still a little confused on oscilloscopes

Discussion in 'General Electronics Chat' started by gjo, Oct 31, 2013.

  1. gjo

    Thread Starter New Member

    May 22, 2013
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    When using an oscilloscope do I have to connect ground clip near component I a am measuring on secondary side of isolation transformer or can I just connect the ground clip near negative terminal of transformer, since this will earth reference entire line?
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Think of the oscilloscope like a multimeter's red and black leads. It measures the voltage between the probe tip and the probe clip. Normally, it is good practice to connect both the probe tip and clip. However, there is more to the story.

    The problem arises because there is a 0 Ω connection inside the oscilloscope case from the probe clip to the Green wire ground pin in the 'scope's power cord. If you indiscriminately connect the probe clip to a AC line powered circuit, you run the risk of shorting the line to the building safety (earth) ground, blowing up the 'oscope and the circuit under test...

    If the circuit you are testing is completely isolated (transformer-isolated or battery-powered) from the building safety (earth) ground, then it is ok to connect the probe clip to any node in the circuit.

    If the circuit you are testing is connected to the building safety (earth) ground, then the probe clip can only be connected to the grounded node (nowhere else, because connecting it there shorts that other node to ground).
     
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  3. MaxHeadRoom

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    Jul 18, 2013
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    Oh Oh, we are now up to four threads by the same poster for the same question!! :(
    Max.
     
    Last edited: Oct 31, 2013
  4. MrChips

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    Oct 2, 2009
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    No, this is a common question. We need a sticky for this if there isn't one already.
     
  5. WBahn

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    Mar 31, 2012
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    Oh, please, not yet another sticky that no one is going to read and that is going to clutter up the top of the forum lists.

    I really wish we could take most of the stickies in all the forums and put them in a single thread that is well indexed and then have a tight limit on the number of stickies in a given forum, most of which should redirect to a post in the Sticky Thread.
     
  6. MikeML

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    Could be because no one wrote out an understandable answer... ;)
     
  7. MaxHeadRoom

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    Jul 18, 2013
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    Or it could be the way the questions have been posed?
    Regardless, why not pursue any misunderstanding in a single thread?
    This way may be it can be deciphered as to where the confusion is? ;)
    Max.
     
  8. Austin Clark

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    Dec 28, 2011
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    Could I theoretically connect a scope directly to my outlet, as long as I am sure to connect my ground clip to neutral ?
     
  9. WBahn

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    You don't want to connect the ground clip to the neutral. There can be several volts difference between the neutral and the earth ground.

    If your scope has a range than can handle ~400Vpp, then you can do it.

    But I wouldn't recommend it unless you have a very good reason or a scope that you aren't too attached to -- it's too easy for a little thing to go wrong.
     
  10. Dr.killjoy

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    Apr 28, 2013
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  11. Austin Clark

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    Dec 28, 2011
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    Interesting, where would the voltage between neutral and ground come from? Neutral is connected to ground at some point, so shouldn't they be electrically connected and thus have 0V across them?

    I suppose if you have current flowing through neutral to ground, the resistance of the wired connections could create a voltage. Is this what causes the voltage difference?
     
  12. MikeML

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    Yes. Suppose your scope draws 100W off the AC line. On a 15A branch circuit, there could be up to 1800-100 = 1700 W of other stuff plugged into different outlets on the same branch circuit. The ~15A of line current will cause an IR drop back to the panel. That IR drop will appear as a common-mode noise signal between the probe clip and the neutral wire in your outlet. You have a "ground-loop"

    If that branch circuit is protected by a GFI, the act of connecting the probe clip will trip the GFI...
     
    Last edited: Oct 31, 2013
  13. WBahn

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    Yep. There should never be any current flowing in the ground, but all of the current has to flow in the neutral (except for the 240V that goes directly from phase-to-phase (180° split phase, not 3-phase phase-to-phase).

    From what I've been able to tell the NEC recommends that total voltage drop at the furthest load not exceed 3% of the voltage at the breaker. That works out to 3.6V for a 120V circuit. Now, presumably the neutral is coming up and the hot is coming down, so the neutral to ground voltage would be about half of that. Call it 2V for rough round figures.
     
  14. vk6zgo

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    Jul 21, 2012
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    NO!, NO!, NO!, NO! Don't muddy the waters!

    In theory,all those points which are shown as connected are at the same potential.
    In Circuit Analysis,this is a reasonable assumption,but in the nasty "real world",we don't have ideal conductors.

    Because the power wiring in your house has real values of resistance,the Neutral pins of individual power sockets may be at different potentials w.r.t the Neutral point where it enters the house,depending on the current flowing in the run of cable feeding that socket.

    Normally,there isn't any current flowing in the Earth line,so the voltage w.r.t the incoming Neutral looks very close to zero volts.
    We can measure between Neutral & Earth with a DMM & read the voltage drop along that particular Neutral line with a fair degree of accuracy..

    If a high current device is operating on that line,(think electric kettle),the voltage drop will be more than if a small device like a 'phone charger is using it.

    If,perchance,the Earth connection between your 'scope,back to where the Neutral & Earth are connected together,looks like a lower resistance,it may take "the lions share" of the Neutral current for whatever device you have just hung across,which may not be healthy for your Oscilloscope!

    But here is the really nasty bit---not all power sockets (we call them GPOs in Oz) are wired correctly!!!

    In Australia,it is mandatory that the LH pin is Active,& the RH pin is Neutral---but this was,up to the mid 1970s,just a "recommendation",which means there may well be quite a few GPOs still in service with reversed connections.

    if you connect the Earth clip to what "should be" Neutral,& it isn't,at best,you will see a blinding flash & the RCD will drop out,at worst,you will do yourself or your Oscilloscope a profound,maybe fatal injury.

    Of course,your country may have had strict rules longer than Oz,but do you
    really want to trust the "Sparky" who fitted that power point many years ago?

    The thing is,if you just want to check for the presence of Mains Active on the power input to a DUT,you don't have to connect the earth clip to anything.
    You will see the 50 or 60Hz Mains quite adequately relying on the 'scopes own connection to Earth.
    OK,you are measuring w.r.t. the Earth/Neutral connection back where the power line enters the house,so you have metres of cabling involved,which will stop you seeing any high frequency artifacts,but most of the time that won't matter.
     
    Last edited: Nov 3, 2013
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  15. Metalmann

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    Dec 8, 2012
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  16. WBahn

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    Ground loops and noise pickup can be very tricky and frustrating things. It is often the case that you get a cleaner signal with the probe ground clip disconnected than with it connected, even to a node that should be at the common point of the scope and everything else. Does that mean that you are not getting distortion due to leaving the probe ground unconnected? Nope. You almost certainly are. It just means that you might be getting significantly MORE noise from ground loops (and the antennas they create for EMI pickup) than you are dealing with as a result of the lack of a good probe ground connection.

    Depending on your goals in the measurement, it might be enough just to check the apparent quality of the signal with and without the ground connection and use whichever one lools better. Of course, if your goal involves getting good noise estimates, then neither approach is probably good enough and you need to spend some time addressing your grounding situation.
     
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