I've spent the last few days reading everything I can find on common rail & DCC. The case against it for a multi-booster layout is confusing, but I get enough of it to recognize that it's probably a bad idea.
For a single booster layout, which I am building, I'm not so convinced. Most sources are also against common rail even with only a single booster but don't make a case that's convincing.
I'm starting to think about my wiring plan and believe that I can save a lot of wire, and more importantly for me - work under the layout. I'd like to use common rail unless someone can talk me out of it.
The one source that has me believing that common rail with a single booster is ok is this excerpt from the PSX circuit breaker manual.
The PSX Circuit breaker is designed to operate by opening one side of the two input leads when an overload is detected.
J1-3 is one of the inputs from the previous PSX.
J1-1 is one of the outputs to the next PSX.
J2-1 is one of the outputs to the power district.
This tells me that if you use the PSX breakers, you're going to get common rail for all the power districts feed by each booster anyway.
And BTW, I'm building a spare bedroom, G-shaped layout in N-scale. I have an 8-amp Digitrax and their 20-amp power supply. I will not be leeching power off the DCC system for any purpose other than running trains. I don't care for sound. I will never need more than one booster.
I have the right to remain silent. By posting here I have given up that right and accept that anything I say can and will be used as evidence to critique me.
When our club was built in the early eighties it was common rail and fourteen blocks with one reverse loop, common rail. HO code 100 hand laid and hand laid turnouts.
When we went to DCC with NCE five amp Power pro which easily handles ten sound locos we wired with #14. We also dropped feeders regularly for the common rail.
Murphy belongs to our club we found out over time so we took not chances with the common rail not having feeders every so often.
All the soldering was done with Sal ammoniac solder and flux before I arrived and over the years that started to cause issues at some joints. The ones who did the soldering said that was not the issue. Go figure. I prefer rosin.
Rich
If you ever fall over in public, pick yourself up and say “sorry it’s been a while since I inhabited a body.” And just walk away.
Duplicate post
carl425I've spent the last few days reading everything I can find on common rail & DCC.
my understanding of common rail is for DC blocks, where only one rail is isolated to create a DC block
based on this, common rail is not applicable to DCC since there aren't blocks.
it's recommended that feeders go to both rails, minimizing losses thru the rails (i.e. copper wire has less resistance than rail). doesn't make sense to add feeders to just one rail.
both rails should be isolated between districts protected by circuit breakers since it's conceivable that there are shorts between rails of difference tracks (e.g. laying metal objects across multiple tracks).
of course electronic components have common grounds and should not be confused with common rails
greg - Philadelphia & Reading / Reading
Carl,
Get rid of the PSX and use a Digitrax BXP88 instead. It gives you eight isolated, solid-state breaker-protected power districts (with the added bonus of detection and Transponding, if you ever care to use them) and by default uses a common "Rail A" for all eight sections.
See the illustration on Page 7 of the BXP88 Instructions.
Stevert Get rid of the PSX and use a Digitrax BXP88 instead. It gives you eight isolated, solid-state breaker-protected power districts (with the added bonus of detection and Transponding, if you ever care to use them) and by default uses a common "Rail A" for all eight sections. See the illustration on Page 7 of the BXP88 Instructions.
If it can. it's not in the current documentation. The PM42 supported turning a section on and off via DCC commands. I would expect the BXP88 can do this, but it isn't documented yet.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
gregc my understanding of common rail is for DC blocks, where only one rail is isolated to create a DC block
That is one approach that was sometimes used back in the day. It is by no means a universal approach to DC cab control, nor is it considered the best approach by most experianced DC operators.
gregc based on this, common rail is not applicable to DCC since there aren't blocks.
Well that assumes you are not doing detection and signaling, because you need blocks for detection and signaling even with DCC.
I like the idea of having the circuit breaker and the detector being the same piece of equipment.
One of the things I have always disliked about DCC is the idea of large amounts of current, needed for multiple trains, available at the rails in the event of a short.
With a system of blocks for signaling, and each detector also being a circuit breaker, current could be limited to the amount required for a single train, just like we do with DC.
Sheldon
Digitrax does not recommend common rail wiring.
For best results with DCC, running a power bus using heavy gauge wires under the track and feeding both rails at regular intervals from the bus will be the most reliable method of wiring. Nickel silver rail has much more impedance than regular copper wire, this technique minimizes the rail's impedance.
For example, C83 rail is equivilent to 26AWG copper wire. See https://dccwiki.com/Rail_Size for details.
I would also have to question why so much current for an N scale layout. That is a lot of power for N scale.
I said it before, I was never a fan of commoon rail, I never followed those Atlas books. Even as a kid I had some inkling that coommon rail wasn't the best way to do things. My Dad wired our layout with gaps in both rails, never common rail, and I did the same when I started building my own layouts.
betamaxDigitrax does not recommend common rail wiring.
Take a look at the manual linked to in Stevert's post. At least in the case of the BXP88 they REQUIRE common rail.
betamaxI would also have to question why so much current for an N scale layout. That is a lot of power for N scale.
It's left over from an much more ambitious multi level HO layout that I had to abandon when I contracted this neurological disease thet took my balance. The new N-scale layout is all within reach from swivel chair.
Besides, like the NRA always says, "it's better to have it and not need it than need it and not have it".
RR_MelWhat about loops and turntables, they don’t like common rail.
None of the above here.
ATLANTIC CENTRALWell that assumes you are not doing detection and signaling, because you need blocks for detection and signaling even with DCC.
Exactly. Also, power districts are just blocks with another name.
ATLANTIC CENTRALI like the idea of having the circuit breaker and the detector being the same piece of equipment.
The more I research it, the more convinced I am that this feature is awesome.
ATLANTIC CENTRALOne of the things I have always disliked about DCC is the idea of large amounts of current, needed for multiple trains, available at the rails in the event of a short.
The BXP88 we're talking about here limits current to 3 amps in any one "block".
ATLANTIC CENTRALWith a system of blocks for signaling, and each detector also being a circuit breaker, current could be limited to the amount required for a single train, just like we do with DC.
Congratulations! You're as smart as Digitrax.
rrinker If it can. it's not in the current documentation. The PM42 supported turning a section on and off via DCC commands. I would expect the BXP88 can do this, but it isn't documented yet.
I checked with Digitrax tech support (great turnaround on my email inquiry, BTW) you can't cut power with the BXP88.
They also told me that multiple BXP88's can share the same common rail if you only have one booster.
carl425 betamax Digitrax does not recommend common rail wiring. Take a look at the manual linked to in Stevert's post. At least in the case of the BXP88 they REQUIRE common rail.
betamax Digitrax does not recommend common rail wiring.
i think the terminology is confusing. it's similar to saying DCC is AC and Lionel transformers were AC. AC means different things and in specific contexts has specific meanings.
ATLANTIC CENTRAL gregc based on this, common rail is not applicable to DCC since there aren't blocks. Well that assumes you are not doing detection and signaling, because you need blocks for detection and signaling even with DCC.
of course rails are isolated in DCC for various reasons.
any block detection circuit measures the current thru one rail. Multiple independent detection circuits don't all need use the same rail.
in the Digitrax BXP88 case, there is a single circuit board with multiple detectors in the same unit and can accept that they may all need to be to the same rail (polarity), but am not sure (depends on circuit). I think the detection circuits may all have the same rail polarity, hence require that the opposite rails for each block be the same polarity (i.e. common).
Could one of the BXP88 detectors be wired to a reversing section where the rail polarity is opposite the adjacent mainline rail which other detectors in the same BXP88 is monitoring? would a separate detector be required for the reversing section.
Greg,
Saying the current is measured in one rail seems a less than accurate description as well. Current is measured in the specific circuit, anywhere in the circuit.
I use inductive detectors, and I run both block feeders thru the coil in opposite directions, so I detect both rails. There are several reasons for this, one is to get overlapping detection in my interlockings, the other is increased sensitivity.
The detectors I use do work with DCC, as does my signal system.
A number of the features rely on not using common rail.
Again, it seems in today's world of model trains, common rail provides no advantages with DCC or DC.
gregcCould one of the BXP88 detectors be wired to a reversing section where the rail polarity is opposite the adjacent mainline rail which other detectors in the same BXP88 is monitoring?
The BXPA1 is the auto-reversing version of the BXP88.
Rather than speculating, why don't you look at the picture on page 7 of this manual.
http://www.digitrax.com/media/apps/products/detection-signaling/bxp88/documents/BXP88_rev.0_1.pdf
Between different booster - NO COMMON RAIL Digitrax instructions DO say this.
The "common rail" for the 8 sections of a BXP88 are not common between multiple booster. Detectioon almost always uses just one rail. So that rail MUST be gapped to define the detection sectioons. The other rail does not need gaps because it is carried through and indeed inside the BXP88 the input for that rail just directly connects to the output for that rail, It's a common rail in the sense that it is common to THAT ONE BXP88. If you need 16 detection sections and get a second BXP88 - that 'common' rail MUST be gapped between the BXP88's.
Common rail between boosters results in an electrical sneak path between the INPUTS of the booster and the high power output, especially if running any locos with split pickup (loco one side, tender the other) when they cross the gaps. Digitrax and others make special order boosters with optoisolator inputs to prevent this from being a prooblem, if you really must insist on using common rail. But why would you? Even on an old large DC layout - one wire feeding the common rail for a rooom size layout iss NOT going to work well on DC either, so there must be multiple feeders supplied. To change from common rail takes a few minutesd with a Dremel to cut gaps in the formerly ungapped common rail, and a snip with wire cutters to break the bus wire into two sections. Add oner wire to feed from the second booster and that's really all there is to it. Everybody likes to make things so complicated for some reason.
I'm not a fan of clustered devices like the BXP88 - you end up with short wires from the booster to the BXP66 and then lots of long, heavy wires running out all over under the layout. With a more 'point of use' device, like most of the current transformer type detectors, it's much more obvious to see that you are only breaking the feed oon one side of the track to insert the detection device and the other side stays continuous, until the next booster.
rrinkerIf you need 16 detection sections and get a second BXP88 - that 'common' rail MUST be gapped between the BXP88's
This is NOT CORRECT.
I asked Digitrax yesterday if the common was common for just one BXP88 and this is what they told me:
"If you only have one booster then they would all connect together"
I understand that all bets are off in a multi-booster installation, but note in the title of this thread it clearly says "single booster". In my opening post I explained in detail why I will never need an additional booster. I was even questioned for having too much power.
I also don't understand why folks assume that common rail implies one feeder for that rail for the whole layout. I will solder a feeder to every rail every 3 feet or so. Maybe we should call it common bus instead of common rail.
rrinkerEverybody likes to make things so complicated for some reason.
My personal opinion is that more wires is more complication. Implementing "breaker districts" requires a sub-bus for each. With common rail, you only need half as many sub-busses.
rrinker I'm not a fan of clustered devices like the BXP88 - you end up with short wires from the booster to the BXP66 and then lots of long, heavy wires running out all over under the layout.
You can still distribute the BXP88's. They can also be daisey chained rather than a home run to the booster for each. I'm thinking one on each of the 4 walls of the layout room.
I'm about convinced that the DCC establishment decided that the user base was too stupid to be trusted with the caveat of "common rail is only OK in the case of a single booster" and because it is a problem with multiple boosters, it was eaiser to make just one rule for everybody - NO COMMON RAIL.
Yes, common bus may be more correct.
I have known lots of guys who converted from DC to DCC and did not any extra power drops beyond the block feeders they already had, one pair every 12' to 30' feet depending on the layout.
Most did have all their rail joints within a block soldered.........
No matter what the math says, this every 6' business seems like over kill.
But what do I know, my little trains don't have brains.....
carl425 gregc Could one of the BXP88 detectors be wired to a reversing section where the rail polarity is opposite the adjacent mainline rail which other detectors in the same BXP88 is monitoring? The BXPA1 is the auto-reversing version of the BXP88. Rather than speculating, why don't you look at the picture on page 7 of this manual. http://www.digitrax.com/media/apps/products/detection-signaling/bxp88/documents/BXP88_rev.0_1.pdf
gregc Could one of the BXP88 detectors be wired to a reversing section where the rail polarity is opposite the adjacent mainline rail which other detectors in the same BXP88 is monitoring?
i did
for the different types of detectors i'm familiar with, they monitor the current passing from some source (i.e. booster) to one rail, thru the loco or car drawing current, and returning to the booster thru the other rail. either rail.
those detectors do not need to have booster connections for both rails, only for the rails isolated to create a (detection) block.
The designs I'm familiar with do not require a connection to the other terminal of the booster/bus connected to the other rail of the track (the rail described as the common rail on pg 7 of the digitrax manual).
for the detectors i'm familiar with, the isolated rails on one rail can be the non-isolated rail for a different detector board and each board could be connected to opposite booster/bus terminals.
and for the multiple detector boards I'm familiar with, that have a single connection to the booster/bus, the rail isolated for all the detectors on that detection board must be the same rail (same side of the track), the one corresponding to the booster/bus connection to that multi detector board.
and i believe this is what pg 7 of the digitrax manual is describing, why the isolated rails for all the detectors on a particular BXP88 must be the same rail. they don't need to be the same rail for a different BXP88
since this is not possible when there is a reversing loop, a different board is required. If there were multiple detection blocks within the same reversing section, a single BXP88 could be used, but only for the blocks in the reversing loop.
Looking at the wiring schematic for the BXP88 on page 7 of the manual, two things stand out.
Cost reduction by eliminating 3 terminals and their associated circuitry on the PCB, and simplification of the wiring.
Using a common rail for the detection sections, they reduce the wires required by 3, and simplify the installation at the same time. This may be appealing to those who consider electricity magic as well.
Common bus is indeed a much beter term for it. But many worked their way up through the Atlas project books and almost all of them showed ONE WIRE to the 'common rail'. Granted, most plans in those books were 4x8 or smaller The only place they ever had feeders to both rails was for revese sections where both rails need to be gapped.
Yes, you can distribute BXP88s, but unless you don;t use all 8 sections of each one, that's still a lot of wiring centralized back to each node. 8 is still better than the BDL168's with SIXTEEN detection sections on one board. But 8 sections is still typically going to be 8 train lengths or more. That can be a LOT of railrooad distance. In a double track area, with a pair of crossovers, it does happen to work out nicely to have 8 detector sections - west crossoover, east crossover, east main between, west main between, east main east of the interlocking, west main east of the interlocking, east main west of the interlocking, west main west of the interlocking - 8 things to detect.
rrinkerBut 8 sections is still typically going to be 8 train lengths or more. That can be a LOT of railrooad distance.
I have an unfair advantage there. Because I'm "twice around" with double track ( the lower lap is staging), any point on the layout would have 4 detection blocks within 8 inches.
I also will need 26 detection blocks which will give me the flexibility to distribute the 6 spares over the 4 DXP88's to make things as effecient as possible.