Chapter 5 ___________________________________________Theory of Operation
VAISALA__________________________________________________________29
close to the window. The heater foil keeps the hood free of
snow and ice deposits by covering the entire hood area.
The electronics part of the LM21 device can be divided into
three different sections:
- Power supply
- Measurement circuits
- Microcontroller
The input voltage range is wide (from 10 VDC to 38V DC,
or alternatively from 8 VAC to 28 VAC) and the hood and
window heaters are operated with 28 VAC.
Both communication interfaces are protected against ESD
spikes and surge voltages. Lightning induced overvoltages
and surges are conducted to a protective ground.
LMB201 Luminance Measurement Board
The LMB201 board contains all the electronics, the
photodiode, and the necessary connectors for cable and
heating elements.
Operating Voltages
The input voltage of the power supply section ranges from
10 VDC to 38 VDC, or alternatively from 8 VAC to 28
VAC. For full power hood heater operation additional 28
VAC is required. When powered from FD12P or MITRAS
the hood heaters can be operated with the 24 VAC available
but with lower power. The external power supply
requirement is about 6 W for basic operation and an
additional 50 W for a full power hood heater operated at
28 VAC.
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