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When is a short not a short?


gordon s

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Thanks for all your replies guys, most appreciated. I guess the thing that is still confusing me (apologies if you have given the answer and I haven't understood it) is when the auto reverse module was still connected, my Wavetek meter said there was no short when connected across the bus in continuity mode.

 

Repeating the exercise with the continuity device, the buzzer sounder to show there was a short. So we have the same test with two pieces of equipment giving totally different responses.

 

Am I misunderstanding the function of each device or is there a fundamental difference that I am missing? It defeats the object if a device shows there is a short when there isn't or vice versa. Until now I have always used the meter without problem, but that meant everything took time as I could only access one side of the board at the time. Clipping the continuity device across the bus on one side of the board whilst working on the other made life a lot easier.

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Thanks for all your replies guys, most appreciated. I guess the thing that is still confusing me (apologies if you have given the answer and I haven't understood it) is when the auto reverse module was still connected, my Wavetek meter said there was no short when connected across the bus in continuity mode.

 

Repeating the exercise with the continuity device, the buzzer sounder to show there was a short. So we have the same test with two pieces of equipment giving totally different responses.

 

Am I misunderstanding the function of each device or is there a fundamental difference that I am missing? It defeats the object if a device shows there is a short when there isn't or vice versa. Until now I have always used the meter without problem, but that meant everything took time as I could only access one side of the board at the time. Clipping the continuity device across the bus on one side of the board whilst working on the other made life a lot easier.

 

The problem is that you are trying to measure continuity with an elecronic module connected. This will only lead to confusing results depending on:

- The polarity of the test signal

- Whether the test signal is AC or DC

- The amplitude of the test signal

- The components in the module

 

Simplified to its extreme, if the electronic module was just a diode then a DC test would show continuity or open circuit depending which way you connect the probes. An AC test would always show continuity.

 

Another thing to be aware of is that testing with a 9V battery connected the wrong way round could destroy the electronics depending, again, on a number of factors.

 

The moral, as you have discovered, is if you want to test the track continuity, test the track continuity, and nothing else.

 

Keith has already pointed out that if you have insulating gaps in the track then simply leaving a buzzer connected across the track as you wire is no use whatsoever. An insulated (isolated) track section could be wired in reverse and the buzzer will not sound. You will only find the short when you try to run a train and the wheels bridge the gaps.

 

A number of solutions spring to mind:

 

- Make up some jumpers with croc clips to jumper the insulation gaps on each new section of track as you wire it.

 

- Make up a test wagon with pickups linking all the wheels on each side and run it around the track regularly.

 

- Again from Keiths post, connect the buzzer to one end of the red bus wire then check for continuity to each newly wired piece of track on the red side. Repeat for the black bus, etc.

 

Andrew Crosland

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As a former telephone engineer (cable jointer and exchange construction) I can tell you that time spent proving each circuit worth while. For DCC connect the meter/buzzer to one busbar and check continuity to each piece of rail on one side touching the other rail should give no response. Do the same from the other bus bar for the other rail. It takes less time than trying to work out where the shorts are later on. Typically you may find a droper has been connected to the wrong busbar especially if you are working upside down.

Ive worked with people who didn't check with a meter

Don

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