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Jon Titus
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Blogger
Test Leads Not Above the Fray
Jon Titus   2/16/2012 1:01:07 PM
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It's no surprise the ohmmeter's test leads went bad.  With all the flexing and bending these types of leads go through they eventually fail.  I once spent an hour trying to find a problem with a power supply only to discover the leads had an intermittent connection.  The same thing happened the other day with the leads from our TV audio-out jack and an infrared transmitter I use with a pair of wireless headphones.  After the sound died I thought about how to troubleshoot the transmitter circuit but checked the wire first. Turned out the audio line had an intermittent. I wish all problems were as easy to solve.

Rob Spiegel
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Blogger
Re: Test Leads Not Above the Fray
Rob Spiegel   2/16/2012 1:35:33 PM
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That's a good story, Jon. If you write it up with a bit more detail, we could use it as a Sherlock Ohms blog.

Alexander Wolfe
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Blogger
Re: Test Leads Not Above the Fray
Alexander Wolfe   2/16/2012 3:21:46 PM
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This is a kinda/sorta practical example of that physics dictum that the observer changes the experiment. In this case, it was that how and where you were making the connection to the ohmmeter was changing the result.

I've definitely experienced stuff like Jon Titus reports, where test-meter leads flexing and bending cause intermittents and failures. Most often, I've had this happen in jury-rigged audio setups.  I think most homebrew users fall victim to this stuff, because we either don't have the proper cables at hand or don't want to spend the time and money to wait to obtain them.

What's unusual in this case is that a big construction operation was using what amounts to a junkbox-picked cable for an important connection, and it took some thinking before anyone noticed this.

William K.
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Platinum
case of the plunging insulation resistance readings.
William K.   2/17/2012 10:21:56 AM
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There is nothing in the posting to indicate that the test leads were other than adequate for the application, so the assertion of jury-rigging is a guess. The preferred implemantation would have been to run the leads but not connect them, and do an initial "reality check" test first. I became a very firm believer in that technique after a co-worker on a project published a report claiming an amazing discovery, and being unable to duplicate his results until I had a ground connection fail. IT seems that his results were also caused by a ground connection failure, which he had failed to recognize. 

The frequent verification of equipment status may be considered excessive by some, but acting on incorrect results is usually the alternative, which is much more embarassing.

A bit more confidence in the effectivness of the heating treatment would have meant that the measurement was made directly after the first heating, and much effort and time would have been saved. "Confidence is not the same as arrogance." 

Larry M
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Platinum
Re: Test Leads Not Above the Fray
Larry M   2/17/2012 11:48:58 AM
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I've definitely experienced stuff like Jon Titus reports, where test-meter leads flexing and bending cause intermittents and failures. Most often, I've had this happen in jury-rigged audio setups.  I think most homebrew users fall victim to this stuff, because we either don't have the proper cables at hand or don't want to spend the time and money to wait to obtain them.

This is why scopes have the 1-volt, 1 kHz square wave tap on the front. When you have an unbelievable result you can touch the probe to it for a sanity check. An easy way to remind yourself that you are using a 10x probe or detect that the compensation adjustment is way out of adjustment.

kenish
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Platinum
Re: Test Leads Not Above the Fray
kenish   2/17/2012 1:07:13 PM
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Every data collection activity starts with checking probes, leads, etc. and recording serial numbers.  Otherwise there's a risk of spending a lot of time collecting erroneous data or being unable to trace the "bad" equipment to back-correct the data afterwards.  Don't ask me how I know !

As the article demonstrates, the characteristics of the probe need to be understood.  Recently a colleague was getting very strange results measuring a mundane logic circuit with an ultra high-speed scope and probes.  Pulled up the probe manuals online...turns out it had a 20k ohm input resistance.  Highly optimized for GHz measurements but not suitable for everyday tasks.

GlennA
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Gold
Re: Test Leads Not Above the Fray
GlennA   2/17/2012 4:12:19 PM
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Many years ago I worked in a shop where the other technicians liked to wrap the test leads TIGHTLY around the meter.  Every time I had to use the meter I told the other techs not to wrap the leads that way - that they were going to fatigue and break the connections at the jacks - and to always check a known live circuit before trying to test an unknown circuit.  And when the leads eventually failed - the 'metered dead' circuit turned out to be 'live' - I had my 'I told you so' moment.  But do you think the replacement leads were then coiled loosely with the meter ? - no - they didn't learn a thing.

Embedded uPs
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Iron
Not Connected
Embedded uPs   2/22/2012 12:09:37 AM
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I had a similar non-work experience the other day.
I had left my motorcycle sitting outside over the weekend where we had rain, snow and a heavy frost.  It was below freezing Monday morning but I had to ride it home.
After running the battery down trying to start it, I got out the jumper cables, but they didn't seem to be charging the battery or helping me to start it.
Went inside to get the meter and started checking voltages.

Because of space constraints, I had focused mainly on the clearance between the positive clamp and the bike's frame.  Because the clamps were oversized for a motorcycle, I had clamped the negative side of the jumper cables across the battery terminal and the adjacent insulating boss on the battery.
I finally realized that only one side of the jaws of the jumper cable was "live" and it was the side that I had clamped on the boss.

Turned it around and the bike started right up!

Tool_maker
User Rank
Platinum
Re: Test Leads Not Above the Fray
Tool_maker   2/23/2012 7:35:50 AM
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"junk box cable". What a great discription. I have found many times that shortcuts and improvisation are rarely reliable methods. I once worked at a company that posted monthly mottos at the time clock. So everyday we got some words of wisdom. One month the post read, "If you can't find the time to do it right, you will have to find the time to do it over."

Whenever a supervisor asked us to throw together some quick "workaround" to get production up and running, that motto was thrown up. After about a week management took that sign down and put the last month's back up. But I have never forgotten the wisdom dispensed.



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