link between the boosters.

--Randy

[cwclark]

e-mail me and i have a lot of circuits drawn in micro soft word i can send you...including a diagram of common rail wiring

[mcouvillion]

Dave,

Are you beginning to see why I don't like common rail. If you intend to eventually go DCC, please take the time NOW to wire a two-wire bus to everything. It makes it so much easier!

Mark C.

[Anonymous]

Hey,
I know your a beginner and all, I was! But as I progressed into planning the wiring, I became frustrated, so I choose EASYDCC! The good thing though, was that I had MR from 1997 when they published it out. My cousin gave them to me! Well hope your situation gets better!

[Anonymous]

QUOTE: Originally posted by rrinker

That is EXACTLY your problem, you cannot use common rail if you use an MRC power pack plus an MRC cab control running off the very same power pack. Both supplies must be isolate from each other
--Randy

It's not exactly a power pack and a cab control. It's something weird MRC sold. It's basically identical to my Tech II 2400 but it has no cord and no transformer. It get's it power off the AC out of the 2500. I have a MRC 1370 and a Large Bachman Spectrum Power supply. I'll try subbing in one of those to see if that is the problem. If it get's too ugly I'll ask Santa for DCC. But Cab control has been around forever so I should be able to make it work. Like I said Each block has both wires running to it, so I could put insulated joiners in the common side and see if that fixes it also.
Dave

[Anonymous]

What rrinker is saying is correct. The whole problem is that the second controler is supplied power from the step down tranformer of the other power pack.

You need two entirely independent power packs for common rail wiring.

Replace the add on controller with a fully independant power pack and your problem will go away.

[rrinker]

Go ahead and ask Santa for that DCC, it's a lot more fun. Sure, cab control has been around forever, and it indeed does work, but once you operate two trains on the same stretch of track with completely independent control and NO switches to constantly flip, you'll never go back.
But if you supply that MRC cab thingy with a different power pack than the main MRC one, the common rail wiring should work. But like I said, common rail and DCC don't mix very well, if you intend to go that way in the future anyway, plus if you DIDN'T use common rail, the control setup you have now would work for cab control.

--Randy

[Anonymous]

so Randy, the difference between common rail and regular Cab control is total isolation of the blocks, you use a DPDT to throw both the lines for each block or can I still use the Atlas controllers as long as I seperate the grounds from one another ? Thanks, Dave

One last thought probably not worth the trouble but If I used a 1:1 isolation transformer between the packs, it should work also.

[rrinker]

You would have to use DPDT toggles, not the Atlas Contollers - just runnign seperate grounds to each block ends up being the exact smae thign as common rail, except the rail is not carrying the commin conenction, there are seperate wires. All that buys you is less voltage drop - even with common rail you should have multiple feeders on the common side.
You can use common rail wiring and the Atlas parts so long as each power source is seperate and not tied together OTHER than at the AC socket. The transformer inside already isolates the secondary side. The only time you'd need an isolation transformer would be in if you are using a variable transformer type power supply (does anyone still use those? MRC made one many years ago, other modellers used the Variac brand and made their own).
Bottom line, just ditch the common rail and run the extra wires. In the end, while it means two wires from each block back to the control panel, it ends up being EASIER to understand. Especially if you actually use a different color wire for each side. [:D]

--Randy

[Anonymous]

Thanks RRinker and others you hit it on the head. Tonight I came home and hooked the MRC 3000GS up to the AC of another transformer an MRC 1370 and all is well. So please tell me why common rail doesn't play nice with DCC. I'm sure I'll eventually go DCC, the only thing holding me back is I have (7) engines only (2) of which are DCC friendly. We are talking "N" scale, I'm not in the hobby right now to have to grind spaces in frames to make room for decoders. Otherwise I'd buy a starter DCC set. So If I just want to flip all the switches to ON and go DCC, I need DPDT's ? Thanks, Dave

[rrinker]

For a good explanation of DCC and Common Rail, go here: http://home.neo.rr.com/mrwithdcc/ and click on Tips, and then Commons.

Short form: with multiple boosters and common rail, it is possible to get DOUBLE the track voltage across your decoders, instantly frying them. It is also possible to get high current flowign through parts of the command station/boosters that aren't meant to take high current, like the cab bus or command bus, unless all boosters are optoisolated. It's OK if you never have more than one booster, and don't have any reversing sections using an auto-reverse module.

--Randy