Handbook for Sound Engineers

Power Supplies 679

necessary to accomplish this, Fig. 19-7A. Its imped-
ance curve is shown in Fig. 19-9A.

An ideal constant-voltage power supply would have
zero impedance. For well-designed voltage-regulated
power supplies, the internal output impedance will
range from 0.001–3: for frequencies from dc–1 MHz.

The actual impedance is a function of the load and the

type of equipment being fed by the supply.

A constant-current regulated power supply is

designed to keep its output current constant, regardless

of the changes in load impedance, line voltage, or

temperature. For a change in the load resistance, the

output current remains constant to a first approximation,

although the output voltage changes by whatever

amount is necessary to accomplish this, Fig. 19-7B. Its

impedance characteristics are given in Fig. 19-9B.

A constant-current supply would have infinite

impedance at all frequencies. However, these ideals are

not achieved. Therefore, a practical power supply has a

very low impedance at the lower frequencies, and the

impedance rises with frequency. The constant-current

supply has a rather high impedance at the lower

frequencies and decreases at the higher frequencies.

A constant-voltage, constant-current regulated

power supply, Fig. 19-7C, acts as a constant-voltage

source for comparatively large values of load resistance

and as a constant-current source for comparatively

small values of load resistance. An automatic crossover

(or transition) between these two modes of operation

occurs at a critical or crossover value of load resistance

(RC) where

(19-20)

where,

Vs is the voltage-control setting,

Is is the current-control setting.

19.4.1 Simple Regulated Supplies

A simple supply consists of only the control element

and the reference element. The solid-state zener diode

has almost replaced the gaseous tube reference element

because it is smaller and has better regulation, wide

voltage range, and wide power range. Referring to the

basic design in Fig. 19-10A, the zener diode is con-

nected in series with the limiting resistor R 1 and in par-

allel with the output. As a rule, the zener diode current

IZ is chosen for a value of 10% of the load current IL.

The value of the series resistance R 1 can be calculated

using the equation

(19-21)

where,

Vs is the voltage source,

Vout is the output voltage,

Figure 19-8. Examples of various control elements.

Figure 19-9. Typical internal impedance characteristics for
a regulated power supply.

RS

RS

Vin

Vin

Vin

Vout

Vout

C. Switching regulator.

B. Shunt regulator.

Vout

A. Series regulator.

Vout = Vin (Rs × Iload)

Vout = Vin [R(Is + Iload)]

Vout = Vin

Ton 

Toff

Ton

001 .01 0.1 1 10 100 1k 10k 100k 1M 10M

001 .01 0.1 1 10 100 1k 10k 100k 1M 10M

105

104

103

102

101

100

Hz

Impedance—

7

Hz

101

100

10-1

10 -2

10 -3

Impedance—

7

B. Constant current power supply.

A. Constant voltage power supply.

RC

Vs

Is

= -----

R 1

Vs VL

1 out

• IL+IZ

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