Biomedical Engineering Reference
In-Depth Information
FIGURE 2-7
Schematic diagram
for a dual rail mains
power supply.
FIGURE 2-8
Schematic diagram
of a generic voltage
regulator.
offer excellent regulation, low dropout (LDO) voltage (the difference between
V
in
and
V
out
)
, and good input noise rejection. Fixed three-terminal regulators like the LM7812
(
12 V) and their relatives operating at other voltages are easy to
use, require few external components, and provide good performance for loads of up to
about 1 A (depending on the voltage drop). Adjustable three-terminal regulators like the
LM317 (for
+
12 V) and LM7912 (
−
ve voltages) have become the regulators of
choice for many designs. They are low cost, require few peripheral components, as can
be seen from the schematic in Figure 2-9, and are capable of adjusting the output voltage
between 1.2 V and 37 V for a 40 V input voltage.
Because all linear regulators produce output voltages that are lower than the input,
some power,
P
disp
, is dissipated. This is equal to the product of the average current,
I
out
,
and the difference between the input and the output voltage
+
ve voltages) and LM337 (for
−
P
disp
=
I
out
(
V
in
−
V
out
)
(2.1)
A wide range of linear regulators is available commercially for most applications, as
can be seen from Table 2-1. For more information on these and other devices, examine
the catalogs of electronics mail-order companies such as Element14 (previously Farnell)
or RS-Electronics.
FIGURE 2-9
Schematic diagram
of a voltage
regulator based on
the LM317.
