Oscilloscope cables and ground...

Discussion in 'General Electronics Chat' started by Spacerat, Aug 3, 2015.

  1. Spacerat

    Thread Starter New Member

    Aug 3, 2015
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    Hello Forum,

    I have an old analog oscilloscope. The probes have a ground clip that should be attached to the ground, reference conductor in the analyzed circuit. What if I don't connect that ground probe to the circuit ground? The oscilloscope also has a ground and a waveform is traced on the scope screen anyway, correct? Does it mean that we are using the ground point for the oscilloscope which can be at a different potential than the ground in my analyzed circuit?

    I get often confused with the different types of ground that can occur in a circuit (floating, mass, etc.) I know that voltage is always measured between two different physical points. One of the points (conductors) in the circuit can be elected to be the ground and the potential of all other points can be higher (positive) or lower (negative) than that reference point....

    Thanks,
    Spacerat
     
  2. dl324

    Distinguished Member

    Mar 30, 2015
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    If you don't use the ground lead, you'll have a large ground loop (from circuit ground to earth ground to scope earth ground to scope circuit ground). Would wreak havoc on high frequency measurements.

    Scopes are usually connected to earth ground; they should only be "floated" if you know what you're doing.
     
  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    That is the important thing. Next is that the scope doesn't necessarily care what those two pints are. If for example you have a small audio amplifier with +15V and-15V power supplies. If you measure the output with the scope ground connected to system ground (the cenpter point between the two power supply voltages, you will see the audio symmetrical about the ground reference line on the scope. But if you move the scope ground clip to the -15V power supply, now you will see the audio riding on a +15V pedastle. That is because the circuit output is 15 V above the negative rail.

    As mentioned, it is important to make sure that your circuit or device can handle a scope ground clip that is connected to earth ground. For normal electronics powered through a fully isolated power supply, this is not a problem because the transformer isolation breaks up the possible ground problem.
    ak
     
  4. crutschow

    Expert

    Mar 14, 2008
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    Don't confuse earth ground with circuit ground (common). They may not be connected together.
    The scope is normally connected to earth (safety) ground through the ground plug on the power cord.
    Your circuit common may or may not connected to that ground. That is why you need to connect the scope ground to circuit ground.
    If if the two grounds were connected together through the power line ground, there likely will be some noise between them and the long lead connection will corrupt high frequency signals.
     
  5. Spacerat

    Thread Starter New Member

    Aug 3, 2015
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    mmm...thanks to everyone. I did learn something but I am still a little confused about the various types of ground.

    earth ground: an actual connection between a metal part in the circuit and planet Earth.
    common ground: a point (not necessarily connected to planet Earth) that is used as a reference for the potential of all other points
    floating ground: ?

    The oscilloscope is connected to earth ground via the outlet. When we use the both clips on the oscilloscope probe (the smaller is the ground probe connected to the common ground), we are measuring the voltages in the circuits in the correct way. We we only correct the larger clip to different points in the circuit and leave the group probe unconnected, we will get the wrong voltage values since the ground in that case will be Earth ground...is that right? Sort of?

    thanks,
    Spacerat
     
  6. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    SpaceRat, Where do you live? (Different countries tread the safety ground differently).

    Does your 'scope have a three wire line cord or a two wire cord?
     
  7. dl324

    Distinguished Member

    Mar 30, 2015
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    The ground lead should always be connected to circuit ground as this will eliminate any issues that could be caused by ground loops. There are times when a scope should be floated; if you don't know what they are, then you probably don't need to do it.

    As I and other responders have said; connecting the scope to earth ground is for safety.
     
  8. Spacerat

    Thread Starter New Member

    Aug 3, 2015
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    I am here in the US, north carolina.

    Thanks for the inputs. Could someone try to give me a brief explanation of what a "ground loop" is? Is it a good or bad thing?

    thanks.
     
  9. dl324

    Distinguished Member

    Mar 30, 2015
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    It's generally a bad thing. Try this wikipedia article for starters.
     
  10. MikeML

    AAC Fanatic!

    Oct 2, 2009
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  11. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
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    I hope I don;t confuse the OP, but we can think of GROUND being one of three things.

    EARTH or Protective ground. In a house in the US, the "third prong" ground, and Neutral are connected to the EARTH at ONE PLACE.
    This ground is designed to handle faults. i.e. The metal on your washer. If hot should come in contact with that metal, the current will follow the protective ground path.

    GROUND as a Reference. No appreciable current flows into this POINT. Your CATV provider generally references this point.

    In a house, The ground as a reference and as protective ground share. This isn't good, but that's the way it is.

    GROUND as a COMMON point. Many times it is connected to protective ground and ground as a reference. In a 12 V Negative ground car, it's a common point.

    In many cases the signals are separated within a piece of equipment and combined at only ONE POINT,

    What we don't want is a loop to this common point

    If the old scope has a 3-wire cord, then the GROUND pigtail is connected to earth. If the circuit you are testing has no other path, you have to provide one.
    Some circuits like a switched mode power supply will do bad things when you start probing around.

    Generally, it is NOT CONSIDERED SAFE to isolate the scope, because the metal case can have a lethal voltage to ground. It's ALWAYS desirable to isolate the circuit your testing. It is safe to use an isolated scope probe.
     
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  12. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
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    Never heard of isolated probes and will have to check them out.. But Dave Jones a really cool video on this and isolating the test circuit is always great process since something is less likely to go wrong with it..
     
  13. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
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