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Why is my D.C. ammeter reading fluctuating?

  1. Jul 12, 2015 #1
    Why is the reading on my D.C. ammeter fluctuating? As in the needle won't maintain at one spot, it'll sway left and right. Also, how do i ensure that the fluctuations are minimized?
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  3. Jul 12, 2015 #2


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    Are you looking at a motor? Or is this with nothing being measured?
  4. Jul 12, 2015 #3


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    What scale, how much current, how much fluctuation, what kind of meter. Have you cleaned the contacts.

    Read the current through a resistor from a power supply that does not fluctuate. If it reads steady, the the answer to your question is obvious. The current is varying.
    If it fluctuates with the fixed current, I don't know what to say other than clean the contacts.
  5. Jul 12, 2015 #4


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    What is the circuit?
  6. Jul 12, 2015 #5
    Call Ghost Busters! :rolleyes:

    Or perhaps check for a nearby magnetic field. Assuming it's an old style magnetic deflection meter, a varying magnetic field could cause that.

    How come I can never find a ghost? :headbang:
  7. Jul 12, 2015 #6


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    this question is so vague that in its present format, cannot be answered

    Try asking again with a much better posed question telling us all about how you are trying to measure current and in what circuit
    a sharp and well lit photo or 2 wouldn't hurt for us to see what you are doing

  8. Jul 13, 2015 #7
    Hi Dave!

    First of all, apologies for not making my statement a little bit more clearer. The experiment that I'm currently doing is measuring magnetic force with a basic current balance where the current is supposed to always stay at a constant 0.4 A. The thing is, it always fluctuate and I do not know why. Here is the set up:

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  9. Jul 13, 2015 #8


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    oh OK, so this is related to your other thread ? .....


    if so, you probably should have keep it all together and saved the confusion

    so how much is the current fluctuating ?

    some things I could think of that would cause fluctuations

    1) variations in the power supply ... that is ... it isn't stable enough for what you are trying to do
    2) its acting as a magnetometer and picking up variations in the Earth's magnetic field .... of which there's been a few over the last week or so
    3) other sources of magnetic fields near your gear are causing problems with the setup

    can I see a better pic of that "Basic current balance " section

  10. Jul 13, 2015 #9


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    Does it fluctuate if you remove the "basic current balance" part from the circuit and just use rheostat and your amperemeter?
    How much does it fluctuate, and on which timescale?
  11. Jul 13, 2015 #10

    jim hardy

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    so many good thoughts thus far.
    Troubleshooting is an iterative process wherein one tries to draw a circle around the problem then tighten that circle like a noose.

    Binary search is usually the quickest but it's not always convenient to cut a system exactly in half.. we look for convenient ways to partition the system under study.
    You have two active electronic devices in your setup, both employ feedback so theoretically both are capable of control system instability.
    So i'd ask - do my fluctuations stem from my active or my passive devices?
    Without changing anything, i'd switch off the electronic scale. If that settles current , it's very interesting. If it doesn't settle current, well , you've eliminated half your active devices., quaternary search ?
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