Common ground bus

[CP5415]

I'm getting to the point making a cab control operating system on my DC layout.

Is it safe to use a common ground wire for 2 or more power packs & have a on-off-on toggle switch routing the positive??

Kinda like this

 

Thanks

Gordon

[fwright]

The only requirement for the scheme you propose is that each cab must have a separate transformer.  This allows the ground to "float" with respect to an external reference, depending on the position of the direction switches.  If the transformer is shared, then setting the direction switches opposite shorts the shared transformer.

I have used the very scheme you propose on all my layouts to date.  I simply run a 16 gauge bare wire looped bus around the layout perimeter, and tie all the common rail connections directly to the bus.  This works in the Lionel AC (transformers must be phased in AC) world as well as DC.  If you loads are heavier, you may want to use heavier bus wire to avoid voltage drops.

Fred W

[CP5415]

Thanks Fred,

I've already used a 14 gauge wire for the ground bus & have separate MRC power packs to power the layout with plenty of 22 gauge wire to run the positive side to the tracks, switches & power packs.

I forgot to ask in the origional post, the direction of current / locomotive direction relies on the + current direction doesn't it, as the grounding side is only a ground?

Gordon

[fwright]

CP5415

Thanks Fred,

I forgot to ask in the origional post, the direction of current / locomotive direction relies on the + current direction doesn't it, as the grounding side is only a ground?

Gordon

The +/- on the powerpack output is nominal.  The polarity will be reversed when you flip the direction switch on the powerpack.  This does not matter in the common wiring scheme as long as each power pack has its own transformer - the common is not necessarily a zero voltage with respect to earth (external reference).  This is true in many DC circuits - the common return is just that, not a zero chassis reference.  Boats and cars, on the other hand, normally do tie the negative battery post to ground, forcing that to be a zero reference. 

In fact, you can use either terminal on either power pack to tie to the common return.  Using the nominal (-) terminal is the most similar to conventional 12V DC wiring (cars, boats).

As an aside, my MGA had a positive ground, and a tube radio (and electrics by Lucas - Prince of Darkness).  I could not replace the radio because all available transistor radios used a negative chassis ground.  Available solid state turn signal flashers had the same issue so I had to stick with the original vacuum operated unit.  Wiring the car lights for towing required completely disconnecting the two batteries (a 6V battery under each seat was stock) so that the car ground could match the tow vehicle.

Fred W

[gregc]

is the drawback to a common ground that the two supplies are now in series, and when one is reversed, the non-common rails may now have twice the supply voltage across them if shorted?

greg

[rrinker]

 That's what happens if you feed two cabs from a common power supply. By having 2 transformers, two rectifiers, and two throttles, you avoid that.

Personally I've never been a fan of common rail wiring, even the layouts I built when I was a kid, I gaped both rails at each block and ran feeds to both rails, no common. Yes, that means DPDT tggles for 2 cab block control, but it's not really twice the wiring - 2 conductor wire is all you need. One wire, 2 connections.

                                      --Randy

[Ron High]

I use common rail but instead of running back to the control panel and switching that side which is unneeded ,run a common rail bus around your layout.  Gap both rails and connect each block on the common side to the bus  at each block on the layout. This keeps the wiring simpler at the control panel and causes less confusion  on gaps and wiring to the track blocks.

Ron High