T101,
the power transformer has two secondary windings;
one for the Model
197
and the other for the Model
1972
Analog Output/IEEE option or the Model
1973
IEEE
option.
The bridge rectifier
(cRl07)
functions as a fullwave rectifier
for both the plus and minus supplies.
R131
limits current to
the
15V
Zener
(VR104)
and to the batteries
(if
installed) for
charging.
VR104
acts as a pre-regulator to the
+
1OV
supply.
R132
limits current to the
15v
Zener
VR105. VR105
acts as a
pre-regulator to the
-9V
supply.
4.7
MODEL
1978
BAlTERY
OPTION
Maximum battery charging rate is achieved when the instru-
ment is connected to line power and the on/off switch is off.
Fullwave rectified voltage from
CR107
is
applied to
R102
and
BTlOl
to
charge the batteries.
QlOl
acts as a current sink
if
the charging current rises above 150mA. The batteries are of
the quick recharge type and will charge in
8
to
10
hours.
With
the instrument turned on the batteries will trickle charge at
approximately 40mA.
With
the battery pack installed, the negative supply is
generated using a
CMOS
voltage inverter
(UlOl).
The output
of
the inverter is applied to
CRlOl
and
C101
for filtering.
Low battery detection is accomplished by the comparator
(U102)
and the microprocessor. A voltage level of
11.6V
across
BTlOl
signals and end of useful battery life. The trip
level for the comparator is set by
R103
and
R104.
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