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Oscilliscope and Ground

Discussion in 'Electronic Basics' started by [email protected], Mar 10, 2005.

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  1. Guest

    I've been trying to learn how to use an oscilliscope. But I get a
    little confused about when and when not to use the ground clip on the
    probe.

    I tried taking measurements from a 9-volt DC power supply that I built.
    It is basically a two-prong power cord into a transformer that knocks
    the voltage down to about 12-volts. Then it goes through a bridge
    rectifier to make it pulsating DC, and then through some capacitors to
    smooth the signal and a voltage regulator, and then I have my positive
    and negative output connectors.

    I tried measuring the AC voltage before the transformer. I just used
    the probe without the ground clip and it gave me an accurate reading.
    Why didn't I need to use the ground clip? And if I did, where would I
    connect it to? Would ground in this situation be the Earth? My power
    cord plug doesn't have a ground.

    Then I tried measuring the DC voltage at the outputs. Since I didn't
    need the ground for measuring the AC voltage, I didn't use it when
    measuring the DC voltage. I just touched the probe to my positive
    output. But this gave me an AC waveform. Why would it give me an AC
    waveform? Then I touched the probe to the positive output and the
    ground clip to the negative output and that displayed the 9V DC voltage
    I was looking for.

    I figured I would need to use the ground clip when measuring my DC
    voltage output, but why didn't I need it for measuring the AC voltage?

    - Clint
     
  2. The ground clip on the scope probe should have a direct connection to
    the case and safety ground prong of the scope. (check this with an ohm
    meter with the scope unplugged.) So it is safe to connect the probe
    ground only to nodes that are either isolated from ground (any node in
    your low voltage circuits that have been isolated from ground by the
    small supply transformer) or to nodes that already have an an earth
    ground connection by some other route.

    If you connect it to any node that has some stiff voltage with respect
    to earth ground, then the connection will constitute a short circuit
    across that voltage and cause large current to pass through the probe
    shield and scope ground conductor, possibly destroying the probe and
    possibly injuring you. If in doubt, make an AC and DC voltage
    measurement between any node you are thinking of grounding (with the
    probe clip) and the clip. If more than a fraction of a volt shows up
    (there can be slight ground noise differences between points that have
    a connection to ground), investigate further before making the direct
    connection.

    The purpose of the clip is to provide a high frequency path for the
    signal across the scope front end amplifier, back to ground, without
    having to go out the scope safety ground prong, around the
    distribution wiring and back up the power cord of the equipment being
    tested. Or, for isolated equipment, to provide a ground reference
    point for the scope, with which to measure some voltage.
     
  3. Rick Fox

    Rick Fox Guest

    Correct, the Earth is the ground. Even though your plug doesn't have
    the ground connection, the 'live' or 'hot' side of the supply is still
    referenced to it. The oscilloscope is also connected to the Earth
    ground through its plug, so you already have a ground connection.
    Trying to connect the 'scope's ground in this situation to anything
    other than the Earth ground (which it already is anyway) would be
    hazardous. You did it right.
    The reason you need the 'scope's ground lead on the other side of the
    transformer is that there is no return path for the current otherwise,
    no 'zero-reference'. Without it, the 'scope will measure AC voltage
    induced onto the wires with reference to Earth (you can touch just a
    large piece of metal for the same effect). Not much use.
    A last tip - never 'float the 'scope' as you may see advised in some
    places. That means removing its own Earth connection from the plug. It
    can be extremely hazardous, putting the whole body of the scope at
    mains potential.

    Rick.
     
  4. Kitchen Man

    Kitchen Man Guest

    Excellent experiment. Even without a ground prong on your oscope power
    cord (how old is that thing?) the scope will sense your building's earth
    ground through the neutral lead. Your transformer primary sees the same
    ground through its neutral. Thus, they are common grounded, and you can
    measure a good ac signal. The DC side is isolated from that common
    ground via the transformer, so you need that second reference to make a
    measurement. You saw an AC signal on the output with the ungrounded
    probe, because that's the voltage the scope could see with reference to
    the building earth ground. Hope that makes sense.
     
  5. mike

    mike Guest

    Ground is a DEFINITION, not a PLACE. People often (mis)use (IMHO) the
    term ground to mean earth. But that only makes sense if you're sitting
    on the dirt.
    Where there's distance, there's impedance. Your measurement problem
    gets much more difficult as the frequency increases. And there's also
    mutual coupling of all kinds...but I digress.


    The best you can hope for is to measure one specific point relative to
    some other specific point. You put the probe ground clip on the point
    you've
    defined as the reference using the shortest possible wire. This is
    often zero volts nominal relative to system ground...which may also be
    nominally the same as building wiring ground...which may also be
    nominally the same as earth ground at the dirt outside. I say nominally
    because if there's current and distance, there's always differential
    voltage. IF that voltage gets really big, you blow out the ground wire
    on your probe and (maybe) electrocute yourself. If it's not so big, you
    get measurement errors.

    Now, if you're measuring low frequencies and don't need extreme accuracy
    and can stand some noise on your reading, you can increase the length of
    the wire on the ground clip to make probing more convenient. For rough
    measurements, you can even clip a wire from scope chassis ground to the
    device chassis ground and skip the probe ground altogether...again
    assuming they're both at the same nominal potential (ground). This is
    NOT always the case. If you care what the waveshape looks like, use the
    probe ground wire.


    You almost always want a wire from somewhere in the scope (ground)
    system to someplace in the (ground) system in the device under test.
    It's possible to make measurements where this is not the case, but it's
    usually an unsafe measurement condition.

    Even if you do have the ground clip on the proper reference node, there
    will also be multiple parasitic return paths that can disturb the
    measurement. It's not uncommon to use common mode measurement
    techniques where you put one probe on the signal, another on the
    reference (ground) and subtract the two. In this case, you can safely
    measure differentials that are not ground referenced.

    If you don't have a defined common (ground) return path for the signal,
    the laws of physics will define one for you...but you have no idea where
    that is or how it affects your measurement.

    While I'm on a tirade, let me address the probe issue.
    Good scope probes are not just a coax with a resistor. They're complex
    networks that often include coaxial wire with a resistive center conductor.
    Depending on vintage, your scope vertical input will probably say
    something like 1Meg, 10pf to maybe 45pf. If you put a 1X probe onto the
    scope, the input capacitance at the tip gets HUGE. It can cause
    significant circuit loading and misinterpretation of the waveform. It
    also makes the grounding problem much worse because you have to charge
    all that capacitance.

    You almost always want to use a 10X probe that's properly compensated
    to the scope input capacitance.

    mike

    --
    Return address is VALID but some sites block emails
    with links. Delete this sig when replying.
    ..
    Wanted, PCMCIA SCSI Card for HP m820 CDRW.
    FS 500MHz Tek DSOscilloscope TDS540 Make Offer
    Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121
    Bunch of stuff For Sale and Wanted at the link below.
    MAKE THE OBVIOUS CHANGES TO THE LINK
    ht<removethis>tp://www.geocities.com/SiliconValley/Monitor/4710/
     
  6. Kitchen Man

    Kitchen Man Guest

    It appears we are in violent agreement. Terms, nevertheless, persist,
    and a building's earth ground might just be called that because it is
    sitting in the dirt. Something to consider.
    Don't you think you're adding a bit more complexity to the issue than it
    deserves? The man's question was pretty simple. If you recall,

    "I figured I would need to use the ground clip when measuring my DC
    voltage output, but why didn't I need it for measuring the AC voltage?"
     
  7. Fred Abse

    Fred Abse Guest

    Who the hell "advises" this?
     

  8. Rich did, and so would I for the OP. His questions
    indicated that he does not yet know enough to make
    sound judgments as to when that safety precaution
    should be set aside and what additional precautions
    to take once the o'scope is floated.

    I agree that 'never' is a strong statement, and that
    with appropriate caveats and precautions, floating
    an o'scope can be done safely. But I would advise
    the OP to walk away with 'never' for now and look
    into it sometime later when he may have the need
    to float his o'scope. There are usually better ways
    to solve the problem anyway.
     
  9. No, no, Fred was asking who the hell would tell somebody to float a
    scope, not who would advise somebody not to do this.

    On the other hand, I've heard about people who planned to do this.
    However, oddly, I've never gotten a report back from them on how it
    went... ;)

    --
    Regards,
    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
     
  10. Fred Abse

    Fred Abse Guest

    I know a few people who have (and some who still do), including people
    working on big industrial equipment where the thinnest piece of metal
    between them and the power station is a hundred amp fuse. You can't tell
    some people anything. Sadly, they're the ones that survive. It's the
    people who are scrupulously careful and have one fleeting moment of
    aberration who have to be scraped off the floor.
     
  11. mike

    mike Guest

    Communication is always difficult in a half-duplex medium like the
    internet. My experience has shown that people often ask simple
    questions about complex issues because they don't know enough to ask the
    question that gets them the answer they really need. Then they get
    many fragments of information that range from correct to wrong to
    downright dangerous.
    I tend to overkill the answer so that there's a consistent message in
    one place.
    Since I'm always right ;-) this is best for all ;-) ;-)

    Scope probing is a very complex issue and deserves treatment in a basics
    newsgroup.

    mike


    --
    Return address is VALID but some sites block emails
    with links. Delete this sig when replying.
    ..
    Wanted, PCMCIA SCSI Card for HP m820 CDRW.
    FS 500MHz Tek DSOscilloscope TDS540 Make Offer
    Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121
    Bunch of stuff For Sale and Wanted at the link below.
    MAKE THE OBVIOUS CHANGES TO THE LINK
    ht<removethis>tp://www.geocities.com/SiliconValley/Monitor/4710/
     
  12. Rick Fox

    Rick Fox Guest

    I gave this advice mainly because when I bought my first 'scope (off
    ebay) it had been floated. The earth wire was cut off inside the plug
    (UK plug).

    Guess what? Finding out about it *was* a shocking experience.

    Rick.
     
  13. If the ground connection gives us an absolute reference, absolutely free
    from noise and absolutely stable within microVolts, there is no reason to
    float the 'scope.

    If that is not the case the noise and voltage changes from the ground
    connection will be added to all measurements.

    Maybe the former owner did not trust his ground connection to be
    perfectly stable and noise free.
     
  14. Fred Abse

    Fred Abse Guest

    More likely he needed to measure above ground and either didn't know about
    add-and-invert, or had only one probe.

    Fault finding on the input side of switchmode PSUs tends to encourage that
    sort of misbehavior.
     
  15. Fred Abse

    Fred Abse Guest

    That's one advantage of molded plugs. Tampering is obvious.

    What I was commenting on was on the lines of "surely nobody actually
    advocates floating 'scopes".
    Plus the revelation of the sort of home it had come from.

    Tek actually used to make a device for floating 'scopes up to 20 or so
    volts. Anything over, it would trip to ground. I've seen one.
     
  16. Rich Grise

    Rich Grise Guest

    Actually, I have "floated the scope", but in very controlled conditions,
    and we (it was a whole class in tech school) followed all of the safety
    rules.

    It's doable, but not recommended, for all of the obvious reasons. Then
    again, I've seen some pretty badly pitted 'scope ground clips! i.e., if
    the scope is properly grounded, what you ground the clip to has to be
    very, very close to zero potential from earth.

    Cheers!
    Rich
     
  17. Clarence_A

    Clarence_A Guest

    I have always used an isolation transformer for the entire test
    station to avoid such problems, but even this precaution requires
    care and use of save techniques.
     
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