Handbook for Sound Engineers

Power Supplies 677

section filter, Fig. 19-5E. S filters have a smooth output
and poor regulation. They are often used where the
transformer voltage is not high enough and low ripple is
required. By using the input capacitor, the dc voltage is
boosted to the peak voltage. The ripple factor for a S fil-
ter is

(19-13)

where,
XC 1 is the capacitive reactance of the first capacitor,
XC 2 is the capacitive reactance of the second capacitor,
RL is the load resistance,
XL 1 is the inductive reactance of the choke.

When the choke is replaced with a resistor, the ripple

factor becomes

(19-14)

where,

R is the filter resistor.

19.3.4 Resistance Voltage Dividers

A resistance voltage divider is shown in Fig. 19-6. In

this system of voltage division, the resistors are con-

nected in series with the particular load they feed. The

resistors are calculated by means of Ohm’s law

. (19-15)

The wattage is computed by

(19-16)

Generally, when a series-resistance voltage divider is

used, a separate bleeder resistor is also used to secure

better regulation. Each section should have a separate

bypass capacitor of 10μF or more to ground. The

bypass capacitors stabilize and improve the filtering and

decouple the various levels. This is particularly true for

the series-type voltage divider.

There are two common types of voltage dividers, the

shunt and the series types. The shunt type shown in Fig.

19-6 is designed to supply three different voltages to

external devices. The upper circuit supplies load L 1 , the

second circuit supplies L 2 , and the third circuit supplies

L 3. All circuits are common to ground.

The total current required is the total current of the

three external circuits, or IL 1 +IL 2 +IL 3 , plus an addi-

tional current called the bleeder current. This bleeder

current flows only through the resistors and not through

the external circuits. It is generally 10% of the total

current.

Resistor R 1 is calculated first, because only bleeder

current flows through this resistor,

(19-17)

where,

V is the L 1 voltage also across R 1 ,

I is the bleeder current.

The voltage at the top of R 2 is the L 2 voltage to

ground. Subtracting the voltage drop across R 1 results in

a voltage across R 2. The current through R 2 is the

current of load L 1 plus the bleeder current

. (19-18)

Resistor R 3 has the current of loads L 1 and L 2 plus

the bleeder current flowing through it or

. (19-19)

The current of load L 3 does not flow through any

part of the voltage-divider system; therefore, it requires

no further considerations.

19.4 Regulated Power Supplies

A regulated power supply holds the output constant

with variations in load, current, or input voltage. Regu-

J 2

XC 1 XC 2

RLXL 1

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

J 2

XC 1 XC 2

RLR

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

R V

I

= ---

P V

2

R

----- -=

I^2 R.=

Figure 19-6. Shunt-type voltage-divider system showing the

current flow in the various branches.

R 1

R 2

R 3 L 3

L 2

L 1

dcin

+

−

+

+

+

R 1 V

I

=---

R 2

VL

2

VL

1

• 
  

IR 1 +IL 1

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

R 3

VL 3 – VL 2

IR 1 ++IL 1 IL 2

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