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Digial Voltmeter DC and AC mode

Discussion in 'Electronic Basics' started by aman, Apr 26, 2005.

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  1. aman

    aman Guest

    I dont understand the real meaning of DC and AC readings on the Digital
    voltmeter. Is it correct to say that if the signal = DC component + AC
    component then DC reading gives the DC component and AC reading gives
    the AC component ?
  2. John  Larkin

    John Larkin Guest

    Usually, yes. On most DVMs, the DC range is heavily averaged/lowpass
    filtered, so the AC component is mostly ignored. And on the AC range,
    it's AC-coupled, and response drops off below 10 Hz or something like
    that, so the AC range ignores any DC component entirely.

    It needn't be so, though: some meters read the *true* RMS voltage on
    their AC range, which includes any DC component; but that's rare.

    I think the old analog Simpson-type VOMs may have responded to DC on
    their AC range. Anybody know for sure? Did they have a series cap, or
    just a rectifier and the meter movement?


  3. Probably - although there may be some meters where this is not so.

    Peter Bennett, VE7CEI
    peterbb4 (at)
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  4. John Larkin

    John Larkin Guest

    Cool: the AC range is DC coupled, but the OUTPUT range adds a series

  5. Jamie

    Jamie Guest

    the DMM (digital Multimeter) will give you DC (direct current)
    from a steady source of energy , like a Battery for example.
    AC voltages are alternating types of energy meaning, take the battery
    and keep reversing the leads on the meter at 60 times per second or
    50 times depending from what country your coming from. this Voltage
    is changing polarity..
    lets look at it from a more simpler view.
    a common cell(1 part of a battery) (carbon) puts out aprox 1.56 volts
    new. imagine putting this cell on a rotating plate on it's side.
    now imagine the probes are on the edge of the plate equally spaced
    as the plate turns with the cell lying on it, the - + terminals are
    getting closer to the probe tips, lets say the meter is reading the
    approaching terminals of the cell giving you a stronger reading until
    they are perfectly lined up with the 2 probes! at this point the reading
    is at its strongest. note that the DMM may have the - or + sign on the
    left of the reading indicating the polarity. now the cell is still
    turning and as it does the voltage is now going down. when the 2 cell
    tips are at the far points of each probe tip, the voltage reading will
    become 0. the cell keeps turning and you then will see voltage start
    increasing again ! but this time the polarity is now the opposite as it
    was before.
    this continues until we get back to the starting point. this is
    call alternating current because the voltage is raising and falling
    back and forth alternating its polarity as it does so.
    Hence the need for a meter to have a different mode to display to you
    the Average or RMS. (root mean Square) reading which is really not the
    highest voltage of the sine wave (PEAK).
    for example.
    10 Volts RMS. would be something like 10.4 if this AC wave was
    converted to a clean DC to represent the peak..
    so when measuring that 120 Volts of house current , the actual peak
    voltage is around 160 volts.
    any ways..
    enough of this.
  6. John Fields

    John Fields Guest

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