Handbook for Sound Engineers

Power Supplies 681

that ultimately controls the regulator output.

Comparator Elements. Compares the feedback volt-
age from the sampling element with the reference and
provides gain for the detected error level. This signal
controls the control circuit.

Preregulator. Monitors the voltage across the series
regulator and adjusts the input Vin to maintain the regu-
lator voltage at approximately 3 volts. This regulator
voltage is held relatively constant regardless of input or
output conditions. This reduces the power dissipated
and the number of transistors in the series regulator, Fig.
19-11.

Current Limiting. A method used to protect the pass
transistor by limiting the current within the safe operat-
ing range. The simplest current-limiting device is a
resistor in series with the load. This, however, affects
regulation by the IR drop across the resistor.
To overcome this, constant current limiting is used.
With constant current limiting, the voltage drop across
the series resistor is sampled. The output voltage
remains constant up to a predetermined current at which
time the voltage decreases to limit the output current.
A third current limiting is foldback current limiting
in which the load current actually decreases as the load
continues to increase beyond Imax. This is usually only
used in high current supplies.

The conventional current-limiting power supply of
Fig. 19-12A is protected from instantaneous short
circuits but long duration shorts can overheat Q 2 ,
leading to its eventual failure. In Fig. 19-12B this circuit
is modified to produce foldback by adding two voltage
feedback resistors, R 3 and R 4. The control transistor Q 1
emitter voltage depends on the power supply output
voltage as sampled by the R 3 , R 4 voltage divider. If R 1

senses a current overload, the drop across it decreases

the output voltage and lowers the emitter voltage of Q 1.

Then Q 1 turns on at reduced current through R 1 , which

limits current flow through Q 2 , as shown in the current-

foldback characteristic of Fig. 19-12B. The foldback

ratio can be adjusted by changing R 3 , R 4 , or R 1 , or all

three.

Overvoltage Protection. Protects the load from over-

voltage. This may be accomplished internally or as an

add-on to the power supply. A crowbar circuit is a typi-

cal overvoltage protector.

The circuit monitors the output voltage of a power

supply and instantaneously throws a short circuit across

the output terminals when a preset voltage is reached.

This is generally accomplished by the use of a silicon

controlled rectifier (SCR) connected across the output

terminals of the supply unit.

Remote Sensing. Adding two extra wires between the

supply and the load produces a remote sensing circuit

that permits the supply to achieve its optimum regula-

tion at the load terminals, rather than at the power sup-

ply output terminals. In this manner, the circuit

compensates for the IR drop in the line from the power

supply to the equipment receiving its voltage. The sens-

ing lines are high impedance and have almost no current

flowing. Therefore, the voltage drop is negligible.

The wire size and voltage drop for regulated power

supplies can be determined by Ohm’s Law or with the

Figure 19-11. Simplified block diagram of a preregulated
power supply.

Pulse

generator

Line

sampling

circuit

Summing

amplifier

Regulator

voltage

sampling

circuit

ac line

Ramp

adjustment

Input

capacitor

Pass

element

Figure 19-12. Current limiting circuits.

A. Conventional current limiting circuit.

Q 2

R 2

Q 1 CR 1

R 1

RLoad

Q 2

CR 1

Q 1

R 3

R 4

Rload

R 1

R 2

6

4

2

(^0) 0.2 0.4 0.6 0.8 1.0 1.2
Output — A
Output — V
6
4
2
(^0) 0.2 0.4 0.6 0.8 1.0 1.2
Output — A
Output — V
B. Addition of feedback resistors to generate
a current foldback output.