Occasional Polarity Flip


D&J RailRoad

D&J RailRoad

Professor of HO
I have a full basement empire that's divided into three power sections; 1, 2 and 3. Section 2 is powered from the command station, a DCS100. Sections 1 and 3 have their own DB150, each in booster mode.
The double track mainline runs through all three power sections.
I have a power buss running under the benchwork for the mainline and power drops to every piece of track.
Each section is completely isolated from the other two with insulators where the tracks join.
The command station and booster output of rail A and rail B are connected to the track with rail A toward the operator and rail B toward the wall.
Over the ten years I have had the layout operating one of the power sections will suddenly invert its power. Not the same section each time though. Measuring voltage from one rail to another shows about 15VAC. Measuring across the section barriers should show the same voltage between the front rail and the back rail. When one of the sections inverts itself, voltage will measure across the section barrier from front rail section 1 to the front rail section 2 or 2 to 3. Same with the back rail.
I'm really not clear on why it does this. The power for each section comes from the command or booster into a Tam Valley soft start module then into the PM42 of each section then into the BDL168 then to the rails. The Soft start is a straight through module that provides a bit of resistance during startup so a fleet of locos on the rails doesn't seem like a short to the command station.
Sometimes, inverting the Rail A and Rail B output of the booster or command station will correct it. Other times that doesn't but instead, inverting the Loconet cable connector will. Sometimes that doesn't work either. Sometimes inverting the power in from the power supply will change it, but not all the time.
Is this common with Digitrax equipment or is someone messing with me?
 
Ghosts???

Isn't DCC current at the track which will show as alternating (AC) voltage? Plus, to measure the volyage in an accurate manner, a RAMP Meter should be used since an normal multi-meter can't measure the DCC voltage correctly since it has a square wave unlike normal AC current.

Greg
 
Last edited:
Reading into the problem and it turns out that there is a occasional problem with the Digitrax command station flipping polarity. It will also correct itself if it feels like it. After letting the problem sit unused for a day, I turned the system on then checked the polarity and it had corrected itself.
 
So you are saying that all 4 outputs of the PM42 change simultaneously? And then I am presuming you have the PM42 set up as a circuit breaker rather than an auto reverse unit. Otherwise it seems all this would be a moot point as any running train should be changing the "polarity" as needed to enter the next district.

Yes, true, but there is still the common side and hot side, sorta like your house breaker panel.
Click to expand...
Not unless you wired it that way. You said you had each section totally isolated with two rail insulators where the power joins, so that says there is no common side.

And now you have me puzzling when the command station sends the boosters a signal on how to start the pulse. Just once at start up? Otherwise it seems it would correct at next sync signal.
 
The DB150 (booster) for the section flipped polarity. It feeds to a PM42 then BDL168 then to the tracks.
There are three sections of the layout. Section 1 is supported by a DB150 (booster), Section 2 supported by a DCS100 (command station) and section 3 supported by a DB150 (booster).
Each section is completely isolated from the other sections with insulators on both rails at the end of each section and buss wires terminating at each section. The only connection between them all is the loconet cable
It's the boosters that occasionally flip polarity. There are no reversing loops on this system. The layout is a large loop around the basement so trains can be run continuously if desired.
 
Do you have auto-reversing enabled on the DCS 100 and DB150's? If you are not using this feature you should disable it. This will cause the command stations and boosters to flip their Phase (Polarity). From the description of your setup you are not using the auto reversing function anyway since you have a PM42 between your districts and command station/boosters.

For the DCS 100, Op switch 03 controls auto-reversing and setting it to close will disable it. See page 44 of online Manual

https://www.digitrax.com/static/apps/products/starter-sets/scfx/documents/SuperChiefXtra_2.pdf

For DB 150. Op switch 03 also controls auto-reversing. Setting it to closed will disable this. See page 41 of online Manual

https://www.digitrax.com/static/app...s/sebx/documents/SuperEmpireBuilderXtra_2.pdf

Even though you have signal passing through other devices it could be shifting phase on startup. By disabling auto reversing it will not shift phase on you.

Once this is disabled you will need to verify that each of your sections are in phase with the other sections

One other thing do you have a reference ground (not earth ground) between your command stations and boosters or are you relying on the Loconet cables to do that? This reference ground is also needed for devices like the PM42 and BDL168's to function properly (at least according to digitrax's manuals). I use PSX circuit breakers and rrcirkits block detectors so I'm not much help to you on the PM42's or the BDL168's.

You could have a grounding loop issue but that is more difficult to diagnose (have to test all loconet cables)

Hope this helps.
 
Last edited:
Yes, I disabled the reversing loop long time ago (T). Actually, the book says closed makes it auto reversing. I check it occasionally in each box when I clear the loco rosters.
Yes, have ground wire between all the items as recommended by Digitrax.
 
D&J RailRoad said:
Yes, I disabled the reversing loop long time ago (T). Actually, the book says closed makes it auto reversing. I check it occasionally in each box when I clear the loco rosters.
Yes, have ground wire between all the items as recommended by Digitrax.
Click to expand...

Digitrax manual is certainly not clear on that. Excerpt below is from digitrax DCS 100 manual. When is says "DCS100's booser is auto reversing" as thrown as factory default, that is certainly confusing. Research on Digitrax.io groups shows you are correct though. Now I gotta check my spare command station. I recently replaced my DCS 100 with a 240 because my signal system was flaking and discovered it was the command station causing the issues. My booster (DB100+) has a physical switch on it for auto reversing (which I don't use) so that one is easy.

DCS 240 manual is much clearer on this that DCS100 manual.
DCS 100 op switches by Chuck Lee, on Flickr


Another thing could be that you CMOS battery is dead in the command station. could it be reverting back to auto reversing on each boot if the CMOS battery is dead because it basically does a factory reset each time its powered down. Doesn't seem likely since you would have to set Op Switch 3 to closed to enable auto reversing if its not a default "on" option.

Gonna pulled the CMOS battery on mine and see what the default truly is.
 
You must log in or register to reply here.



Back
Top