Handbook for Sound Engineers

676 Chapter 19

A choke filter operates best with maximum current
flow. It has no effect on a circuit when no current is
flowing. The critical inductance is the inductance
required to assure that current flows to the load at all
times. An inductor filter depends on the property of an
inductor to oppose any change of current.
To assure that current flows continuously, the
peak current of the ac component of the current must
not exceed the direct current Idc =Idc »RL. Therefore,

(19-8)

and

(19-9)

where,
LC is the critical inductance,
RL is the load resistance.

Filter chokes should be selected for the lowest
possible dc resistance commensurate with the value of
inductance.
The ripple factor (J) for an inductive filter is

(19-10)

where,
RL is the load resistance in ohms,
RC is the choke resistance in ohms,
f is the ripple frequency.

19.3.3 Combination Filters

Combination filters use a combination of resistors,
capacitors, and inductors to improve the filtering. The
simplest is a resistor-capacitor filter and the more com-
plicated is a series of inductance-capacitor (LC) circuits.

19.3.3.1 Inductance-Capacitance Filters (LC)

Inductance- capacitance filters, sometimes called L fil-
ters, use an inductor as an input filter and a capacitor as
the second stage of the filter, Fig. 19-5C. LC filters
operate well under varying load conditions.
The inductive reactance of the choke in an LC filter
section tends to oppose any change in the current
flowing through the winding, creating a smoothing
action on the pulsating current of the rectifier. The
capacitor stores and releases electrical energy, also

smoothing out the ripple voltage, resulting in a fairly

smooth output current.

The ripple factor for an LC filter is

(19-11)

where,

XC is the capacitance reactance in ohms,

XL is the inductive reactance in ohms,

f is the frequency of ripple,

C is the capacitance in farads,

L is the inductance in henrys.

When multiple LC filters are connected together, the

ripple factor is

(19-12)

where,

L is the inductance in henrys,

f is the ripple frequency,

C is the capacitance in farads,

n is the number of sections.

19.3.3.2 Resistance-Capacitance Filters

Resistance-capacitance filters, RC, Fig. 19-5D, employ

a resistor and capacitor rather than an inductor and

capacitor. The advantages of such a filter are its low

cost, light weight, and the reduction of magnetic fields.

The disadvantage of such a filter is that the series resis-

tance induces a voltage drop that varies with current and

could be detrimental to the circuit operation. An RC fil-

ter system is generally used only where the current

demands are low. RC filters are not as efficient as the

LC type, and they may require two or more sections to

provide sufficient filtering.

19.3.3.3 S Filters

A S (pi) filter has a capacitor input followed by an LC

2 Irms

XL^2

3 RL

t---------

LC

RL

32 u Sf

=-----------------

J

RL+RC

32 2u Sf

=-------------------------

J

2 XC

3 XL

--------------=

2

32 u SfCu 2 SfL

=-----------------------------------------

0.01

f^2 CL

------------=

J

2

3

------ -

16 S^2 f^2 LC

= -------------------------------n

0.47

157.9f^2 LC

= --------------------------------n-