732A
Section
3
Theory
of
Operation
3-1,
INTRODUCTION
3-2.
The
information
in this section describes the
theory
of operation
of the 732A. It contains an overall functional
description
followed
by a circuit description of the
732A.
Both descriptions
are supported by a block
diagram
(Figure 3-1).
Component
level descriptions contained
in
the circuit
analysis
are referenced to the detailed
schematics
in Section
8 of this manual
3-3,
OVERALL FUNCTIONAL DESCRIPTION
3-4.
The
732A dc
Voltage
Referance Standard is
a
highly
stable lOV,
12 mA
power supply.
Refer
to
Figure
3-
I,
AC line
input power is full
wave
rectified
and fed
to
a
two
stage
voltage
regulator. The
first
stage,
or Pre-
regulator
converts
the
raw
dc to
32V
dc.
The second
stage,
or Regulator
converts
this voltage to 18.5V dc which
powers
the
Oven
Controller and
the Reference,
3-5.
The
Voltage
Monitor disables the
Oven
Controller
and latches the
IN
CAL
indicator off
when
the
output
of
the
Regulator
is insufficient for proper
operation.
The
RESET
terminal
is
used
to restore
the
IN CAL
indicator
to
the
ON
condition after
standardization
of the
instrument
has
been performed.
3-6,
If
ac
line power fails or is not
available,
an internal
sealed, lead-acid
battery
maintains operating
power
to
the 732A.
When
ac
power
is
available, a battery charger
charges the battery.
This is
indicated by the BTRY CHG
indicator,
3-7,
When ac
power
is
not available,
the battery
may
be
charged
by
an
external
ac
or dc
source
connected
at
the
rear
panel
POWER
INPUT
connectors.
The
external
source
can
also
supply
operating
power
for
the
instrument.
The battery
voltage can
also be
measured
at
the
rear
panel
connectors.
3-8.
CIRCUIT
DESCRIPTION
3-9.
The
information
in this section
describes
the
circuitry
of the 732A
to the functional block
diagram
level.
Refer to the detailed
schematics
in Section
8.
3-10.
Power Supplies (A3
and A4)
3-11.
The
732A has two
cascaded
regulators.
The Pre-
regulator
(A3Q1) is a simple
emitter
follower regulator
that
damps
the full
wave
rectified
power from the bridge
rectifier to
approximately 32V
dc.
3-12.
The
Regulator (located on A4) supplies
operating
voltages to
all of the circuitry in the 732A
except
the
battery charger.
During battery operation, the
battery
drives
the Regulator
input.
3-13.
The Regulator (Ql,
Q2, Q3,
Q4)
is a
conventional
series
pass transistor error-amplifier
design
that
regulates
the
32V
to
18.6V
dc.
3-14.
Voltage Monitor
3-15.
The
Voltage
Monitor
circuit
(Q5, Q6,
Q7, Q8)
checks the regulator
output
and disables the instrument
when
the supply voltage
falls
below
a critical value.
When
this happens, the Oven Controller
is disabled and the IN
CAL indicator is latched off. The
reset
circuit is used to
turn the IN CAL indicator back
on
after
standardization
has
been
re-verified
by qualified
personnel
The
Voltage
Monitor is located on the A4 Regulator
PCB.
3-16.
Transistor
Q8
is turned on by the
voltage
drop
across the
Regulator circuit series-pass
transistor.
This
causes
switching
transistor
Q5
to
saturate,
supplying
power
to
the Oven
Controller circuit
and the
IN CAL
indicator circuit.
When
the output falls below
that
needed
for normal
operation,
Q8
and
Q5
turn off, shutting
down
the Oven Controller
and
removing
drive
from
Q7,
a
Programmable Unijuction
Transistor
(PUT). This
3-1
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