Teledyne GFC-7000T - Trace CO2 Analyzer - manual - Manual (Page 223)

Theory of Operation Model GFC7000TA Carbon Dioxide Analyzer
Teledyne Analytical Instruments 205
9.3.4.4. Photo-Detector Temperature Control
The synch/demod board also contains circuitry that controls the IR photo-detector’s thermoelectric
coolers. A drive voltage, PHT DRIVE, is supplied to the coolers by the synch/demod board which is
adjusted by the synch/demod board based on a return signal called TEC control which alerts informs the
synch/demod board of the detector’s temperature. The warmer the detector, the harder the coolers are
driven.
PHT DRIVE is one of the Test Functions viewable by the user via the front panel. Press or TST>
until it appears on the display.
9.3.4.5. Dark Calibration Switch
This switch initiates the Dark Calibration procedure. When initiated by the user (See Section 6.13.6 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 synch/demod board circuitry.
9.3.4.6. Electric Test Switch
When active this circuit generates a specific waveform intended to simulate the function of the IR photo-
detector but with a known set of value which is substituted for the detector’s actual signal via the dark
switch. It may also be initiated by the user (See Section 6.13.5 for more details).
9.3.5. 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.
9.3.5.1. 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.
9.3.5.2. GFC Wheel Motor Control
The GFC wheel operates from an 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. A physical switch located on
the relay board can be used to turn the motor off for certain diagnostic procedures.