Handbook for Sound Engineers

268 Chapter 10

tance, the time constant is defined as the time it takes
for the current to reach 62.3% of its maximum value.
The time constant can be determined with the following
equation:

(10-43)

where,
T is the time in seconds,

L is the inductance in henrys,

R is the resistance in ohms.

See Section 10.2.1 for a further discussion of time
constants. The effect of an inductor is the same as for a
capacitor and resistor. Also, curve A in Fig. 10-14
shows the current through an inductor on buildup and
curve B shows the current decay when the voltage is
removed.

Right-Hand Rule. The right-hand rule is a method
devised for determining the direction of a magnetic field
around a conductor carrying a direct current. The
conductor is grasped in the right hand with the thumb
extended along the conductor. The thumb points in the
direction of the current. If the fingers are partly closed,
the fingertips will point in the direction of the magnetic
field.
Maxwell’s rule states, “If the direction of travel of a
right-handed corkscrew represents the direction of the
current in a straight conductor, the direction of rotation
of the corkscrew will represent the direction of the
magnetic lines of force.”

10.3.3 Ferrite Beads

The original ferrite beads were small round ferrites with
a hole through the middle where a wire passed through.
Today they come as the original style plus as multiple
apertures and surface mount configurations.

The ferrite bead can be considered a frequency-
dependent resistor whose equivalent circuit is a resistor
in series with an inductor. As the frequency increases,
the inductive reactance increases and then decreases,
and the complex impedance of the ferrite material
increases the overall impedance of the bead, Fig. 10-27.
At frequencies below 10 MHz, the impedance is less
than 10:. As the frequency increases, the impedance
increases to about 100: and becomes mostly resistive
at 100 MHz.

Once the impedance is resistive, resonance does not
occur as it would using an LC network. Ferrite beads do

not attenuate low frequencies or dc so are useful for

reducing EMI/EMC in audio circuits.

10.3.4 Skin Effect

Skin effect is the tendency of ac to flow near the surface

of a conductor rather than flowing through the

conductor’s entire cross sectional area. This increases

the resistance of the conductor because the magnetic

field caused by the current creates eddy currents near

the center of the conductor. The eddy currents oppose

the normal flow of current near the center, forcing the

main current flow out toward the surface as the

frequency of the ac current increases.

To reduce this problem, a wire made up of separately

insulated strands woven and/or bunched together is used.

Commonly called Litz wire, the current is equally

divided between all of the individual strands which

equalizes the flux linkage and reactance of the individual

strands, reducing the ac losses compared to solid wire.

10.3.5 Shielded Inductor

Some inductor designs are self-shielding. Examples are

toroid, pot core, and E-core inductors. Slug cores and

bobbins may require shielding, depending on the appli-

cation. It is impossible to completely shield an inductor.

10.4 Impedance

The total impedance created by resistors, capacitors,

and inductors in circuits can be determined with the

following equations.

T L

R

=---

Figure 10-27. Impedance of ferrite beads. Courtesy of

Vishay Dale.

800

600

400

200

(^0) 1 10 100 1000
Frequency—MHz
Z,
R,
X,—
7
600 7 ±25%
Z
R
X