Lab Exam Tips

Discussion in 'Homework Help' started by testing12, Apr 3, 2011.

  1. testing12

    Thread Starter Member

    Jan 30, 2011
    80
    2
    Hello everyone,

    I will be given one of the 4 circuits below. The components are in a SEALED box. MY objective is to determine which circuit I have and to meaure the values of R, C or L. At my disposal I have a signal function generator, a scope, 2 DMM's and a DC power supply.

    Here is what i know and I can Expoloit these but was looking for some additional tips from the pros.

    1) Capicitor acts like open circuit to DC and short circuit to really high frequency.

    2) Inductor acts like short circuit to DC and open circuit to high frequencies,

    The above 2 points should help me eliminate the 2 series circuits.

    Next,

    3) I can determine if i have a capicitor or inductor by connecting the scope accross the signal generator and add a resistor in series with the box and connect channel 2 of the scope accross the resistor. (NOTE we are told R in the box will be large, so my added resistor (ie 10 ohms) should not affect readings very much)

    If output leads = cap if lags = inductor.

    4) Now to find the impedance to get the actual values of the R and C or L. I can observe a lissaous pattern on the scope with the configuration in step 3. This will help me obtain the phase angle. (Question is, what frequency signal should i use to get an easy to read pattern?)

    V= IZ and i can ogtain I(max) with my resistor in series (ie VRmax/R) then take I and divide by the signal function generator Vmax. (question is what voltage should i use?)

    also Z = R + (Xc or XL) = Z(magnitude)<phase shift.

    Ok, thats all i got, any other tips you can suggest?
    Thank you very much.

    [​IMG]
     
    Last edited: Apr 3, 2011
  2. dewasiuk

    New Member

    Feb 14, 2011
    24
    0
    Some tips I recommend:

    1) Do a frequency sweep and take note of your boundary conditions. What happens to the CCT at a very low frequency (ω=>0)? What happens at a very high frequency (ω=>∞)? You could use that info to draw a rouch reactance or or impedance vs. frequency plot and use that to predict any resonances if they occur.

    2) Use your scope channels to determine any phase shifts. "ELI the ICE man" will help you alot here.

    In my opinion, I don't think the DMM will help you very much if your CCT could potentially include frequency dependent components.

    Good luck!
     
  3. testing12

    Thread Starter Member

    Jan 30, 2011
    80
    2
    Great thanks for the tips.
    Instead of ELI:


    CIVIL

    ^^^^

    :)
     
  4. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    Well I'd say measure the dc resistance with the DMM first.

    Open circuit implies circuit 1

    Short circuit implies circuit 4

    Any intermediate value implies circuit 2 or 3
    Values of resistance on DM = R : you need to determine 2 or 3 by connecting to the DC supply and the DMM (now switched to volts). Remove the DC supply. If the DMM reading disappears immediately it is 2 if it slowly subsides it is 3

    I will leave you to concoct a measurement scheme for the missing component or it is not an exam.

    PS If I was your examiner and wanted to make things really difficult I would make you use a wet finger instead of all that expensive gear.


    :eek:

    go well
     
  5. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    Note that unless the inductor used has a very low resistance, an ohmmeter might not a measure circuit 4 as a dead short circuit.

    By the way, if you are sitting an examination, you might want to take care with your spelling: the word is capacitor, not capicitor.
     
  6. testing12

    Thread Starter Member

    Jan 30, 2011
    80
    2
    Thanks everyone. :)
     
  7. testing12

    Thread Starter Member

    Jan 30, 2011
    80
    2
    Hi everyone,

    We had a test run today and it went terrible. I think the frequency we used was WAY too high.
    I need help to determine R and C in the digram below, i keep getting values that are off and i think its due to the frequency selected. More will be posted on this tomorrow.

    i know phase shift = arctan (Xc/R) or z = (Xc^2 +r^2)^1/2
    and Zeq = (r*Xc) / (r+Xc)

    [​IMG]



    EDIT:
    An easier way would be to use the admittance. Since in parallel its simply the sum of the individual admittiances. Still the numbers did not work out.

    Please advise.
     
    Last edited: Apr 8, 2011
  8. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    Is this new work?

    All I can tell from what you have told us is that you applied a 14.9 kHz signal to a parallel combination of an unknown resitor and and unknown capacitor.

    Why did you not simply charge the cap up to some known DC voltage and observe the discharge time (constant)?
     
  9. testing12

    Thread Starter Member

    Jan 30, 2011
    80
    2
    Ok here is what I have:
    I managed to get the capacitance correctly but the real portion of the resistance i still cannot obtain. Can someone pease advise.

    Measured values:


    [​IMG]

    Actual Values:
    [​IMG]
     
  10. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    This looks pretty ill-conditioned. The total impedance was not that much more than the 20Ω external to the black box, so that when you subtract that away small uncertainties in the measurements can turn up as big errors in the results.

    Another source of difficulty is the fact that the magnitude of the capacitor reactance is very much less than the resistance in parallel with it. This means that the parallel resistance has little effect on the total phase angle, compared to the effects or the external 20Ω and even the capacitor's loss angle (which you probably do not know).

    You are right to say that the test frequency was too high. A lower frequency would have given a bigger total impedance, so that the external resistor would have been a smaller part of the total. The capacitor reactance could also have been made more comparable with the parallel resistance.
     
    testing12 likes this.
  11. testing12

    Thread Starter Member

    Jan 30, 2011
    80
    2
    Is it safe to give up on this poor attempt, and try again with a lower frequency. I have put alot of time into this, maybe i should book some more time in the lab instead and try again.

    Adjuster thank you very much for your time, its greatly appreciated.
     
  12. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    That would be my guess, if you can spare the time from other things.

    Before you go in, try to estimate what frequency to use, bearing in mind any limitations of the equipment you may be using. You may find that the frequency needs to be inconveniently low for this method to give a really good estimate of the parallel resistance, assuming your 64100Ω value is correct. I also wonder whether the voltage across your 20 ohm monitor resistor would be sufficient in this case?

    Perhaps you should measure at a lower frequency than before, but also consider other means, like measuring DC resistance with an ohm meter. As previously advised by studiot, measuring the RC discharge time constant is another valid method. It can only tell you the RC product though, you would need to find R or C independently.
     
    testing12 likes this.
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