Impedance

AC

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?

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