R convert the input power to the necessary voltages required by the
R controller electronics.
R (a) Microcomputer
R
R The microcomputer section consists of a microcontroller and
R asssociated components to provide the required decision processes
R of the ice detector.
R A watchdog timer circuit is used to monitor the correct operation
R of the microcontroller function. A time out of this circuit will
R cause the microcontroller to enter the hardware reset state: this
R enables the FAULT output (FAULT indication) and disables the ICE
R and SEVERITY outputs.
R (b) Probe oscillator
R
R The probe is driven at its resonant frequency by a closed loop
R oscillator circuit. This circuit consists of a drive coil and a
R feedback coil. These coils are mounted concentric with the
R unexposed half of the probe tube. The drive coil produces a
R magnetic field which causes the magnetostrictive probe tube to
R elongate. The relaxation of the tube is picked up by the feedback
R coil which drives the input of the oscillator operational
R amplifier setting up the resonant circuit.
R (c) Heater control
R
R The heater control consists of a silicon controlled rectifier
R with zero cross switching that controls the flow of current
R through the strut and probe heaters.
R D. Detector Inputs
R (1) Test inputs
R
R To perform initiated test, the test input must be activated for at
R least 500 ms. The test input can be initiated several times between
R energization and de-energization of the aircraft electrical circuits.
R (2) Reset inputs
R
R The reset input is activated to reset the severity counter and cut
R off the SEVERITY signal.
R In the ice detection system, the reset input receives the wing anti
R ice valve status. When this valve is open, the SEVERITY signal is
R reset and its processing is inhibited as long as the wing anti ice is
R supplied from the aircraft.
R **ON A/C 001-049, 051-099, 101-149, 151-199, 201-299, 301-399,
R E. Lighted Icing Indicator
A lighted icing indicator is installed in lieu of one winshield center
panel retainer bolt. It is visible by both pilots. It provides a visual
indication when icing conditions are encountered.
Its illumination is controlled by a switch INT LT/ICE IND & STBY COMPASS
(2LF) for night visibility.
R **ON A/C 401-499,
R E. Detector Outputs
R
R Each ice detector generates three signals:
R
R (1) ICE signal
R
R The ice detector sends the ICE signal when a thickness of 0.5 mm of
R ice is accreted on its sensing element. This signal is maintained for
R 60 seconds. If new ice detections occur within 60 seconds, the ICE
R signal is maintained for 60 seconds after the last detection.
R
R (2) SEVERITY signal
R
R The ice detector generates the SEVERITY signal when a number of ten
R elementary ice detections is reached.
R The threshold for the SEVERITY signal corresponds to approximately 5
R mm of ice accreted on the most critical protected surface of the
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