Oscilloscopes: Grounding

Discussion in 'General Electronics Chat' started by Steve1992, May 13, 2007.

  1. Steve1992

    Thread Starter Senior Member

    Apr 7, 2006
    100
    0
    To safely measure a floating high voltage, two channels and ADD and INVERT functions are used to measure a differential voltage.

    Can you safely measure the same voltage with the one channel by using an isolating transformer for the scope?

    Thank you.
     
  2. Tube Tech

    Active Member

    Jan 11, 2007
    46
    0
    Yes, but you have to isolate the scope from the environment, and yourself from the scope. Put the scope on plastic or a dry towel, wear insulating gloves, don't let anything touch the scope.
     
  3. John Luciani

    Active Member

    Apr 3, 2007
    477
    0
    If the isolation transformer breaks the ground connection you are creating a
    shock hazard. The scope ground is connected to earth ground for safety. If you disconnect the ground the scope chasis will float to a voltage that is dependent on the leakage current of
    the Y capacitors in the input power filter. Different scopes float to different voltage levels depending on the capacitors.

    What you want is an isolation amplifier on the input of each channel. Check out the
    scope manufacturers catalog. These amplifiers can be expensive. It may be cheaper
    to do the channel subtraction.

    (* jcl *)
     
  4. Steve1992

    Thread Starter Senior Member

    Apr 7, 2006
    100
    0
    If I were to use an isolating transformer on the unit under test.
    I can safely scope away with the one probe then?

    But wouldn't then, the probe gnd clip complete the earth loop, removing the isolation given to me by the transformer?
     
  5. John Luciani

    Active Member

    Apr 3, 2007
    477
    0
    Removing the earth ground connection creates a shock hazard. Removing the
    connection from the UUT instead of the scope just moves the shock hazard. The
    hazard is not removed.

    Connecting the scope probe ground clip will connect your circuit to earth ground.

    There is a Tektronix application note at
    http://www.tek.com/Measurement/cgi-...at/eng/amplifiers.html&FrameSet=oscilloscopes

    (* jcl *)
     
  6. chesart1

    Senior Member

    Jan 23, 2006
    269
    1
    Hi,

    I have isolated an oscilloscope from ground to make a measurement but it wasn't high voltage. The circuit I troubleshot had a common that was isolated from ground. Therefore I was worried about accidentally grounding the circuit.

    If the scope is truly disconnected from ground, I assume the ground clip on the scope probe would not be connected to an external circuit ground. You stated that you were measuring a differential output, not a grounded output.

    I fully agree with the above cautions concerning an isolated scope.
     
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