Isolating oscilloscope versus isolating test circuit

Discussion in 'General Electronics Chat' started by mnevans93, Apr 8, 2016.

  1. mnevans93

    Thread Starter New Member

    Apr 8, 2016
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    Hello everyone,

    New to the forum, and I'm going to start this post by saying I'm a mechanical engineer with no formal electrical training. Try to not laugh too hard if the problem is obvious :oops:

    I had a circuit set up to test a unit at work. The basic circuit was as follows:

    120 VAC 2 prong power cable ---> Bridge rectifier ~
    Bridge rectifier + ---> Multimeter to measure current ---> Large coil ---> Bridge rectifier -

    Plugging this into the wall worked fine. The coil received rectified DC voltage and current was about what I expected.

    Then, I tried to set up an oscilloscope to look at the voltage waveform the coil was receiving. I was told to use an isolating transformer on the oscilloscope for this purpose to prevent a ground loop. I plugged the 'scope into the isolating transformer, connected the probe to the + coil lead and the ground clip to the - coil lead. As soon as I plugged in the 120 VAC power cable connected to the rectifier, there were sparks and a breaker tripped. There were no visible shorts, as in alligators touching or anything like that.

    Later, I tried plugging the rectifier power cable into the isolating transformer and plugging the scope into the non-isolated wall socket, and this time, it worked. What changed?

    I read some other posts on similar topics. One in particular caught my eye, which was a post by MrChips in the thread "At what point Isolation transformer needed":

    To my understanding, I was doing the initial option to "float" the scope, but I still tripped a breaker. The second option is the only one that ended up working. Any insight into why this happened would be greatly appreciated. I would like to understand what caused it for future reference. Thank you in advance for your patience with this mechanical engineer! ;)
     
  2. mnevans93

    Thread Starter New Member

    Apr 8, 2016
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    Sorry for the double post, but it seems that I can't edit my initial post after a certain amount of time.

    I wanted to add that I understand this situation is different than the one in the quote because of my use of a bridge rectifier, but I'm not sure what that might do to the circuit to cause this issue one way (with the 'scope isolated) and not the other (with the test circuit isolated).
     
  3. crutschow

    Expert

    Mar 14, 2008
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    It sounds like the isolation transformer doesn't isolate the scope's safety ground (connected to earth ground through the 3rd power-plug pin), which is connected to the scopes chassis and input circuit ground. That makes a short-circuit between the mains and the scope ground for a bridge circuit, which you observed.

    Alternately, using the transformer for the circuit under test means there is then no path from the mains to the ground, so it works fine and is relatively safe.

    Incidentally whoever said you could isolate the scope is flat wrong since that means the circuit still has lethal voltages to earth ground. :eek: Apparently this person is working on a Darwin award.
    The isolation transformer should always be connected to the circuit under test.
     
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  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Most scopes connect their ground point to the AC power ground, or that 3rd pin on the plug.

    The engineering department here keeps a drawer full of those ground pin isolators, or the "cheaters" as anyone with any sense calls them. They break the ground connection when they need to make measurements similar to what you are doing.

    At least they insist on using onlye the bright orange adapters so hopefully someone will see them before touching anything.

    It is not a recognized practice, but sooner or later we all sucume to the temptation to do it. Just don't leave your now lethal setup for anyone else to stumble over.
     
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  5. crutschow

    Expert

    Mar 14, 2008
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    But those should only be used to breakup a signal ground loop (which I've done myself at times), but definitely not for working on unisolated line powered circuits (which is an absolute no-no).
     
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  6. ian field

    Distinguished Member

    Oct 27, 2012
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    Isolating the scope is a really bad idea - most have all metal case with the same potential as its signal ground.

    Some people take off the earth wire in the mains plug - a few only did it once................

    Always use the isolating transformer on the equipment under test.
     
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  7. mnevans93

    Thread Starter New Member

    Apr 8, 2016
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    Thank you for all the responses.

    I think my main (hah) struggle is a lack of understanding of how exactly ground and ground isolation works, so I'm still not sure I'm understanding you all correctly.

    From what I gather after reading your responses, isolating the scope doesn't actually remove the direct path to earth ground it has from the ground clip to the third pin, but it cuts the connection between signal ground and earth ground, so when I attached the oscilloscope probe and ground clip to the DC outputs of the rectifier, the DC voltage could only go to earth ground rather than the signal ground, causing the breaker to trip. I think I understand this part... correct me if I'm wrong.

    Now, when you isolate the line powered circuit, you are essentially removing its center tap and the rectified DC voltage goes through the oscilloscope normally. Is this due to the scope's signal ground having a lower impedance than the earth ground, whereas floating the scope removed the signal ground path connection altogether?

    And lastly, why does removing the center tap affect how the current wants to flow through the scope, as in, why is it also bad to have neither the scope nor the unit under test? I kind of just took people's word for needing isolation without really understanding why.
     
  8. ian field

    Distinguished Member

    Oct 27, 2012
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    With any equipment using rectified mains; the lowest negative potential is equal to the negative peaks of the AC waveform. If you connect the scope ground clip to that - it'll go bang.

    Isolating the scope is a seriously bad idea, because then its chassis will be at the peak negative potential of the mains AC waveform.

    You need to run the equipment under test via an isolating transformer - then you might live long enough to see if this thread gets closed for discussing transformerless power supplies.
     
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  9. mnevans93

    Thread Starter New Member

    Apr 8, 2016
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    Right, and that's what we are doing now. I'm simply trying to understand what's going on at a circuit level so I can avoid this in the future entirely. I've had a couple incidents with electricity, this one not being the first. Had a run-in with 440 VAC that was particularly terrifying, so I'm trying to do more research to prevent things like this from happening again. I appreciate your insights.
     
  10. crutschow

    Expert

    Mar 14, 2008
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    The crux of the danger is that all mains AC is referenced to earth ground so anything connected to the mains has the potential for current to flow from the circuit to anything in contact with ground (such as yourself or the scope common which is connected to earth ground through the 3rd prong safety ground).
    An isolation transformer breaks the connection between the mains and earth ground. Its output voltages are only referenced to the other output winding(s), so touching an output winding and earth ground does not have a path and there is no shock hazard to ground.
     
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  11. ian field

    Distinguished Member

    Oct 27, 2012
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    Rules are rules, and not discussing transformerless power supplies is one of them.

    Personally I don't understand the logic of closing a thread because someone strays outside a certain parameter - in most cases a TS is going to go ahead and do it anyway, and far more likely to harm themselves without the advice and warnings they'd get on this forum.
     
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  12. mnevans93

    Thread Starter New Member

    Apr 8, 2016
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    I would assume it has to do with liability and legal issues, because if the TS hurts themselves or someone else, and it somehow comes back to the forum that they were given advice here, even if it was misinterpreted, legal issues could ensue. I didn't mean to break the rule to begin with and I apologize. I also don't mean to be clueless when it comes to electricity and wiring, but... yeah.

    Thank you for bearing with me regardless!
     
  13. ian field

    Distinguished Member

    Oct 27, 2012
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    You could also argue that the forum denied them advice that might've saved their life................
     
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