BEKA BA374C, Manual

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12. BACKLIGHT
The BA374C can be supplied with LED
backlighting to improve display contrast when the
transmitter is installed in a poorly illuminated area.
The backlight is segregated from the measuring
circuit and has been certified as a separate
intrinsically safe circuit.
The backlight must be powered from the safe area
via a Zener barrier or a galvanic isolator as shown
in Fig 15. Any EEx ia IIC certified Zener barrier
or galvanic isolator may be used, providing the
output parameters do not exceed:
Uo = 30V dc
Io = 159mA
Po = 0.8W
For guidance the system certificates list some of
the devices which may be used.
Fig 15 Backlight power supply
The display brilliance depends upon the current
flowing through the backlight which is determined
by the supply voltage and the end-to-end
resistance of the Zener barrier or output resistance
of the galvanic isolator. Brilliance will not be
significantly reduced until the current falls below
20mA.
Backlight current = Vsupply -18
End-to-end resistance of barrier
#
# or output resistance of galvanic isolator
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