083730300A DCN8101 Theory of Operation 159
6.4.3.3 PHOTO-DETECTOR TEMPERATURE CONTROL
The sync/demod board also contains circuitry that controls the IR photo-detector’s
Thermal Electric Coolers (TEC). A drive voltage, PHT DRIVE, is supplied to the
coolers by the sync/demod board which is adjusted by the sync/demod board based on a
return signal called TEC control which alerts the sync/demod board of the detector’s
temperature. The warmer the detector, the harder the coolers are driven.
PHT DRIVE is one of the functions viewable on the Dashboard (may need to be
configured: Setup>Dashboard).
6.4.3.4 DARK CALIBRATION SWITCH
This switch initiates the Dark Calibration procedure. When initiated by the user (See
Section 4.5.1 for more details), the dark calibration process opens this switch,
interrupting the signal from the IR photo-detector. This allows the analyzer to measure
any offset caused by the sync/demod board circuitry.
6.4.4 RELAY BOARD
By actuating various switches and relays located on this board, the CPU controls the
status of other key components. The relay board receives instructions in the form of
digital signals over the I
2
C bus, interprets these digital instructions and activates its
various switches and relays appropriately.
6.4.4.1 HEATER CONTROL
The two heaters attached to the sample chamber housing and the GFC Wheel motor are
controlled by solid state relays located on the relay board.
The GFC Wheel heater is simply turned on or off, however control of the bench heater
also includes circuitry that selects which one of its two separate heating elements is
activated depending on whether the instrument is running on 100 VAC, 115 VAC or 230
VAC line power.
6.4.4.2 GFC WHEEL MOTOR CONTROL
The GFC Wheel operates from a AC voltage supplied by a multi-input transformer
located on the relay board. The step-down ratio of this transformer is controlled by
factory-installed jumpers to adjust for 100 VAC, 115 VAC or 230 VAC line power.
Other circuitry slightly alters the phase of the AC power supplied to the motor during
start up based on whether line power is 50Hz or 60 Hz.
Normally, the GFC Wheel Motor is always turning while the analyzer is on.
6.4.4.3 ZERO/SPAN VALVE OPTIONS
Any zero/span/shutoff valve options installed in the analyzer are controlled by a set of
electronic switches located on the relay board. These switches, under CPU control,
supply the +12VDC needed to activate each valve’s solenoid.
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