Conductive Plywood?

[jamie92208]

Has anyone ever had a problem with a short circuit through a baseboard.

 

I've just started wiring up the fiddle yard on Lancaster Green Ayre.  Unlike the rest of the layout it is built on 9mm ply.  We wired the first b oard up then started testing and got a short showing across a point.   After spending many frustrating hours we can't bottom it.  All wires have been taken off the track concerned and nothing else is connected to it.  However when we put 12v to the two stock rails we get between 2 and 4volts appearing at the frog, which should be isolated.  I then took one of the stock rails off and still gotr 2.5volts at the frog when i enerbgised the two point blades.  I even got 2 volts showing on the heads of the screws that used to hold the stock rails.  The points are constructed on pcb and all pieces of rail have 1" brass screws soldered to them as droppers.

 

I then tested the points on the next board, which as yet has no wiring and found the same happening at three of the 4 point frogs.  It is possible that both boards came out of the same 10  by 5 sheet of ply and I am wondering if one or more of the layers of veneer has become conductive.

 

I suppose that there are two questions:-

 

1.   Has anyone come across this before

 

2.   Does it matter if there is a little leakage as the frog will normally be live at the full voltage when the track is energised.

 

Jamie

Rather puzzled and baffled and heading for the cupboard to open a bottle of malt.

 

 

 

 

 

1.  Has anyone else ever come across this.

[Black Sheep]

Rather puzzled and baffled and heading for the cupboard to open a bottle of malt.

 

 

 

 

 

1.  Has anyone else ever come across this.

becoming puzzled and baffled and heading for the malt, probably

 

I did so last night after becoming puzzled at running out of cement.

 

Can't help with the conductivity of wood, you've not got any dribbles of solder or anything causing it that's quite small and hard to see?

[Mark Forrest]

Is the pcb of the double sided type?

[jamie92208]

Is the pcb of the double sided type?

Thanks  to Black Sheep and yourslef for the interest.

 

a)  Yes I've been over all the gaps and double checked for things like solder dribbles.

 

b)  No it's normal single sided pcb. 

 

I've double gapped some of the sleepers and there is no voltage on the isolated pieces.  It seems to be coming via the brass screws that go through the board.  All wires have been  disconnected from them so we are thinking of cutting the brass screws to see if it makes any difference.

 

Jamie

[amdaley]

Did you test the points for shorts before fitting them to the baseboards ?

 

The only way wood would be conductive would be if it was wet.

 

Have you tested a spare piece of the ply for conductivity ?

 

Are all the sleepers gapped ?

[jamie92208]

Did you test the points for shorts before fitting them to the baseboards ?

 

The only way wood would be conductive would be if it was wet.

 

Have you tested a spare piece of the ply for conductivity ?

 

Are all the sleepers gapped ?

Thanks.

a)  No the points were built in situ on the baseboard, 9mm ply, then 3mm cork then paper templates then the PCB.

b) I've tried testing the ply but my small meter doens't register a short.  I think there might be a semi conductive layer over part of it as one of the points doesn't ahve the voltage leak,.It has been in the clubroos for the past 2 weeks but has been in my shed for about a year so i did wonder about a bit of damp in it.

c)  Yes the sleepers are all gapped and the ones supporting the frog are now double gapped on both sides. 

 

Jamie

[amdaley]

The reason I asked was the ply damp or wet was because it has been mentioned before that some people have got shorts after ballasting because of the water content of the pva. Once the ballast dries the shorts go away. So if the ply was damp you might get a short.

 

Is there a DCC system connected to the layout anywhere ?  If there is that will give you what appears to be a short.

 

Is there any stock on the layout anywhere or a tool or something across the track anywhere ?

 

Disconnect the board from the rest of the layout.

[kevinlms]

Interesting problem. Quite likely the ply is damp.

 

One way of testing would be to make sure the track on the ply has EVERYTHING removed from it, especially locos etc and testing with an insulation tester or Megger (connect to the 2 rails). Perhaps you can borrow one from somewhere, and preferably with some one helping who knows how to use it.

 

They usually use 500 or 1000 volts & will soon show up any low resistance.

 

 

Edit sorry that should be between both rails & between each rail & each frog.

[martin_wynne]

However when we put 12v to the two stock rails we get between 2 and 4volts appearing at the frog, which should be isolated.

 

Hi Jamie,

 

Stop measuring volts and measure resistance. What is the resistance between the stock rails and the frog? My guess is that you will find it several thousand Kohms and nothing to worry about. A short of any kind would be less than 1Kohms. A real short would be only a few ohms, or zero. 

 

A few stray volts on a high impedance digital multimeter can come from almost anywhere -- the ink on the template likely contains carbon for example. Or you could be holding the leads with sweaty fingers. It won't affect the working of the system.

 

regards,

 

Martin.

[jamie92208]

Hi Jamie,

 

Stop measuring volts and measure resistance. What is the resistance between the stock rails and the frog? My guess is that you will find it several thousand Kohms and nothing to worry about. A short of any kind would be less than 1Kohms. A real short would be only a few ohms, or zero. 

 

A few stray volts on a high impedance digital multimeter can come from almost anywhere -- the ink on the template likely contains carbon for example. Or you could be holding the leads with sweaty fingers. It won't affect the working of the system.

 

regards,

 

Martin.

Thanks  very much for that Martin.  I will certainly check the actual resistance this evening.  When we checked it last Friday it was several Kohms.   Your answer also answers part 2 of my question as I wasn't sure what would actually cause a short and stop the layout working.   I will also test some of tghe trackwork with a loco tonight. 

 

Thanks to everyone else for the replies.   For everyone's info the board is on it's own with no stock on it and nothing connected to it.

 

Jamie

[Enterprisingwestern]

Of course it could be a faulty meter, when was it last tested and calibrated?

 

Mike.

[chaz]

Martin is quite right. A voltage measured may well be of little consequence - if you have a high resistance the current flow will be tiny, and on a model railway can be ignored. I suggest you do a more practical test. Connect a PP3 battery to the track. A fresh battery should move a loco, if it does you don't have a problem.

 

Chaz

[Edwin_m]

The resistance must be pretty high as you are measuring voltage under no load so even the small current used by the meter is dropping the voltage significantly.  Measuring voltage without a load (eg a moving loco drawing current) tells you very little of any use. 

[Fat Controller]

Is it possible that the plywood has been treated with copper salts in solution? These are used as a wood preservative for timber for external use- on pressure-treated fenceposts, green streaks may be seen where they leach out. Under certain conditions, notably high humidity, I could see these being conductive.

[jamie92208]

Is it possible that the plywood has been treated with copper salts in solution? These are used as a wood preservative for timber for external use- on pressure-treated fenceposts, green streaks may be seen where they leach out. Under certain conditions, notably high humidity, I could see these being conductive.

 

That's a good idea Brian.  The board is on exterior grade 9mm ply and has been in my shed for most of the last 18 months so may have become a little damp. All the other scenic boards use 6mm with a 3mm layer where the track is to keep weight down.  I have just consulted with my co-conspirator  (Supersonic of this parish) and we are going to try running some locos tonight with Croc clips for leads and see what happens.   We did test run stuff in the yard when we laid the track and things ran OK.  It appears tnat a sensitive meter may be identifying a problem that we can forget about.  Whatever the solution is it's better to find it our now rather than later.

 

Thanks again for all the helpful replies.

 

Jamie

[jamie92208]

To everyone who replied, thanks very much.  I took a loco down tonight through the snow and it ran perfectly so we have decided to just keep going as we were regardless of the very slight current leak.

 

Jamie

[Katier]

For the record your not the only one, I noticed similar things with my layout although in my case I noticed a high resistance when I was expecting open circuit. So Martin says because it was way off a hard short I just ignored it.