Isolation transformers "no" says Tektronix

Discussion in 'General Electronics Chat' started by pastinsain, Sep 5, 2012.

  1. pastinsain

    Thread Starter Member

    Aug 25, 2012
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    I just came across this info from Tektronix about isolation transformers and the oscilloscope

    http://www.cbtricks.com/miscellaneous/tech_publications/scope/floating.pdf


    Tektronix does " not" recommend using an isolation transformer.
    Instead they sell high priced isolation components for use with their oscopes.

    I did a google search on the topic and still no answer.

    any advise or opinions?
     
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Oops, I guess I already did the ground defeating technique. :D

    They are right, it may be dangerous because, depending on where you connect the ground probe to, other easily accessible parts of the oscilloscope may carry a possibly dangerous voltage with respect to ground.

    How about isolating the DUT from the grid via a isolation transformer and not the oscilloscope?

    You must always be aware of possible dangerous situations. Sometimes you cannot have a ground connection to your oscilloscope, for example when investigating certain noise issues.

    If you want to stay on the save side do what they recommended.

    Maybe you tell us what exactly is your measurement problem.
     
  3. JMac3108

    Active Member

    Aug 16, 2010
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    It was common practice at the last place I worked to use isolation transformers with oscilloscopes to look at AC power supplies. But this is extremely dangerous exactly as TI says. If you defeat the safety ground on your oscilloscope using an isolation transformer or a cheater plug, then you connect your scope probe ground to ground in your AC circuit (perhaps at the ground point of a bridge rectifier), you now have a ground reference that is actually at a very high voltage. This means that the metal case of your oscilloscope is also at this same high voltage. If you are touching the case of the oscilloscope and at the same time touch the metal case of another instrument that has a proper earth ground, you may get a serious, even lethal, shock.
     
  4. JMac3108

    Active Member

    Aug 16, 2010
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    The best thing is to sketch out a diagram of your power system including any instrumentation and carefully identify any unsafe condition. If you don't know how to do this, then you shouldn't be working with high voltages.

    When I was a young technician just out of the army, before I went to college, I was working for a military contractor. Once on the night shift I was performing DC Hipot testing on some wire harnesses using an automated tester. There were sometimes connection problems and you had to reseat connectors. The problem was that you had to turn off the system, reseat the connector, then start the test over. I had the brilliant idea one night (or so I thought) that I could reseat the connectors when the high voltage was present on the cable as long as I sat on my stool that had rubber feet so I was not grounded. Well it worked, and I was happily able to simply pause the test that failed, reseat the connectors, and continue the test. I saved a lot of time. Then the inevitable happened ... I had one hand on the metal backshell of a connector on the unit being tested, then reached over and touched a metal toggle switch on the front panel of the tester. After I got up off the floor and realized that thankfully I was still alive, I learned a very valuable lesson.
     
  5. pastinsain

    Thread Starter Member

    Aug 25, 2012
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    Last edited: Sep 5, 2012
  6. BillB3857

    Senior Member

    Feb 28, 2009
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    Depending upon your scope, many have the feature to invert the signal on Chan-2 and add to Chan-1. This gives the same effect as the high priced Tektronicx add-ons.
     
  7. pastinsain

    Thread Starter Member

    Aug 25, 2012
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    I just purchased the tektronix 475a and yes ive read about the signal inversion using both channels but have not tryed that one one yet.

    I just wanted to use isolation for both the oscope and DUT . and rubber gloves,shoes and rubber mat anything else I left out?
     
  8. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    I never actually tried this but it is said that this setup has very poor common mode rejection.
     
  9. BillB3857

    Senior Member

    Feb 28, 2009
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    It really depends upon the quality of your scope. Some are better than others. The way to check is to put both probes on the same signal. If everything is up to snuff, you should only see a straight line.
     
  10. pastinsain

    Thread Starter Member

    Aug 25, 2012
    160
    2
    this taken from the same tektronix pdf file
    ""
    This is dangerous, not only from the
    standpoint of elevated voltages present
    on the oscilloscope (a shock hazard
    to the operator), but also due to
    cumulative stresses on the oscilloscope’s
    power transformer insulation.
    This stress may not cause immediate
    failure, but may lead to future dangerous
    failures (a shock and fire hazard),
    even after returning the oscilloscope
    to properly grounded operation!
    Not only is floating a ground-referenced
    oscilloscope dangerous, but
    the measurements are often inaccurate.
    This results from the total
    capacitance of the oscilloscope chassis
    being directly connected to the
    circuit under test at the point where
    the common lead is connected
    ""
    well thats interesting!
     
  11. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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  12. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    From my experience it never caused any trouble. It's just you need to be totally aware of what you are doing. But then if you use it with the ground connection you also need to be careful, otherwise you may damage the device you want to repair/measure.

    Thta's why I only use a handheld Fluke oscilloscope for common every day measurements (maybe not very precise but ok). I don't have to worry about ground connections.
     
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