(resurrecting one of my old threads)
I was thinking about this topic the other day, and I came across this article on DCCWiki:
Braking Districts use various Digital Command Control technologies which create a braking section for supported decoders. This allows automation with signals by stopping trains where a signal is against them.
dccwiki.com
It discusses various options including "Brake on DC" which it rather dismisses but it gave me a few clues on how I could do some more investigation. As mentioned in earlier posts the information from Massoth is limited and rather vague.
As far as I could tell, it should work like this. You identify a section of track on your DCC layout where you want to trigger Brake on DC. Create an isolated section (both rails not just one) and set up a switch to switch the power to that isolated section from DCC to DC.
It is very important that the isolated section is longer than the length of your loco (or consist) AND BRAKING DISTANCE. This is important as you must not bridge DC voltage track and DCC voltage track, otherwise you may/will damage your DCC command station and/or booster. If your loco (or consist) spans the different sections DC current will flow through your loco power pickups and potentially back to your command station*. Assuming your decoder supports Brake on DC, then you set CV 29 to turn DC mode off. Run the loco on your layout and when it is FULLY in the isolated section switch the power from DCC to DC. The decoder recognises the change in voltage type and tells itself to set speed to zero. The loco will then come to a stop. When the isolated section is set back to DCC, the loco will resume at the speed step it entered with.
(* this is what the Lenz LT100 device - if you could get one - would prevent happening - see post #6)
WARNING: DO NOT TRY THIS YOURSELF UNLESS YOU REALLY KNOW WHAT YOU ARE DOING!
I thought I would give this a try to see if theory and reality are more or less the same. My indoor layout is wired in such a way that I can switch the whole layout from DCC to DC. This is so I can run analogue as well as DCC fitted locos. That means in order to test the theory, I didn't need to create an isolated section, and I didn't need to worry about that section being long enough as my entire layout was the "isolated section". So I set a loco to "DCC 28 speed steps, no DC" by setting CV 29 to value 2, and set it going. At a suitable straight section with fingers crossed I flicked the DC switch. Sure enough the loco came to a halt - rather abruptly. This was because I was applying 0 volts to the DC section - doh! Trying again with 12 volts DC then the loco again came to halt when I flicked the switch, still quite abrupt but not a dead stop as before. Tried it again this time with lights and sound turned on. To my surprise after the halt in the DC section the lights and sound stayed on.
I wondered if I could make the halt less abrupt. Thinking about how this is working, assuming the decoder is telling itself to set speed to zero, it is probably using the CV 4 braking speed to control how quickly it brakes. I changed CV 4 from 8 to 32 and tried again. Sure enough the braking was much more leisurely, which of course means it took a longer distance to stop. This is probably why the DCCWiki calls this method "unpredictable". (Strictly speaking it is predictable but it depends on knowing the entry speed of the loco and the braking speed parameter.) I wondered how much DC voltage I would need, and whether polarity mattered. My tests showed that polarity does not matter, but you need at least 6 volts for the decoder to detect it and to power lights etc.
To summarise my findings:
- Brake on DC works more or less as expected
- The decoder needs to have DC mode turned off
- The isolated section length needs careful planning to avoid damage to your DCC kit
- The DC voltage needs to be at least 6 volts (12 volts better) but polarity does not matter
- Lights and sound stay on in the DC section
- Loco resumes when DCC restored
I would agree with the DCCWiki conclusion that Brake on DC is problematic and unpredictable, but in controlled conditions it could be useful - that is, if you can set the max speed and braking speed then the braking would be predictable.
I will try setting up an isolated section on my layout to try this out for real, probably using a loco magnet / track reed switch detector to trigger the switchover from DCC to DC.
I should add that my tests were done with a Massoth XL motor and lights decoder and Massoth S sound decoder connected via SUSI cable to the XL, and a Lenz LZV100 command station running 19 volts (input). DC was provided by a PIKO analogue controller supplying 12 volts (output).
Finally a quick 30 second video to show what the automatic braking looks like. The switch to DC occurs 19 seconds into the video.