MANUALP/4000_SERVICE_0712 July2012
24
SECTION 6 THEORY OF OPERATION
Power to the enclosure is supplied through a proprietary combination fused,
filtered, and switched input module.
The controller board incorporates an onboard step-down transformer that
supplies the necessary low voltages for the board. A further transformer is
utilised to provide 12v AC for the door heater. CO
2
inject solenoids, enclosure
heater, door heater and fan motor make up the other controlled devices within
the enclosure.
The versatility of the enclosure is based on the Contherm ZP22 microprocessor
control system. Multiplexed analog inputs that sense the Temperature, and CO
2
are used to interface the sensor circuitry to the controller .
All off board controlled devices are interfaced via optically isolated triac modules.
A proximity switch is incorporated in the front panel to sense the status of the
outer door. The motor and heater are deactivated whenever the door is open.
The temperature in the enclosure is sensed by a solid state temperature sensor.
The microprocessor supplies PID controlled heater outputs that control the
temperature conditions of the enclosure. Carbon dioxide sensing is by EITHER
Thermal Conductivity OR by an INFRA-RED sensor.
After first turning on power to the unit a RESET operation is performed, this
operation tests the condition of retained data in the EEROM which contains any
set points or correction factors and then by computing a CHECKSUM
determines if the data has been corrupted.
If the data is found to have been corrupted, all the control points are set to
factory preset values and all calibration factors are reset to their defaults.
Every 10 milliseconds after initial turn on a timer interrupt occurs, this signal is
derived from the internal clock and is used for time keeping functions.
Relative Humidity is sensed by a capacitive sensor probe. The humidity is not
controlled in the enclosure but the value of the current enclosure RH can be
accessed via the CO
2
button. The RH is used to correct the CO
2
algorithm for
humidity interferences.
NB: A relative humidity sensor is NOT FITTED on INFRA-RED Models as these
do not require any correction for humidity.
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