Digitrax Back-EMF

Discussion in 'DCC & Electronics' started by platypus1217, Mar 1, 2009.

  1. platypus1217

    platypus1217 Member

    Has anyone ever set up the back-emf features on a digitrax decoder? It seemed like a bit of work going through the three CV's and indvidually incrementing them to find the correct value as described in the instructions. Is it worth it?

    I tried turning them up pretty high and I still had to crank the throttle up to get it to move. I was expecting that the back-EMF would help the motor get started at slower speeds. My understanding was that when the engine starts off at a low voltage that isn't quite enough to turn over the motor the decoder should keep increasing the voltage until the motor turns over and generates some back-EMF that is consistent with low voltage that is being applied to it. Is that how it is supposed to work?
  2. steamhead

    steamhead Active Member

    I'm no expert, but.....Back EMF is a feature of the decoder which allows it to monitor motor speed and automatically compensate power to the motor to maintain a constant speed.

    What you want to achieve is controlled by the variable VStart, which is CV #3.
  3. diburning

    diburning Member


    To change the start voltage/speed, it's CV2. Atlas units are generally around 10, Athearn can be 50+, proto 2000 should run ok out of the box. I'm not too sure about other brands but the numbers above are general numbers. My Athearn SD50 is at a value of 73 to get it to crawl at speed 1.
  4. steamhead

    steamhead Active Member

    Diburning..is right. It's CV2. I mis-read my CV card....:cry:
  5. platypus1217

    platypus1217 Member

    I got the top speed of my two locos matched up ok last night. But they have different starting inertias so once you go below 3/4 throttle they start to get out of synch.

    I was half tempted to go out and get a scale speedometer and USB interface so that I could write a program to do it for me. Just put the train on the track and tell the computer the scale speed curve that I want the engine to have and then let it do the work of figuring out what values to use in the speed table.

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