I have a stupid DCC/DC question

Polski Sep 10, 2014

  1. espee4441

    espee4441 TrainBoard Member

    24
    6
    12
    I am just trying to figure out how calibrating between AC and DC can be done. The decoder is set up to some sort of speed table initially, so I guess the only thing left is how does the DCC side get calibrated to the DC side? From the looks of it, the decoder that draws the highest voltage would be the one that defines the rest in the fleet to follow.

    That would make my new GP40P-2 as the default for all others, which would mean tweaking the r-t-r decoders to start at notch one around seven volts or so. I guess it takes a serious voltmeter for starters to see how each one performs.


     
  2. montanan

    montanan TrainBoard Member

    974
    925
    21
    I a DC only and have found out how much more sensitive DCC locos are to dirty track. When operating in the DCC mode, full voltage is to the track whereas when operating in the DC mode the locomotive may not be getting full voltage. Found that out with that first little Mogul. There were spots where my DC locomotives were having no problems at all but the DCC loco would cut out from time to time. After a good track cleaning, the DCC locomotive no longer had any problems.

    The DCC guys at my club reinforced that. I am a DCC novice and have no plans at all to change over to DCC.
     
  3. Greg Elmassian

    Greg Elmassian TrainBoard Member

    242
    30
    11
    You always "lose" a few volts in a decoder-equipped locomotive, the track power is not hooked directly to the motor, but normally must go through a full wave bridge rectifier and then through the output transistors and then to the motor.

    This is why an HO DCC layout typically runs at 16 volts or so, when pure DC would run the loco fine at 12-13 volts.

    Greg
     
  4. montanan

    montanan TrainBoard Member

    974
    925
    21
    Greg is so right. I keep a volt and ammeter in line with my DC throttle and it usually can take up to 6 volts to "wake" a DCC locomotive up and then slightly more to get it moving. My old DC Atlas Alco locomotive that I picked up when they first came out with the Kato drive are some of the smoothest running locomotives I have seen. They can start creeping so slow at just barely over one volt so slow that you can time them with a calendar between ties.
     
    Hardcoaler likes this.
  5. Greg Elmassian

    Greg Elmassian TrainBoard Member

    242
    30
    11
    That 6 volts is actually driven by the fact that the microprocessor needs to be running as well.

    But you won't lose 6 volts to the motor, the motor will usually get 2 or 3 volts less than the track voltage.

    Also remember all decoders put out PWM, i.e. full track voltage pulses of varying width (duration)... (minus the 2 or 3 volts)...

    So running on DC... when the micro finally wakes up at 6 volts, it will probably provide pulses of 4 or 5 volts, and then these pulses will increase in voltage in step with the track voltage rising.

    Because the "top voltage" to the motor is limited by the DC track voltage, slow speed starts under DC won't be as good (starting with 4 volt pulses) as slow speed starts on DCC, where there is ALWAYS 16 volts (HO example) available to the motor for nice high voltage pulses.


    Greg
     
  6. nstars

    nstars TrainBoard Member

    22
    0
    17
    As far as I know it's DCC with sound which requires 6 volt before the loco starts moving. The 6 volt is necessary to power the sound amplifier. A DCC locomotive without sound will already run at much lower voltages.

    Marc


    Verzonden vanaf mijn iPad met Tapatalk
     
  7. Greg Elmassian

    Greg Elmassian TrainBoard Member

    242
    30
    11
    There are decoders that have microprocessors that start operating at lower voltages, but MOST operate at 5 volts.

    Actually the sound amplifier can operate at a much lower voltage, since it is only an amplifier, not a microprocessor.

    But of course the same microprocessor runs the digital part of the sound, so it's back the the microprocessor again.

    Greg
     
  8. nstars

    nstars TrainBoard Member

    22
    0
    17
    Most loco's with modern decoders (like ESU Lokpilot, Zimo and CT Elektronik) will run at 2-3 volts. Just check for instance the N scale Bachmann 44 ton loco. According to Model Railroader it already runs at 3V and I would hardly consider the Bachmann decoder as modern. Sound loco's will start at higher voltages as they do need considerable more power to run the motor and produce sound at the same time. And power is current multiplied by voltage. Note, older decoders had the habit to revert back to 14 speed steps under DC which explains the jerky ride.

    And don't use a DC controller with heavy pulse (like for instance MRC controllers) as it can create lots of problems with decoders up to the point where a DCC loco will only start at high voltages or won't start at all on an analog layout. Only use controllers with minimal pulse.

    Marc


    Verzonden vanaf mijn iPad met Tapatalk
     
    Last edited: Sep 7, 2017
  9. nstars

    nstars TrainBoard Member

    22
    0
    17
    Apologies for this message, I made a mistake which resulted in a response too many.

    Marc
     
    Last edited: Sep 7, 2017
  10. Greg Elmassian

    Greg Elmassian TrainBoard Member

    242
    30
    11
    I'll have to check my recent Zimo decoders. I also have CT Electronik. They are not so common over here in the US.

    Completely agree about pulses, although most of the PWM controllers are just generic PWM... when you mention "minimal pulse" do you mean a controller that uses DC and some pulse that minimizes at higher speeds? Can you give an example of a controller that does that?

    Again here in the US, most stuff is MRC and it's kinda crummy...

    Greg
     
  11. nstars

    nstars TrainBoard Member

    22
    0
    17
    The optimal solution would be pure DC, hut that won't be great for loco's without decoder. We have discovered that a controller with rectified AC output delivers the best compromise for use with loco's with and without decoders. That pulse is mild, won't be seen as a digital signal by a decoder and still give a loco without decoder a push.

    Marc


    Verzonden vanaf mijn iPad met Tapatalk
     
    Last edited: Sep 9, 2017
  12. Greg Elmassian

    Greg Elmassian TrainBoard Member

    242
    30
    11
    So you mean rectified sine wave DC, unfiltered.

    [​IMG]
     
  13. nstars

    nstars TrainBoard Member

    22
    0
    17
    That's the pulse we use for analog. Btw, we normally run DCC, but we can run analog for testing purposes.


    Verzonden vanaf mijn iPad met Tapatalk
     
  14. Greg Elmassian

    Greg Elmassian TrainBoard Member

    242
    30
    11
    Over here, most people would not call that pulsed DC, but unfiltered, rectified AC. But I agree with you, technically this is pulsed DC.

    It's also easy to supply! :)

    Greg
     
  15. nstars

    nstars TrainBoard Member

    22
    0
    17
    No problem with calling it unfiltered, rectified AC and it is indeed very simple. We use an old schematic published in MR as a base for a cheap transistor controller.

    Marc


    Verzonden vanaf mijn iPad met Tapatalk
     

Share This Page