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SECTION 8.
LOW-VOLTAGE POWER SUPPLY (A3-A6)
8.1 DESCRIPTION
The Low-Voltage Power Supply consists of four
hoards: 1) Battery Charger board (A3),
2)
Control
board
(A4), 3)
Output board
(A5)
,
and
4)
Switcher
hoard
(A6). This
power
supply
converts an
ac
or
a
de
voltage
input to
the
de
voltages required to
power the
.
System
Analyzer.
In ac
mode,
the
power
supply oper-
ates as
an otf-line,
half-bridge
converter
for
inputs of
100 to
130
Vac
or
200
to 260 Vac
at
50
to
400Hz.
In
de
mode,
the power
supply operates
as a
push-pull
con-
verter for
inputs
of
11
to
19
Vdc.
When connected
to
an
ac
power
sou rce,
the
unit
automatically switches
to the
ac operating
mode. The
Battery
Charger
is
included
in
this power
supply
for
charging
the
optional external
battery pack. A standby
mode
allows
the
operator
to shut off
power
to
all mod
-
ules
in
the System
Analyzer
except the
OCXOtrCXO
and
the
Battery
Charger.
The Low-Voltage Power
Supply
contains
protec-
tion circuitry that
will
shut
down
or
lock
out
the power
supply for the following
conditions:
1)
a
power-supply
short circuit, 2)
high and low input voltages, and
3)
high
internal
temperatures.
A block diagram
of
the Low-Voltage Power
Supply
is
shown at the
end
of this section
in
Figure 8-1.
8.2
THEORY
OF OPERATION
8.2.1
BATIERY CHARGER
BOARD
(A3)
8.2
.1
.1
General
The Battery
Charger
board
contains the rectifier
circuitry
for the ac
ott'-line
switching
power
supply
and
part
of
the
input
filtering.
A
line transformer supplies
the external
battery
pack with charging
voltage,
sup-
plies
bias
voltage
to the
control
circuitry
for t
he
power
su
ppl
y,
and
powers
the 10-MHz Frequency-Standard
Interface
board
(Al6).
A
separate -5V
regulat or
for
the RF
Synthesizer
module
(A9)
is also
included,
along
wi
th
a 6.2V zener diode (VR3) that provides
an
outer
voltage damp
on
the
+!)V output
line
of
the Low-
Voltage Power
Supply.
A
schematic of
the
Battery
Charger
board
is
shown
at
the
end of this
section
in
Figure
8-2,
the
printed
wiring
board and parts
list
in Figure
8-3,
and
th~
assembly
and
parts list
in
Figure 8-4.
8-1
8.2.1.2
Rectifier/Doubler
The
bridge rectifier (BRl
)
and
the filter
capacitors
(C1
,
C2,
C5, and C6) rectify and
filter the raw
ac
input
to a
de
output
on
the V
c +
and
V c- lines. Some
of
t he
filter
capacitors are on the A3
board;
the
rest (C3,
C4,
Cl2,
and
Cl
3)
are on the Switcher
board
(A6).
A nom-
inal
230-Vac
input uses the
AC INPUT
and 220
RETURN
lines,
which
a re
connected to the
inputs
of
the bridge
rectifier.
The
bridge
rectifier
provides
full-
wave
rectification
of the
ac
input,
and
the filter
capac-
itors filter
the
signal to a
nominal
level
of
325
Vdc.
For
a nominal115-Vac
input,
half
of
the
bridge rectifier
is
used
(the
AC
INPUT line) in conjunction
with
the
fil-
ter capacitors
(the 110 RETURN
line)
to
provide
a
full
-wave
voltage doubler.
The
voltage doubler
then
produces
a
nominal325-Vdc
output.
Thermistors
RTl
and
RT2 limit
the initial
in-rush
current to
the
filter
capacitors.
8.2.1.3
Line
Transformer
T he
line t
ransformer
(Tl)
has two primary
wind-
ings
and
a
single, center-tapped secondary.
The
two
primary
windings
are switched
on the System Moth-
erboard (Al9)
by
the
110/220
switch
-
in
series
for
220V
operation and
in
parallel for
llOV
operation.
This
provides
the
same
voltage on
the
secondary
winding
for
operation
at
inputs
of
both 110
and 220 Vac.
The
sec-
ondary
winding
provides
two outputs. One output,
AC
VOLTAGE SENSE/BATTERY
CHARGER,
is full-
wave rectified
by
bridge
rectifier
BR2
and
filtered
by
capacitor
C4
to a
level
of 27
to 35
Vdc.
The
other out-
put, FREQUENCY
STANDARD
SUPPLY,
is half-
wave
rectified
by CR8 and
filtered
by
capacitor
C3
to
a
level
of
15 to 16
Vdc.
8.2.1.4 Battery
Charger
The
Battery Charger circuitry
is
selected by
the
POWER
ON
line
coming from t he
front panel. A
no-
connect
on the POWER ON line will turn
transistor
q:2
on, which
ena bles
transistor Ql.
Transistor Ql is
biased for
constant
current operation (1.2A)
by
resis-
tor
R7
and
diodes
CR5,
CR6,
and
CR7.
8.2.2 CONTROL
BOARD
(A4)
8.2.2.1
General
The Control
board
co ntrols, protects,
and
regulates
the
Sy!'tem
Analyzer's
Low-Voltage
Power
Supply.
On
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