Beechcraft 20138 - The Field Regulationcircuit

3-3
THE
FIELD REGULATIONCIRCUIT
Figure
3-5
shows
a
simplified schematic of
the
Model
20138
regu-lator
section.
This
section
performs
the
functions of voltage
sensing
and field
current
control.
2OI38A
CR6
IN4004
FROM 28V
"" '
B
TO
FIELD
R14 C4
RIO
R13
Q4
R15
R11
MúULATED
VOLTAGE
AojusT.
CR4
CS
C3
6.2V
R16
R17
FIG.
3-5
SIMPLIFIED
REGULATORCIRCUIT
Circuit
Operation: If
the
buss
voltage is
below the preset
voltage
level, the
action
will be as follows.
The
emitter of Q3 is
maintained at
6.2 volts
by
Zener diode
CR4
and its biasing
resistor R13. The
voltage
at
the base of Q3
will be set
by the voltage
divider
R10.
R11,
and Rl2.
With
a
low buss
voltage, the base
of Q3
will be
at,
or
below
the emitter
voltage,
therefore, Q3
will
be non-conducting and its
collector will
be
at
the buss
potential.
3-8