Handbook for Sound Engineers

Power Supplies 675

When a dc voltmeter is connected across the unfil-
tered output of a rectifier, it will read the average
voltage. As an example, assume a dc voltmeter is
connected across a half-wave rectifier. Because of the
inertia of the meter pointer movement, the meter can not
respond to the rapidly changing pulses of the half-wave
rectified current but acts as a mechanical integrator. The
pointer will be displaced an amount proportional to the
time average of the applied voltage waveform.
The average voltage (Vav), as read by the dc volt-
meter, is

(19-6)

where,

Vp is the peak voltage,

S is 3.1416....

The ripple factor is

(19-7)

where,

Idc is the output dc current,

f is the ripple frequency,

C is the filter capacitor in farads,

RL is the load resistance in ohms.

Capacitor filters operate best with large filter capaci-

tors and high-resistance loads. As the load resistance is

lowered, the ripple increases and regulation decreases.

Filtering efficiency is reduced, and the internal

leakage is increased when the capacitor’s power factor

increases. Electrolytic capacitors should be removed

when their power factor reaches an excessive value. In

an ideal capacitor, the current would lead the voltage by

90°. Capacitors are never ideal because a small amount

of leakage current always exists through the dielectric.

Also, a certain amount of power is dissipated by the

dielectric, the leads, and their connections. All this adds

up to power loss. This power loss is termed phase differ-

ence and is expressed in terms of power factor (PF). The

smaller the power factor value, the more effective the

capacitor. Since most service capacitor analyzers indi-

cate these losses directly in terms of power factor,

capacitors with large power factors may be readily iden-

tified. Generally speaking, when an electrolytic capac-

itor reaches a power factor of 15%, it should be

replaced. The filtering efficiency for different values of

power factor can be read directly from Table 19-2.

19.3.2 Inductive Filters

An inductive filter employs a choke rather than a capac-

itor at the input of the filter, as shown in Fig. 19-5B.

Although the output voltage from this type of filter is

lower, the voltage regulation is better.

Figure 19-5. Capacitive, inductive, and S filters.

Rectified

ac

Rectified

ac

Rectified

ac

Rectified

ac

Rectified

ac

A. Capacitor filter.

B. Choke input filter.

C. Inductance-capacitance or L filter.

D. Resistance-capacitance filter.

E. π filter using inductance and capacitance.

R

RL

RL

RL

RL

R

C C

C

C

L

L

CRL

L

Vav

Vp

S

----- -=

Table 19-2. Filtering Efficiency versus %Power Factor

Filtering Efficiency % PF Filtering Efficiency % PF

100 0.000 35 0.935

90 0.436 30 0.955

80 0.600 25 0.968

70 0.715 20 0.980

60 0.800 15 0.989

50 0.857 10 0.995

45 0.895 5 0.999

40 0.915

J

Idc

4 S 3 fCVdc

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