# Impedance

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

1. ### clementGuest

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

2. ### Lord GarthGuest

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. ### Robert C MonsenGuest

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