Theory of Operation
Introduction
4
4-3
Introduction 4-1.
This section presents theory of operation for Models 734A, 732B, and 732B-7001. The
section begins with an overall functional description that describes basic theory for each
of the three instrument models covered by this manual. The functional description
explains how Models 734A and 732B-7001 relate to the 732B DC Standard.
Following the functional description is a detailed circuit description of the Model 732B
Standard. An overall 732B block diagram and separate block diagrams for the reference
circuitry and battery charger/ac power supply are supplied. Refer to the schematic
diagrams for the component-level circuit descriptions.
Note
Although theory is provided, no troubleshooting information or parts
breakdowns are provided for components and assemblies inside the oven
(the small box within the foam insulation). In order to guarantee the output
specification, reference assemblies are not repairable. A faulty reference
must be replaced as a whole unit. Refer to Section 6 for part number
identification and ordering information.
Overall Functional Description 4-2.
Model 732B is a highly stable 10V, 12 mA dc power supply to be used as a standards
laboratory primary dc voltage reference. Refer to Figure 4-1 for an overall block
diagram.
The 10V reference circuit can be thought of as a very accurate and stable series regulator
that reduces the +11.5V regulated supply voltage to the accurate and stable output
available at the front panel 10V and 10V COM binding posts.
The heart of the 10V reference is a pretested, selected reference amplifier (Ref Amp) and
a beta string wirewound resistor set. Refer to "732B Circuit Description" for more
details. The reference circuit is inside a temperature-controlled oven. The oven
temperature controller is a voltage-to-temperature closed loop amplifier. The set point is
established by a voltage divider at approximately 6.43V. This 6.43V corresponds to a
temperature of 45°C inside the oven.
A precision thermistor mounted close to the reference amplifier on the Reference Hybrid
(HR1) substrate enables the user to monitor changes in the oven temperature through the
rear panel MONITOR/EXT BAT IN connector. Instructions for using this connector are
detailed in Section 3. At an oven temperature of 45°C, the thermistor resistance is
approximately 41.3 kΩ.
Four front panel indicators show operating status. The Power Supply (A5 Battery
Charger) assembly lights the AC PWR indicator when the 732B is connected to ac line
power. The Power Supply also controls the IN CAL indicator to warn that an excessively
large drop in 11.5V operating voltage or a large change in oven temperature has
occurred.
Another circuit on the Power Supply monitors the battery voltage and controls the LOW
BAT indicator. A timer IC on the Front Panel assembly (mounted on the binding posts)
serves as an oscillator for LOW BAT causing it to blink.
The dual-mode charger lights the CHARGE indicator when in the constant-current
(high-charge) mode. The charger turns off the CHARGE indicator and switches to a
float-charge mode when the battery is charged to approximately 90% capacity. The float
charge completes and maintains full charge.
Refer to "732B Circuit Description" for a component-level circuit description that begins
with the 10V reference circuit and works outward to the oven control and power supply.
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