Thanks for the warning, Randy.
The TCom bus is for 'brute force and ignorance' devices - lamps, switch machines, semaphore signals (also twin-coil devices) and a few relays. The closest any of that comes to electronics is an occasional diode.
All of the electronics (occupancy detectors and power packs with op-amps) will be or already has been connected to RCom, the common rail bus. Two totally separate sets of circuits that don't meet anywhere downstream of the 120VAC house supply.
Chuck (Modeling Central Japan in September, 1964)
The only caution I have from an electronics point of view is having twin coil switch machines using that same common. Twin coil machines will generate large inductive spikes on the line when they throw, as well as brief high current draws (potentially leading to short voltage sags due to line loss). So be careful if that common line is common with circuits powering electronic devices. If all the other lines sharing the common are just for things like structure lighting, it shouldn;t be an issue, though you might get some light flicker.
I would suggest 2 commons - one for the twin coil machines and their inductive load, and one for everything else.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
Happy to help Chuck, us "roll your own" types have to stick together.....
Sheldon
Many thanks to all who answered, especially to my fellow MZL user who reminded me of the additive nature of the outputs.
A bit of clarification - the wall wart transformer (secondary isolated from primary, 21VAC output on house input) is all that survived an encounter with my hammer-wielding grandson. (Sitting in a pile of black plastic shards and circuit board fragments; "But, Granpa, I just wanted to see what was inside!") I've tested it, isolated from everything else, and it does a good job of juicing twin coil switch machines with coils isolated for half-wave DC power on the hot side.
My intention was to connect coil common of all my switch machines to a layout-wide bus, TCom. This will allow different power sources to activate the same machine from different panels. (This supply powers some of my hidden staging and the end of the railroad colliery. The JNR and TTT main panels will have a separate switch machine power source but have to be able to set routes from the far end of the peninsula.)
By piggybacking the return leg of the circuit that powers interpanel communication lamps onto TCom I'll simplify the interpanel wiring and avoid having to provide a separate return from CTC to each of the zone panels. It will also simplify adding lamp power transformers for panels not yet built and structures that have yet to find homes on the layout.
Note that all of this is totally separate from power to the rails. The closest TCom comes to RCom (common rail) is about 1.5 inches - two studs on a terminal block, in (so far) two dozen or so places.
Again, thanks to all...
I am using two wall-wart transformers this way to get +/-12 VDC for Tortoises.
One thing to watch out for though- if the common "neutral" gets disconnected then there will still be a path from the high leg to the low leg through the powered devices and the current will find the easiest path to flow, potentially blowing out devices not rated for the combined voltage. We had this happen on a much larger scale at our house years ago- the underground neutral wire corroded and suddenly burned out. The high leg to low leg voltage blew out everything electronic in the house. Fortunately the power company was responsible for the underground wiring and they paid to replace/repair everything.
You can put a fuse on each of the legs to protect the circuit and devices being powered. But NOT on the common "neutral", and make sure it always stays connected.
Huntington Junction - Freelance based on the B&O and C&O in coal country before the merger... doing it my way. Now working on phase 3. - Walt
For photos and more: http://www.wkhobbies.com/model-railroad/
tomikawaTT I would like to know if it's safe to connect one secondary terminal of each of several AC transformers to a single AC common return bus. At present I have a 21V wall wart transformer powering twin-coil switch machines and a 12.6v 3 amp center-tapped filament transformer powering panel indicators and signals. Future plans call for using several additional small AC transformers for the same purposes and for structure and 'scenery' lighting. I would like to connect all of them to a single layout-wide bus (#12 solid copper wire). I doubt that the load will ever reach 9 amps, especially since my switch machines only generate momentary spikes and a lot of the connections will actually be for half-wave DC circuits. I am comfortable with common rail, but connecting AC sources is a new concept for me. Chuck (Modeling Central Japan in September, 1964)
I would like to know if it's safe to connect one secondary terminal of each of several AC transformers to a single AC common return bus.
At present I have a 21V wall wart transformer powering twin-coil switch machines and a 12.6v 3 amp center-tapped filament transformer powering panel indicators and signals. Future plans call for using several additional small AC transformers for the same purposes and for structure and 'scenery' lighting. I would like to connect all of them to a single layout-wide bus (#12 solid copper wire). I doubt that the load will ever reach 9 amps, especially since my switch machines only generate momentary spikes and a lot of the connections will actually be for half-wave DC circuits.
I am comfortable with common rail, but connecting AC sources is a new concept for me.
Chuck,
The short answer is that it will be fine.
But remember this, the voltages between the different hot sides of the various transformers will be added together.
So, for example, you take two 24 volt AC transformers, you tie one leg of each together as a common return. Just like the AC system in your home, lets call the hot leg of one trandformer A, and the other B, and the common N, or Neutral.
The voltage from A to N will be 24V, the voltage from B to N will be 24V, the voltage between A and B will be 48 volts.
With three transformers, A, B and C, the voltage between and two hot legs will be 48 volts. All three provide a 48 volt three phase connection that could run a three phase motor if one existed for that voltage.
Using transformers of different voltages may have some small effect on exact delivered voltage from hot to neutral. The lower voltage may bump up some, the higher voltage may dip some - depends on the transformers.
Even though Ed R used DC, it works the same as his original power supply in MZL.
In electrical distribution any new voltage created by a transformer is considered a "derived" source. It is only connected to the source by way of the induction of the transformer - until it is grounded the earth like the system on the primary side.
The only other question would be the exact internal wiring of the wall warts - but simple testing with a meter should confirm that the outputs are completely isolated from the the inputs.
Not needed. I missed the word secondary.
Rich
If you ever fall over in public, pick yourself up and say “sorry it’s been a while since I inhabited a body.” And just walk away.