732A
removes
the drive from
Q6,
shutting off the
IN
CAL
indicator
on
the
front panel
When power is
restored,
Q7
remains
latched
off until its emitter is connected
monemtarily to
the COMMON
output terminal via the
RESET connection, accessible through the front panel.
3-17.
Reference
Circuit,
AS
3-18.
The Reference
Circuit (A4Q12,
QL
Q2,
Q5,
Ul,
U2) reduces the
18.6V
output of the Regulator to
precisely lOV.
The
Reference
circuit is
a
highly
stable
series-pass voltage regulator. The entire reference supply
(except the pass transistor) is
enclosed
in
an oven
to
provide the
consistent
thermal environment necessary for
the stability of the output.
3-19.
U2,
the Ref-Amp,
is
a transistor and zener
diode
mounted
on a
common substrate.
This
construction
compensates
for
ambient temperature changes, thus
U2
has an extremely
low
temperature coefficient.
The
Ref-
Amp
compares the
lOV output to its
internal
zener
reference
to
derive
an error
voltage which
is amplified
by
op amp
Ul.
U1
drives
the
series
pass
element
(Ql,
A4Q 1
2).
Q2
provides current limiting to protect the series
pass element under short circuit conditions. Variable
resistor
R20
allows
a small
adjustment
(±50
uV)
in the
output voltage of the Reference. Larger
adjustments
can
be made by jumper
changes on the Calibration
PCB
Assembly, A7.
3-20.
Output Divider
3-21.
Two precision
resistive
voltage
dividers
divide the
precise lOV output down to
IV
and
L018V,
Each of these
dividers is adjustable
over
a limited range to
allow
calibration. Both dividers are enclosed in the oven
with
the reference,
3-22.
Oven Controller
3-23.
The Oven
Controller (A4Q13, A4Q14,
Q3,
Q4,
U3, U4, U5)
maintains
the internal temperature of the
oven at
a
nominal
temperature
of
48
±
2®C. The Oven
Controller is a high
thermal
gain,
proportional control
circuit. The
Oven
Controller
circuit
is partially located on
the
A5 Reference
PCB
assembly,
inside of the oven. The
oven driver
and
output transistors
are
located
on
the
A4
Regulator
PCB assembly.
3-24.
Thermistors RTl,
series
connected
RT2, and RT3
are
connected
in a bridge
configuration
with R28 and
R29,
and
are located inside the
oven.
U3 buffers the
bridge output
and
drives
differentialamplifier/integrator
U5
which drives
the oven driver
and output transistors
(A4Q13, A4QI4)
and
subsequently
the oven
heaters.
U4
shapes
the
overall
loop frequency
response.
3-25.
Battery
Charger
3-26.
The battery charger
determines
the
state of
the
charge
of the internal
battery
and
sets
the charging
current
accordingly:
constant
current
charging
for deep
discharge
or
constant
voltage
trickle
charging
for charge
maintenance.
The Battery
Charger circuit
is
located on
A3.
3-27.
Transistor
Q2
is
a
current
source
that
supplies all
the charging
current. Transistors
Q3
and
Q4
form a
schmitt
trigger.
Transistor
Q6
supplies a
constant
voltage
output
for
trickle charging and
thus
maintains
the battery
at
full
charge.
Three thermistors
monitor
the ambient
temperature
(RTl) and the battery
temperature
(A6RT1,
A6RT2)
and
adjust the charging rate
accordingly.
3-28.
During
initial
charging,
Q3
enables
Q2
and
the
high
charge rate.
When
the battery voltage
rises
to
approximately 32V,
Q4
turns off, shutting
off the
constant current
charge.
The battery is then constant
voltage
charged
by
Q6
(approximately 27V
at
23"^C),
Potentiometer
RIO sets the threshold point
of
this
transition
and
hence the end of charge current.
At
this
point,
Q6
supplies
a constant
voltage
trickle charge to the
battery
and
R20
sets
this voltage
level. Thermistor
A3RTl compensates
the constant voltage
charging for
variations
in the ambient
temperature.
Thermistors RTl
and RT2, located on the battery
PCB,
and
Q5
prevent
high
current charging at temperatures below
5°C,
and/or
high
temperatures.
3-2
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