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Homework Help: Multimeter measuring full-wave rectified DC current?

  1. Oct 18, 2007 #1
    I have this question as a pre-lab for a physics electricity and magnetism lab. (I attached the question as an image.)
    I am having difficulty answering the second part:

    A full-wave rectified DC current is just the absolute value of the sinusoidal AC current: http://en.wikipedia.org/wiki/Image:Gratz.rectifier.en.png.

    I can not answer the this question because I do not know the internal workings of the multimeter.
    I can see three possible outcomes:
    1) the multimeter is analog, it uses a galvanometer with the needle having some inertia, so the reading would be some sort of average of the maximum amplitude.
    2) the multimeter is digital, and thus might measure the rms voltage.
    3) the multimeter has no 'inertia', and thus would show the actual time varying voltage V(t).

    Any suggestions?

    Attached Files:

  2. jcsd
  3. Oct 18, 2007 #2


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    The way analog meters work when in AC mode is that they will always read the average value. If you're measuring an AC signal, the meter first rectifies it(either half or full wave), then takes the average value of the rectified signal. It then multiples it by a multiplication (form) factor to display the RMS value.

    There is always inertia with these analog meters.
  4. Oct 18, 2007 #3
    I have no idea if in the question the multimeter is set to DC or AC.
  5. Oct 18, 2007 #4


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    What is the question? I don't see a question in the OP! :confused:
  6. Oct 18, 2007 #5
    The question is attached in the Picture 001.jpg attachment.

    As I understand it now, the question is that you switch a Voltmeter to measure DC voltage. You connect it to a power source that produces a 7.8 V 60 Hz voltage that has been put through a full-wave rectifier.

    What will this Voltmeter measure? Will it give the amplitude of 7.8V or will it measure something else?
    Basically in the question the guy is trying to measure a voltage tht varies with time V(t) with a DC voltmeter.
  7. Oct 18, 2007 #6
    It turn out that the attachment is "pending" for some reason...

    Here is the link to the question that I uploaded:
    http://e.271828.p.31415.googlepages.com/Picture001.jpg [Broken]
    Last edited by a moderator: May 3, 2017
  8. Oct 19, 2007 #7


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    The peak is 7.8V assuming ideal conditions for the rectifier (no diode drop). A dc voltmeter would meter the average value (as mentioned in my initial post). Can you find the average value of this signal?

    Hint: Use calculus to find the average vale of a full wave rectified signal. The wave has a period of pi radians.
  9. Nov 23, 2007 #8
    Is this a full wave bridge? Silicon diodes?
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