dual power pack and DC common rail

Discussion in 'FAQs' started by Santa Fe Jack, Feb 17, 2007.

  1. Santa Fe Jack

    Santa Fe Jack Member

    I read in Andy Speradeo's book that if you intend to wire your layout with common rail (as I do) then one must exercise caution when using a dual power pack. Specifically, if the dual controls are run from the same transformer, then connecting them via common rail would short it out. Only if the dual power pack has separate transformers can they be used on a common rail scheme.

    So, I have a question: Will my dual power packs work? I have:

    MRC Tech II 2800 and and MRC Tech 4 280.

    MRC does not provide sufficient information in its manuals or on its web site for me to tell if these have one transformer or two. does anyone out there have experience sith this?

    - John
  2. rod46

    rod46 New Member

    hi Jack,

    I use the common rail method with an MRC pack and a cheap Bachmann pack (single control per pack). They work great for me. I would think if your packs are dual control there should be dual DC contacts. Maybe? The Atlas wiring book is fairly good at explaining the wiring requirements.

    Rod (rod42)
  3. ezdays

    ezdays Out AZ way

    He may be talking about a dual power pack, not two independent power packs. But, in any event, a dual power pack has more than one output, or secondary winding on the transformer and the output of these windings is what's converted to DC. They should be isolated from each other in either case, so there shouldn't be a problem with using a common rail regardless.

    There is one other concern when you do that, there will be a time when the voltage between the two power pack non-common leads can be twice the rail voltage depending on the polarity. Again, if you're using a two-position toggle switch, that shouldn't be a concern. If you're using a rotary switch, be sure it's a break before make type or you will short out your power packs.
  4. 60103

    60103 Pooh Bah

    The problem is that not all dual power packs have independant transformers; some use one set of transformer windings for both controls.

    Test the power pack: turn both controls part way on. Get a 12V bulb and touch it to all the combinations of the terminals from one control to the other. If it lights up anytime connected from A to B, the controllers are not independant and the common rail is out.

    (changed "wirings" to "windings")
  5. ezdays

    ezdays Out AZ way

    Man, I'd hope they would at least use separate secondary windings. I've got an old MRC 700N dual pack and it has one transformer with separate windings for each output. But you're right, if they don't, you can't use a common rail.
  6. Santa Fe Jack

    Santa Fe Jack Member

    You got it, David. That's exactly it.

    Your test suggestion sounds good. I'll try it out tomorrow. Thanks.

    - John
  7. pgandw

    pgandw Active Member

    I am sure the Tech 2 2800 has independent power supplies; my Dad used one on his layout with common rail wiring and no problems. At the time of Tech 2 production, MRC was the one company that could be depended upon to make dual controls compatible with common rail.

    As common rail has fallen out of favor in recent years in the model railroading "press", I can't be sure about the Tech 4 280.

    Are you sure the manuals on the MRC web site don't address this?

    yours in wiring
  8. Santa Fe Jack

    Santa Fe Jack Member

    I ran David's test. Both controllers tested OK for common rail Whew! :thumb:

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