undertstanding questions

Discussion in 'The Projects Forum' started by braddy, Apr 7, 2007.

  1. braddy

    Thread Starter Well-Known Member

    Dec 29, 2004
    83
    0
    Hi I have two questions I need help with.

    1- I am asked to design a experiment to determine the capacitance of of unmarked capaictor. I suggested the time constant method but I was told to indicate another one. Can someone see something else?

    2-The other questions are for my understanding of the oscilloscope probe :
    -I would like to know why high frequency cause problems to the probe of an oscilloscope?
    -If I am not calibrating a probe and I make measurements, I introduce errors in the AC component not in the DC component because of the probe capacitor, right? Can I have a brief explanation about what is going on?

    Thank you
    B.
     
  2. Johann

    Senior Member

    Nov 27, 2006
    190
    30
    Point 1. Connect capacitor to an ac source of known frequency and voltage (AC) with current meter in series. Measure voltage applied and current flowing and by applying Ohm's Law, you will have the capacitive reactance, Xc. Now apply and manipulate the formula, Xc=1/(2 x pi x F xC) to determine the value of C. (I'm sure you're familiar with this?).

    Point 2. Exactly because the probe consists of 2 conductors running close to one another inside the probe cable and will have a certain value of capacitance. Apply the same formula and see for yourself how the capacitive reactance behaves with increased frequency!:)
     
  3. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    Point 1. You can use a constant current source and measure voltage on the cap after being charged for defined time.
     
  4. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    This suggestion it more along the lines of a slightly crude implemention of kubeek's excellent constant current scheme for addressing point 1.

    The suggestion involves taking a 555 timer chip and setting it up so that the capacitor to be measured can be connected into the timing section. The capacitance could then be calculated be using the formula for frequency of a 555 timer as a function of the capacitance in the timing circuit.

    Measure the frequency then plug the value into the formula and compute the value of capacitance it would take to produce the measure frequency.

    hgmjr
     
  5. John Luciani

    Active Member

    Apr 3, 2007
    477
    0
    1. As was suggested I would use the constant current source. If you have
    an oscilloscope it is easy to implement.
    2. All of the high frequency problems with probes (that I have seen) are due to
    uncalibrated probes, mismatched probe/scope and noise coupled to the ground wire
    of the probe. Verify your probe impedance matches that oscilloscope and
    calibrate your probe. The scope manufacturer provides a table that lists the
    proper probes for your scope. Noise coupled to the ground wire is due to the area
    of the loop formed between the ground wire and the tip. This acts as an
    antenna. See my notes and pictures at
    http://www.luciani.org/eng-notes/ee-notes/ee-notes-index.html

    (* jcl *)
     
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