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Line Voltage Has Offset

Discussion in 'Electronic Design' started by D from BC, Jan 17, 2008.

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  1. D from BC

    D from BC Guest

    I did an oscilloscope diff. test on a wall outlet.
    The sine was offset??

    Outlet ---N------x10 Probe tip 1------Ch 1
    Outlet----G-------------------------- Earth
    Outlet----L-----x10 Probe tip 2-------Ch 2 Inv

    I confirmed with a DMM on DC setting.
    Approx. 10VDC with respect to L.

    Just a bit curious how the line gets offset.

    D from BC
    British Columbia
  2. Phil Allison

    Phil Allison Guest

    "D from BC"

    ** Totally WRONG test procedure.

    To measure a small DC offset in the presence of a large AC voltage with a
    DMM - you must first remove most of the AC voltage with a low pass filter.

    Try using a 1 megohm resistor in series with a 1 uF film cap wired across
    the AC supply - then measure the DC component across the cap with your
    DMM. Allow for the effect of the resistor if you like, it reduces the DC
    reading by about 10%.

    Bet the new result is way under 1 volt.

    ......... Phil
  3. Greg Neill

    Greg Neill Guest

    Could be that some load is creating what is called
    harmonic distortion of the power "signal". This
    can result in slightly non-symmetrical upper versus
    lower halves of the sinewave (or more complicated
    relationships) and a non-zero DC component of the

    It might be interesting to perform a spectrum
    analysis of the waveform.
  4. Phil Allison

    Phil Allison Guest

    "Greg Neill"

    ** You are a gullible damn fool.

    Go away.

    ..... Phil
  5. D from BC

    D from BC Guest

    I think that was probably a classic DMM gotcha. :(
    Too much faith in my DMM. :p
    I just fired myself... :p

    Just got rehired. :)

    I did the RC filter circuit as suggested ..
    Line offset voltage = 0.0001VDC (zilch..)
    That's more like it.. :)

    I dunno wtf I did wrong with the scope set up..
    I repeated the set up and the scope shows no offset.
    Must have goofed a vertical setting.
    Double fooled..

    Embarrassed :(

    D from BC
    British Columbia
  6. Phil Allison

    Phil Allison Guest

    "D from BC"
    "Phil Allison"

    ** However, you DO get major brownie points for TAKING the advice,
    acting on it and then admitting your error.


    You are far from alone in making this particular mistake.

    ....... Phil
  7. Joerg

    Joerg Guest

    Look at the bright side: Nothing exploded, your scope still works and
    the house hasn't burned down.
  8. Jamie

    Jamie Guest


    "I'd rather have a bottle in front of me than a frontal lobotomy"

    "Daily Thought:

  9. Joerg

    Joerg Guest

    Now it wasn't me who said that!

  10. Yet you contiune to climb back up the stairs,just so you can be
    pushed right back down. :(

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
  11. John Smith

    John Smith Guest

    It didn't hurt your head though, did it? I mean you have always been
    like "that", I can tell from past posts ... :)

  12. MooseFET

    MooseFET Guest

    .... or there was a DC offset and now there isn't.
    Was the neighbor doing some sort of "science experiment" involving
    passing large DC currents into a "ground rod"? :) :)

    Do you live in the SanFransisco area near a BART station?
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