CHAPTER 8
THEORY OF OPERATION
In order to fully understand the operation of the Model 48C, a general knowledge of the
electronics, software, and subassemblies is necessary.
ELECTRONICS
The electronics can be broken down into the following subassemblies:
DC Power Supply
Bias Power Supply
Detector and Preamplifier
Input Signal Conditioning Board
Temperature controller
Microprocessor System
A brief description of each follows. Note that all the electrical schematics are given in
Chapter 11 “Schematics.”
DC Power Supply
The DC Power Supply outputs the regulated and unregulated dc voltages necessary to
operate the digital electronics, the Bias Power Supply, the detector and preamplifier, the
Input Signal Conditioning Board, and the Temperature Control Board. It outputs +24
volts unregulated and
±
15 volts and +5 volts regulated. The main transformer can be
configured for use with either 110 or 220 volts ac. The power supply board also contains
the circuitry for driving the optional zero/span and sample solenoid valves and an
additional circuit for driving the chopper motor.
Bias Power Supply
The Bias Power Supply Board provides the necessary bias voltages for the solid-state
detector, its internal thermoelectric cooler, and the IR Source. The high current 18 volt
regulator provides power to the IR Source and thermoelectric cooler. In addition, the 18
volt source also powers an oscillator. The output of the oscillator goes to step-up
transformer T1. The step-up transformer generates the high bias voltage required by the
detector. A voltage sensing circuit is included and sent to the microcomputer for
diagnostic display.
8-1
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