tuned circuit assignment, problems when measuring current through branch

Discussion in 'Homework Help' started by ninjaman, Jan 3, 2015.

  1. ninjaman

    Thread Starter Member

    May 18, 2013
    306
    1
    hello

    I have this circuit
    upload_2015-1-3_23-28-2.png

    this is the question
    4. Consider the circuit below:-
    L1, R1 and C1 are your original 3 components. C2 and R2 are additional components that you will be given (they will be known values).
    1. Connect the circuit to a 500 Hz supply at any suitable voltage and measure the branch currents and
    2. the supply current.
    3. Using complex notation verify these values.
    IMPORTANT
    The final report should be written up in chronological order with details of activities and analysis at each stage. Theoretical and practical values should be compared and variations should be investigated and corrected or justified. Photographic evidence can be used at any stage if considered appropriate


    I have simulated the circuit to try and get some idea of what currents to expect. I then made the circuit and measured the current. for the above example I got 44mA instead of the uA reading in the picture above. that leads me to believe I am doing something seriously wrong. I have had problems with measuring values and getting the correct values. the inductor is a transformer, a large chunky thing. would it work better with an axial inductor? I measured the capacitors with a meter and got the correct value along with the resistors. they were within tolerance. but I cant understand what im doing wrong.
    I have just tried a different meter in multisim and got 1A reading instead
    upload_2015-1-3_23-35-19.png
    so yeah.....what the hell!?!

    any help would be great!!!

    I hope you all had a merry Christmas and happy new year!!!!
    guess what Christmas soon! only another 300 (odd) days to go!!!

    thanks
    simon
     
  2. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,157
    why are you measuring DC amperes in an AC circuit?
     
  3. ninjaman

    Thread Starter Member

    May 18, 2013
    306
    1
    hello Joe,

    no I was measuring AC set to auto and milliamps, this was on one meter. my other meter had a blown fuse which I have to replace.
    should I be using DC amps?

    thanks
    simon
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    If you are asked for AC Magnitude and Phase, it is more appropriate to use Spice to do an .AC analysis, not a time-domain analysis like you did...

    Here is the Magnitude and Phase for each of the branch currents at frequencies near 500Hz.

    When you measure this in the lab, you will be using a real function generator, which likely has a 50Ω output impedance. If you do this sim with and without including R3=50Ω, you get widely different results...

    Notice how easy this is in LTSpice. If you want to know a branch current in any one of several ways of plotting it, you just point to it and ask. No stupid little iconic Function Generators or Current meters to get in your way...

    113.gif
     
    Last edited: Jan 4, 2015
  5. ninjaman

    Thread Starter Member

    May 18, 2013
    306
    1
    thanks mike I don't really understand any of that but it looks nice!!!
     
  6. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    You asked for the branch currents at 500Hz. I showed you the AC Magnitude and Phase of the three branch currents, I(C1), I(C2), and I(L1) in the vicinity of 500Hz. What is there not to understand?
     
    Last edited: Jan 4, 2015
  7. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,157
    ninjaman,

    In your first post you asked "I have simulated the circuit to try and get some idea of what currents to expect. I then made the circuit and measured the current. for the above example I got 44mA instead of the uA reading in the picture above."

    That is why I asked how come your measuring DC amperes (on the top diagram).

    Second question, in your lab, what is the model number of the signal generator that you are using to generate the 500 Hz sine wave? and What is the model number of your meter used to measure the current?
     
    Last edited: Jan 5, 2015
  8. WBahn

    Moderator

    Mar 31, 2012
    17,725
    4,788
    Instead of running a sim to do your thinking for you (I'm not trying to be insulting, honest), have you tried calculating what you expect? What is the impedance of a 10uF cap at 500 Hz. What will the RMS current be if you apply a 5V peak signal across it?
     
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