Debugging DCC bus short

[Graskie]

Accepted best practice has always evolved or changed in many fields, though, TWD. For instance, no rustable steel has been used in trackwork for some years now. I can't possibly see any potential problem, especially wiring every single conductive fishplate in a clean average warm room, although, as I said, I haven't needed to do that.

[Teedoubleudee]

Agreed Graskie, that best practice evolves but that is usually in tandem with newer materials and products. All I'm saying is why would you solder the dropper to a piece of metal that is then connected by touch only to the next rail.

 

Look at this way. Take 10 pieces of trackwork. To join them all together using the soldered fishplate method results in 9 solder joints per rail (18 joints in total). It also introduces 36 points of possible failure over the ten piece network. By soldering directly to the rail as in my suggestion requires 10 solder joints per rail (20 overall - only 2 more solder joints) and virtually NO possibility of a failure due to temperature or movement. You only want to do this job once, isn't it better to reduce probability of failure first time?

[Fishmanoz]

Interesting that DCC bus discussions always cause enough heat to melt points, just ask john. We know that a range of practice works but we also know some practice is better than other. 

 

We now have 3 ways to compare and 2 different ways of making a joint.  Starting with joints, we can have physical or chemical.  The chemical I refer to is soldered where the solder flows onto the pieces being joined forming an intimate bond on each and between them.  It is generally accepted that this is one of the best electrical joints.  Physical bonds are where the 2 pieces are pushed against each other in some fashion.  The problem with them is possibility that corrosion which is an insulator will form over time and cause the electrical resistance in the joint to rise.  Clearly physical bonds are used in all manner of connectors so can be made to work but are invariably gold flashed in high reliability applications to avoid corrosion (note to Graskie - are you using gold rails and fishplates?).  Another physical method that used to be in vogue was called wire wrap where the wire was wrapped onto a square peg using a special tool that maintained tension as the joint was form, with the aim to ensure the corners of the square actual dug into the wire to make it more electrically secure.

 

So when we rely on fishplates for electrical continuity, we run a risk.  This risk is twofold with the possibility of both corrosion (oxidation in this case) and loss of tension in the fishplate over time leading to a higher resistance.  A soldered joint to the rail is better.

 

Notwithstanding all of that, we know that some use single point connections to their track, and usually physical connection via a power clip or track, plus physical DCC point clips and have no problem.  Then we also know that for others like john, there have been significant problems leading to point melting with this method.

 

It is clear that multiple point connection is better, in theory and in practice.  Even if it still relies on physical connection at fishplates, doing it at many of them significantly reduces the risk as the power is approaching all rail joins from 2 directions not one, and no connection relies on a long run of fishplates, point blades and point clips to get to it.

 

But that still leaves soldered joints to every piece of track work as the best practice.  Not many of us will go that far, we all assess the risk of not doing so in some fashion and our individual experiences over time tell us if we got the risk right.

 

And just a note that those using best DCC practice at electrofrog points have no choice but to solder to a number of places on them.  The necessary breaks in the point blades and IRJs mean soldering can't be avoided, and not to fishplates.

[Fishmanoz]

But back to CS, have you found the problem?  Is it a short/swapped wire somewhere?

[choralc]

I can just hear it now: "Gee, don't some people ever read what's posted by the OP? He said this was a practice layout and he had heard good and bad about both methods!" Gotta laugh :)

 

After a restless night tossing and turning I finally got up this morning and took the meter to the patio. A few minutes remembering how to use it and yes I have continuity across the rails, sigh

[choralc]

/media/tinymce_upload/de23da52e5379d1ffa7715fda95f1913.jpg

[Fishmanoz]

No surprise there but good to confirm it.  And if you can't find it by inspection, no way to avoid gradually pulling it apart until that short disappears.  Every time you disconnect something, check for the short with the meter across the rails.

[choralc]

Ray, they're all Hornby insulfrog points. I will save trying electrofrogs for the 'proper' layout.

 

It's one of the reasons I am trialling droppers on fish plates. Pic to follow after moderation.

/media/tinymce_upload/68c5a589c0b72d8d51e34ff0e2bb19c2.jpg

[choralc]

Graham, I used the Elite. Swapped connecting wires was the first thing I did but still got the dreaded error on startup. Then I started up the Elite without connecting wires and then inserted one - okay - then the other - error.

[choralc]

Yes, Fishy I was trying for mutiple point connections to the point of being over engineered.

 

Being a relatively small layout on an 8' x 4' board I tried for continuity by having a dropper where ever I could barring the points as I wasn't going to solder wires to them hence the droppers on fishplates. Three of them on a single point either two blacks one red or two reds and one black except where two or more points were joined together in crossovers or a ladder of points (if that's the right term) in the yard which is where I used soldered droppers to the underside of the rails.

[choralc]

Fish, I'll mow first and then get back to the layout. The sun is in just the right place to make it uncomfortable to work on the layout.

 

One thing about the droppers on the fish plates is that I may be able to slide them off completely without unsoldering them from underneath. If I make sure they are then isolated from the rails I can keep testing for continuity on different parts of the layout until I get to the places that I can't simply remove them and then have to unsolder!

[choralc]

Took off the first pair of dropper-soldered RJs between two pieces of flexitrack and had no continuity on one disconnected side but not so on the other where a crossover is downstream. Now I don't understand as the current surely goes both ways? I'll isolate the crossover next and focus on the outer oval.

[choralc]

Well it's not in the outside oval so I guess I'll go over the inside oval and sidings and disconnect them and reconnect the droppers on the outside and work my way in testing with the multimeter as I go as I should have done in the first place :)

 

And if anyone is thinking of it there is no reverse loop.

 

My good lady says to leave it for today and come back tomorrow and that's all I'll say about that.

[96RAF]

See what NMRA member and professional electrical engineer says about 'anecdotal evidence'. Especially "Hey I wired my layout this way and it worked!"

https://sites.google.com/site/markgurries/home/dcc-general-best-practices/wiring-planing

 

 A good days worth of reading there poliss and superb infromation that answers many of the Q's posted on here over the years.

 

[96RAF]

Took off the first pair of dropper-soldered RJs between two pieces of flexitrack and had no continuity on one disconnected side but not so on the other where a crossover is downstream. Now I don't understand as the current surely goes both ways? I'll isolate the crossover next and focus on the outer oval.

 

 

I'm wondering if your crossover is an X shape (diamond crossing) or a pair of points (turnouts) loop to loop. Normally an X crossover would take an outer loop of 3 to the inner loop without access to the middle loop. Continuity of such track is same as for normal rails L-R.

 

No matter what crossover you have it seems as if your fault finding whilst of necessity painstakingly slow (unsoldering/resoldering, checking, etc) it is very methodical and will eventualy provide the Eureka moment - hopefully sooner rather than later.

Good luck.

[Teedoubleudee]

It may be too late for you now as you sound like you are a long way down the road in your fault finding exercise so hopefully will be shouting "Eureka" any time now but.....

 

This may be useful to you or others in the future.

 

During my service training in electonics, we were tought the "halve it" method of quickly finding a short or open circuit on a continous length of wiring. Basically it means splitting the entire failing length in approximately half.  Testing it, one half should stil be faulty and the other good. Now split the faulty half in two etc etc. This method should prove quicker than starting at one end and working your way along, unless you get lucky!

 

Beware if you have a 'ring' type of circuit though - your first division must be split in TWO places before testing.

 

 

 

[Teedoubleudee]

Off Topic.

 

Fishy, your earlier essay on electrical jointing best practices, and the part regarding the wire wrap method reminded me of my only experience of this. Everybody will be aware of the English Electric Lightning fighter jet of the 1960-1980s era. In the pointy nose of that beast lived the Farranti A.I.23 airborne interception radar. This piece of wizardry employed the wire wrap method of connecting sub components rather than the conventional plug and socket method. Farranti obviously considered this superior for the rigorous pounding it's equipment might suffer when exposed to speeds of Mach 2 and above!

 

[choralc]

Thanks TDD, I will only remove half the connections tomorrow morning. The bus is an open ring with the control connecters roughly half way.

[Teedoubleudee]

Hi Choracl,  Hope you haven't misunderstood me regarding the halving method. No need to remove half of the connections - just split the complete track into two completly isolated sections from each other by pulling the track apart. As yours is not a complete ring, you should only need to split at one point. Ensure that NOTHING else is connected to that part of the track, including the controller, and then check again across the rails.

Terry

[choralc]

Hmm that might be hard to do as I followed a route with the bus to meet the position of the droppers but I'll give it a go. Pic to follow of my bus routing

[choralc]

/media/tinymce_upload/03586b6c52012610c6cd7498b9ec4e27.jpg

[StephenD]

TWD, I think you're 'halve it' method is called a binary search in IT and computers use it to find a particular value in a list of ascending or descending values.  I thought of suggesting it but felt that it would necessitate cutting the bus, perhaps in several places, which was possibly worse than having to unsolder all the droppers.

[Teedoubleudee]

Stephen, you are correct in calling it the binary method (I worked in IT after leaving the forces) but I didn't want to confuse the situation by bringing computers into the conversation !!

 

.

 

Choralc, looking at your picture could you at least break the track at the point it joins the controller (ensuring that neither side is still connected to it? That at least would (should) remove 50% of the layout from your fault finding excercise..

 

You may be able to do this without lifting the track by pushing the fishplates along the rail so that they only touch one of them leaving a gap between them

[StephenD]

I don't understand TWD.  As both halves of the layout would still be connected to the bus, is it enough just to put a break in the track?

[Fishmanoz]

Have to split the bus too.

 

And don't forget that Murphy says it matters not where you split it, there will be at least 2 faults, with at least one in each half!  Hopefully not but remember if both halves still show a short.