Campbell Scientific 0871LH1 Freezing Rain Sensor - Manual (Page 28)

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1 1.6 Drive Coil
The drive coil modulates the magnetic field of the magnetostrictive
oscillator and causes an ultrasonic axial movement of the probe.
1.7 Feedback Coil
The feedback coil senses the movement of the probe and when
employed in the probe oscillator circuit, completes the feedback portion
of the MSO.
1.8 Heater
The probe heater de-ices the probe. It is activated when the nominal
icing trip point of 0.50mm is reached and is turned off five seconds after
the MSO has returned to at least 39,970 Hz (the additional five seconds
allows the strut probe time to shed the de-bonded ice). The maximum
heater ON time is 25 seconds. If the probe frequency has not returned at
least 39,970 Hz by that time, a de-ice failure is declared and the heaters
are turned off. An open circuit of the heater is detected by the
microcontroller.
1.9 DC Power Supply
The DC power supply provides 24 VDC for the heater circuitry. Internal
circuitry converts the 24 VDC input power to 5 VDC for use by the
microcontroller and associated circuits. It employs a large input capacitor
to provide enough time between detection of input power loss and
actual loss of DC power, for the microcontroller to store the current unity
status in the non-volatile memory. The DC power supply provides input
transient protection to meet RTCA DO-160C power input, voltage spike,
and lightning requirements.
1.10 Status Output
The status output provides a ground output when the freezing rain
sensor is operating correctly, and high impedance (200 K
Ω minimum)
when the unit has detected a failure. Failures are detected through
continuous and initiated tests. The Status output is capable of sinking 50
mA and is guaranteed to be no more than 1.5 VDC with respect to Signal
Return when active. This output is transient protected to meet RTCA DO-
160C lightning requirements and to prevent stray high-voltage from
coupling into the unit and damaging the output transistor.
1.11 Ice Signal Output
The Ice Signal output provides a ground output for 60 ± 6 seconds when
the ice detector has detected the presence of ice (frequency drop of 130
Hz, equivalent to approximately 0.50mm ice formation). If the frequency
subsequently decreases by 130 Hz while the Ice Signal output timer is
non-zero, the timer is reinitialized to 60 seconds.
The output is transient protected to meet RTCA DO-160C lightning
requirements and to prevent stray high-voltage from coupling into the
unit and damaging the output transistor.
The ice output has feedback to the microcontroller for software to verify
it is in the correct state for more built in test coverage. The software in the