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Impedance

Discussion in 'Electronic Basics' started by clement, Sep 15, 2004.

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  1. clement

    clement Guest

    Hi: helpful souls out there

    I like to know how can i measure the resistance or an impedance in an AC
    circuit?
     
  2. Lord Garth

    Lord Garth Guest

    You need to know the inductive and / or capacitive reactance and the
    resistance
    in order to calculate the impedance. Z=((Xl+Xc)²+R²)**.5

    Xl=2piFL and Xc=1/(2piFC)

    Isn't this in your text book?
     
  3. Z = V/I

    If you have a signal generator, you can pass a known sine wave through
    the circuit. By comparing the peak of this known voltage with the peak
    of the resulting current using an oscilloscope, you can measure the
    phase difference P between voltage and current in radians. Measure the
    RMS of the current, I. Now, you have the complex current, which is I L
    P, where the L sign is the angle sign for polar form of complex
    numbers.

    Given this, you can figure the impedance in polar form by

    Z = (V/I) L (-P)

    The rectangular form can be computed from this using

    Z = (V/I) (cos(P) - j*sin(P))

    If the imaginary part of this rectangular form is negative, then the
    impedance has an inductive component. If its positive, the impedance
    is capacitive. If its 0, then the impedance is purely resistive.

    Given the imaginary part, you can compute the size of inductance or
    capacitance in the circuit fragment using the standard formulas and
    the frequency of the input sine wave.

    cos(P) is called the 'power factor' of the circuit, because it gives
    how much of the impedance is resistive, and thus how much power is
    used by the resistances in the circuit.

    Regards,
    Bob Monsen
     
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