3-rail AC to 3-rail DC/DCC?

Discussion in 'G / O / S Scale Model Trains' started by 356speedster, Dec 31, 2009.

  1. 356speedster

    356speedster New Member

    OK.. I just managed to loose a long post, by spending too much time writing it so I got logged out in the meantime...

    Anyway... I want to re-wire O scale 3-rail track (AC) to DC for pure DCC operation. Is this as straightforward as I imagine, by just wiring the positive to the center rail and the negative to both outer rails? or will I get into trouble with the switches?

    My focus is smooth slow speed operation (yard switching), and the O scale will be a service line to a DCC operated On30 line so I am hoping to be able to parallel connect both lines and operate all trains from one DCC controller.

    My DCC system deliver 3 amps of power. The On30 locos only draw 0.1 to 0.3 amps each during operation, so I was hoping to be able to run at least one On30 and one O scale small "switcher" from the same DCC controller/system simultaneously.

    I am basically looking for advice on O scale locomotives, the O scale budget is limited, so I am hoping to convert one or two cheap/used diesel switchers to run pure DCC on 3-rails. I need 3-rails because of limited radius (O31). I have no previous experience with O scale AC motors, so I need help to figure out which locomotives that are easier to convert to DC than others. or if the few DCC decoders for AC motors are any good for slow speed operating, and can be used on a "DC wired" 3-rail track.

    Any advice to help me get a couple 3-rail locos running on DCC without having to invest in another $500 command system is highly appreciated.

    BTW: I am not afraid of doing electrical work or a bit of scratch-building to make this work.

    Tore Andre
  2. pgandw

    pgandw Active Member

    Switches themselves will work just fine. You may need to go inside the switches and disconnect the non-derailing feature to prevent cross-connecting the switch machine power supply and your DCC. Your DCC system does not have enough power to throw the switches, and is not recommended for that purpose. Use a separate power supply for all switch machines, either AC or DC.

    1) AC motors run just fine on DC. Most newer (post 1990) 3 rail O locomotives have can motors, which are strictly DC anyway. They use rectifiers/diodes as part of the e-unit for reversing. If the locomotive uses a can motor(s), disconnect/bypass the e-unit when you install the DCC decoder. If there is an AC motor, you will need a specialized DCC decoder to furnish reversing of the field windings in comparison to the armature windings. The AC motors have separate windings for the field.

    2) Most inexpensive 3 rail O locomotives run way too fast for decent switching. Kids like them fast so they are made that way. If the locomotive has 2 motors, you can rewire them to be in series instead of parallel. This will cut the speed in half, and is reported to work very well with Williams locomotives.

    3) Your DCC decoder has to have a sufficient current rating to handle the motor(s). If the engine has 2 motors, you might need a decoder with as much as 4 amp capability. Whether your 3 amp DCC system is sufficient is going to depend on how much the 3 rail O engine(s) draw when running. The smaller engines should be fine.

    4) 3 rail O electronics such as horns, bells, sounds, etc., depend on AC as the primary power. These features are usually part of the e-unit, and will be eliminated when you bypass or disconnect the e-unit. If they are separate, you must disconnect them when putting in the DCC decoder. The DCC decoder must become your source of electronic wizardry instead of any other circuit boards in your engine.

    Headlights may also be a problem - you need to check the current rating of the light functions of the DCC decoder against the rating of the light bulb(s).

    Most DCC decoders don't have enough power for a smoke unit. And your DCC system certainly doesn't have that power to spare. Smoke unit needs to be off, and ideally disconnected.

    Bottom line: the less features your 3 rail O engines have, the better. Finding ones that run slow enough to switch with will be the challenge. Two motors can easily be rewired to run at half speed.

    Though I think that covers most of the bases, you will probably discover some things I missed - hopefully without letting any magic smoke out!

    Enjoy - it sounds like a great plan!
  3. pgandw

    pgandw Active Member

    I thought about my reply some more, and am not happy with advice I gave about switches.

    First, please check with your DCC system manufacturer as to whether or not they allow common return wiring with an AC or DC system. I suspect the answer is going to be no, because it is too easy to make mistake and get the DC or AC back fed into the DCC system, and fry it's brains. The circuit breaker is designed to protect the DCC command center and booster from too much current going out, and might not protect against other power coming in.

    Assuming the answer about common return is no, the implication is that switch machine power must be kept totally isolated from track power, or the switch machine must use track power. As I said in my first post, using DCC power for the switch machines is going to cause noticeable momentary slowing of running trains. And the back emf from the switch machine coils could be disruptive to decoders and DCC commands. I don't recommend using DCC power for the switch machines.

    Unfortunately, Lionel, Marx, and MTH, all ground their switch machines to the switches, even if operated on "constant voltage". This means you have common return wiring whether you want to or not, assuming separate switch machine power. And the Lionel O22 switch constant voltage plugs worry me. All it takes is for the plug to get loose or not fully open the track power contact and you are now mixing switch machine and DCC power.

    To use Lionel, Marx, or MTH switches, I would limit myself to the manual variety for safety of your DCC gear.

    You want to use switches where the switch machine coils have no electrical or metal connection to the rails - the same as in 2 rail scale. I believe Ross and Gar-Graves switches would meet this requirement. Atlas switches also appear to have independent switch machines from their catalog - Atlas uses springs in the points for non-derailing rather then electrical control rails. With these makes, the switch machine power is isolated from track power, and should be perfectly safe with your DCC.

    My other thought on the matter is that unless you are planning an extensive 3 rail O line, I would use the Atlas track in 3 rail O for it's much better looks as compared to traditional tubular track. Atlas has O36 and O27 curves and flex track. Their smallest switch is an O36, which should fit most situations. After all, you do want to show some of the difference between standard gauge and narrow gauge - full scale cars are going to want O36 curves.

    The Atlas track was my plan when I contemplated a combine 3 rail/On30 layout. The factor that dissuaded me form O/On30 was the size of buildings and scenery, not the track or rolling stock issues.

    And if you wanted to/needed to you can use a conventional AC transformer for the 3 rail track and locomotives until you get around to converting to DCC. When the conversion is done, the transformer remains as your source of light and switch machine power.

    my thoughts, your choices

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