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When is a short not a short?


gordon s

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For years I have been using a Wavetek DM9 meter and it's never let me down. Included in the parameter selection is a continuity buzzer that has proven invaluable over the years, but I wanted something more specific so have just bought this continuity tester. It is not responding however as I would expect. The instructions/data sheet is attached.

 

Taking a plain piece of track that is not connected to anything and using the red and black probes shows no short between the two rails. Bridging across the two brings up a short and the beeper goes off. Now moving onto the recent board that I have just completed. The Wavetek meter shows no shorts and a DCC loco runs through all the pointwork faultlessly so I know it is OK. Using the continuity tester I am geting a short indication on every rail and across the bus directly. I know there isn't a short as all works fine.

 

Is this something to do with the sensitivity of the continuity tester? The spec says that 'continuity confirmation' is 'equal to or less than 1.0k Ohms. Putting a meter across the two rails and measuring the resistance indicates at least 20M ohms across the track.

 

So now I'm totally confused as the continuity tester is showing a short when there isn't one. What am I doing wrong? I'm just clipping the red and black leads from the main device to each rail...

 

Any ideas?

 

post-6950-0-04061700-1307270040_thumb.jpg

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Is it measuring the continuity across the power supply? Try disconnecting one feed lead from the track and see if it still occurs.

 

I disconnected every feed to the board. Both a 12vDC supply to the Tortoise motors from a Gaugemaster D Controller and the 17v DCC supply from the Lenz 90 set up. The only wiring left is the main bus, the control bus and the droppers to every piece of rail.

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Far simpler to stick to buzzers, measuring resistance can lead to confusions ... there will be things that will give a resistance reading, you can measure your own finger for example, stick to the buzz, it's simple and it works.

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Try measuring the resistance, you mentioned 20m ohm, but was this from the older meter? On the face of it the continuity meter is measuring a short.

 

Try testing a plain piece of track and slowly re-connect things to isolate the problem.

 

It is not impossible to have a faulty tester, it may read several thousand ohms and be going off, to test do not connect to anything and then put resistors of assorted values across the leads to verify the real range.

 

I have not read the full instructions, they are to small to read easily from the jpeg, but maybe there is a second or selection of sensitivities?

It cannot be anything complex, just a matter of tracing the problem.

 

Stephen.

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Are the AC pulses causing a field between the two wires of the bus, which is showing as continuity, as they are closer than the rails are to each other?

 

On the G scale layout of a friend we were getting interference from the point wiring creating dcc signal interference when it ran in the same conduit, separated them by an inch and all the oddities dissappeared. wires crossing didn't cause a problem but running alongside for a distance seemed to!

No idea why but all we could guess at was that some field was being created.

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Far simpler to stick to buzzers, measuring resistance can lead to confusions ... there will be things that will give a resistance reading, you can measure your own finger for example, stick to the buzz, it's simple and it works.

 

I am only using the buzzer. I took a resistance reading with the Wavetek as I though someone mighask for it. I'm a great believer in keeping something as simple as possible. All I want to do with the continuity tester is hang it across the the main bus so that I can see I'm connecting each rail to the correct supply. Sdaly, as soon as I connect up the buzzer goes.

 

What is on the control bus, Gordon? Note that this device provides ac pulses at 2 Hz and is NOT a direct current measuring unit.

 

This is what I call the 0v supply that feeds the Tortoise motors. The +12v or -12v supply comes from the toggle switch on the control panel.

 

I have disconnected all the feeds to these bus cables.

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I have not read the full instructions, they are to small to read easily from the jpeg, ...

Double click on the instructions image, that will bring them up x3 in size (if not more).

 

 

 

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I have read the instructions now, it is an AC low frequency pulse type and will be affected by capacitance, there may be capacitance of a significant amount to an electronic tester from the bus wires and the insulation on the track, is it PC board, which is notorious for bad insulation to AC at low pulses?

 

Test a piece of plain track, and add the other wires one by one and see what trips the meter, Other than that use a plain multi meter, bulb or a buzzer!!!

 

Logically if the power from the DCC is getting through there is no problem with the track as such.

 

Stephen.

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For years I have been using a Wavetek DM9 meter and it's never let me down. Included in the parameter selection is a continuity buzzer that has proven invaluable over the years, but I wanted something more specific so have just bought this continuity tester. It is not responding however as I would expect. The instructions/data sheet is attached.

 

Taking a plain piece of track that is not connected to anything and using the red and black probes shows no short between the two rails. Bridging across the two brings up a short and the beeper goes off. Now moving onto the recent board that I have just completed. The Wavetek meter shows no shorts and a DCC loco runs through all the pointwork faultlessly so I know it is OK. Using the continuity tester I am geting a short indication on every rail and across the bus directly. I know there isn't a short as all works fine.

 

Is this something to do with the sensitivity of the continuity tester? The spec says that 'continuity confirmation' is 'equal to or less than 1.0k Ohms. Putting a meter across the two rails and measuring the resistance indicates at least 20M ohms across the track.

 

So now I'm totally confused as the continuity tester is showing a short when there isn't one. What am I doing wrong? I'm just clipping the red and black leads from the main device to each rail...

 

Any ideas?

 

 

 

Disconnect the DCC supply to the board & see if you still get a short indication also make sure there's no loco on the board.

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Found it! It is the solid state auto reverse module that is tripping the continuity tester. Now I've disconnected the reverse module, the continuity tester is working as it should.

 

I think that this unit is possibly not what I wanted, although it will be useful tracing wires back to the control panel etc.

 

What do you recommend as a pure continuity tester? I want to clip the continuity tester onto the main bus and have a buzzer sound and LED flash if I am connecting the red rail to the black feed or vice versa.

 

Thanks Geoff for pointing me in the right direction on YMR.

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Double click on the instructions image, that will bring them up x3 in size (if not more).

Did try it, Firefox, (it is a beta version, but up to date), failed to open on double click, but opened on right click, ...mmm.. the vagaries of Windows........

EDIT.......Found it, disabled smooth scrolling app I was trying and it opens normally.......

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I disconnected every feed to the board. Both a 12vDC supply to the Tortoise motors from a Gaugemaster D Controller and the 17v DCC supply from the Lenz 90 set up. The only wiring left is the main bus, the control bus and the droppers to every piece of rail.

 

doesn't quite match with

 

It is the solid state auto reverse module that is tripping the continuity tester.

 

Otherwise I (and others) would have suggested any gizmos attached will cause continuity ... :yes:

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Now the problem's solved the device you have will do the job fine, can't see any reason it should not work and give the indication that you require.

But as a pro electrician, the best approach to bus wiring is a logical approach, do one side and then the other feed in sequence and it's very hard to go wrong.

In telephone exchanges we had thousands of wires to get correctly to destination in a Strowger Exchange type wiring harness, and only occasionally had to resort to a meter on old already installed wiring, not new layouts.

 

Stephen.

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What do you recommend as a pure continuity tester? I want to clip the continuity tester onto the main bus and have a buzzer sound and LED flash if I am connecting the red rail to the black feed or vice versa.
So what you want to achieve is to have something that will detect a short should you create one while wiring? The thing you have bought will do that, but as you have found needs to have any other gadgets disconnected. Your Wavetek will do the same.

BUT. This concept only works if you have continuity through the rails as well as through the bus wires, if there are gaps in the rails then adding a new piece of rail to the bus will not cause a short whether it is red to black or red to red, it will tell you if you connect 2 droppers from the same piece of rail, one to each bus.

Personally I install droppers for all the red rails, label them underneath, then do the same for the black rails, then for the frogs (green?). Then mistakes connecting to the bus are unlikely as they are all labelled, but I test for continuity and shorts as each section is completed just in case and while the section to be searched for errors is fairly small.

Regards

Keith

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snapback.pnggordon s, on 05 June 2011 - 11:52 , said:

 

I disconnected every feed to the board. Both a 12vDC supply to the Tortoise motors from a Gaugemaster D Controller and the 17v DCC supply from the Lenz 90 set up. The only wiring left is the main bus, the control bus and the droppers to every piece of rail.

 

 

doesn't quite match with

 

snapback.pnggordon s, on 05 June 2011 - 12:21 , said:

 

It is the solid state auto reverse module that is tripping the continuity tester.

 

 

Otherwise I (and others) would have suggested any gizmos attached will cause continuity ... :yes:

 

 

 

 

Of course you are right and I apologise if in giving you half the information, I have wasted your time this morning...:yes:

 

It's easily done and I'll explain why. The autoreverse module is fed from from the main bus and sits alone underneath the board. Looking from the top surface, I had disconnected all the main feeds and not seen or considered the reverse module as that had no external feeds and wasn't obvious at the time.

 

So many thanks for your help but unfortunately you have to experience these situations to learn for the next time.

 

Any recommendations for a device that will hook across the bus and stop me making incorrect feeds/droppers in the future?

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Frankly the best is a multimeter, some even have buzzers built in, but an AVO meter or any modern multi meter should do all you want, but as you have the continuety tester I would go ahead and use that.

Stephen.

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Its never a waste of time Gordon.Its all part of learning.:help:Its very easy to get caught even though there are some that will never admit it.

As regards feeds & droppers. I presume you are using color coded wiring B) I just use a buzzer on a couple of longish leads(color coded):yes: and test each dropped/feed as I go along.That way if you make a mistake its always the last dropper/feed that you did.

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Of course you are right and I apologise if in giving you half the information, I have wasted your time this morning...:yes:

 

So many thanks for your help but unfortunately you have to experience these situations to learn for the next time.

 

Any recommendations for a device that will hook across the bus and stop me making incorrect feeds/droppers in the future?

 

I'm happy to help and it's not wasted if you gain knowledge :drinks:

 

I simply use a continuity tester with buzzer, that does everything I've ever needed B) - you will always make mistakes, it's the experience in finding them that makes us make less :lol:

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Frankly the best is a multimeter, some even have buzzers built in, but an AVO meter or any modern multi meter should do all you want, but as you have the continuety tester I would go ahead and use that.

Stephen.

 

Just be careful relying on buzzers built into multimeters. Many modern ones turn themselves off if left without any changes after a preset time.

Just the thing to lull you into a false sense of security. You come back from your lunch break & back to work, accidently put a short across and nothing happens, so you continue on...

 

Best to deliberately put a short on every now & again to prove working. Could be that an aligator clip has fallen off too.

 

:help:

 

Kevin Martin

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Just be careful relying on buzzers built into multimeters. Many modern ones turn themselves off if left without any changes after a preset time.

Just the thing to lull you into a false sense of security. You come back from your lunch break & back to work, accidently put a short across and nothing happens, so you continue on...

 

Best to deliberately put a short on every now & again to prove working. Could be that an aligator clip has fallen off too.

 

:help:

 

Kevin Martin

 

I always touch the probes together before using and every now and again to be sure it's all working ;)

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The easiest thing is to make up your own circuit with a buzzer,a 9volt battery & a couple of leads.

I made one about three years ago now from a cheap buzzer from Maplins or any electronics shop, A pair of leads from an old broken multimeter & the 9volt battery connections from an old bedside table clock. Its still going strong even after 3 years. Nice & light especially if you're under a layout or baseboard & there's no danger of dropping your expensive multimeter.Makes a good noise when you have a short or need to check a connection.

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The easiest thing is to make up your own circuit with a buzzer,a 9volt battery & a couple of leads.

 

We are making one from the Simpsons version of the "Operation" game :laugh:

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