Audio Engineering

152 Chapter 5

5.10 Overcurrent Protection ..........................................................................................

A fundamental problem with any kind of solid-state voltage regulator layout, such as
that of Figure 5.3(a) , is that if the output is short-circuited, the only limit to the current
that can fl ow is the capacity of the input power supply, which could well be high enough
to destroy the pass transistor (Q1). For such a circuit to be usable in the real world,
where HT rail short-circuits can, and will, occur, some sort of overcurrent protection
must be provided. In the case of Figure 5.3(c) , this is done by putting a resistor (R2) in
series with the regulator output and then arranging a further transistor (Q2) to monitor the
voltage across this. If the output current demand is enough to develop a voltage greater
than about 0.65 V across R2, Q2 will conduct and will progressively steal the base current
from Q1.

Set current limit

R1

Q3 D3

4V7

R9

1k0

V in

R10

3k3

c4

100 nF

R7. 10k

R6. 1M0

Q2

C3. 22N

0 V 0 V

R8

R5. 33k 3k3

R4. 33k

R3. 33k

12 V ref

IC1. TL071





Q1

3k3

R2

Set V out

RV1. 100k

C1 C2

470 n100 uF

V out (reg)

Figure 5.4 : Series-stabilized PSU.