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DCC Momentum (CV 3,4) Question...

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DCC Momentum (CV 3,4) Question...
Posted by Anonymous on Friday, April 11, 2008 11:54 AM

I'm having some trouble getting my N scale SD80's to stop and start at the rate I want them to.

The digitrax manual seems to think the value for CV 3 and 4 tops out at 31, but I've testing much higher and it seems to work. When I put in a value higher than 127 it seems to have the same effect as a value of 0.

Another problem I have is when I have CV 5 and 6 set pretty low it still seems as if low throttle seems too fast.

What do you have your CV 3, 4, 6, and 5 set at?

- Thanks in advance

  • Member since
    December, 2004
  • From: Bedford, MA, USA
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Posted by MisterBeasley on Friday, April 11, 2008 1:19 PM

I have most of mine set between 5 and 20.  Of course, I've got a wide mixture of both engine and decoder models.  The CV's should be specific to each decoder, although momentum and start voltage seem to be common among all of them.  It wouldn't surprise me, though, if some had a range of 0-31 and others went from 0-127.

Have you adjusted CV 2, the Start Voltage?  The rule of thumb on that is that it should be set so that the engine just barely starts moving (by itself, no cars attached) when you go from zero to 1 on the throttle.

It takes an iron man to play with a toy iron horse. 

  • Member since
    January, 2002
  • From: Portland, OR
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Posted by jfugate on Friday, April 11, 2008 1:56 PM

What decoder are you using? If the decoder has torque compensation or pulse width modulation, you can often tune those settings to get a very smooth low speed performance (assuming the loco already runs reasonably well and doesn't have a tight mechanism).

I've also found locos that have a slightly tight mechanism can often benefit from back EMF (BEMF) at low speeds. TCS recently added BEMF to almost all their decoder line and you can have BEMF only come on at lower speed settings if you want. I find this works absolutely *great* for the ocassional loco in my fleet that has a tight mechanism. I can get silky smooth low speed performance by using BEMF in this way. 

Joe Fugate Modeling the 1980s SP Siskiyou Line in southern Oregon

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Posted by Anonymous on Friday, April 11, 2008 2:30 PM

 jfugate wrote:
What decoder are you using?

All my loco's are Kato's using 163 series drop in decoders. I left everything except fro the address and CV's 2,3,4,5,6 defaulted.

  • Member since
    July, 2003
  • From: Sierra Vista, Arizona
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Posted by cacole on Saturday, April 12, 2008 7:48 AM
I always start out with CVs 3 and 4 set to a value of 4, and have found that to nearly always be enough momentum.
  • Member since
    October, 2001
  • From: Sumner, WA
  • 242 posts
Posted by MRRSparky on Saturday, April 12, 2008 7:50 PM

Almost all the locos I have (HO) are controlled by Soundtraxx.  I do have a few with Digitrax motor decoders and Soundtraxx sound-only decoders.  I find that with the Digitrax decoders and the LC-series Soundtraxx decoders, I need to have CVs 3 and 4 set for 20 for what to me appears to be realistic response with no train attached.  If you have a loco with 30 or 40 loaded coal hoppers connected, CVs 3 and 4 would need to be set for a lot higher.

For the two Tsunami-equipped locos, CV 3 is set for 40 and CV 4 is set for 30.  Using the CV at 20 was almost equal to having no noticeable acceleration or deceleration delay.  So I guess it depends on whose decoders you are using.

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