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9.5 Display backlights
The BA324ND may be supplied with two different
backlights. The loop powered backlight produces
green background illumination enabling the display
to be read at night and in poor lighting conditions.
No additional power supply or field wiring are
required, but the indicator voltage drop is
increased to 5V. Alternatively, the separately
powered backlight has a bright orange output
which enhances daylight viewing, but additional
field wiring and a power supply are required.
9.5.1 Separately powered backlight
The separately powered backlight is electrically
segregated from the measuring circuit, and has
been certified as a separate Ex nL Group II,
Category 3G circuit.
The Type nL input safety parameters are:
Ui
=
30V dc
Ii
=
100mA dc
Pi
=
1.3W
This allows the backlight to be powered from a
safe area 18 to 30V dc supply as shown in Fig 15.
Providing the supply voltage is less than 30V, the
backlight safety components will ensure that the
maximum input current and power are within the
safety limits.
Reducing the supply voltage below 18V, or fitting a
current limiting resistor in series with the supply will
reduce the display brilliance.
Fig 15 Backlight powered by Zener barrier
9.4.2 Loop powered backlight
This backlight is connected in series with the
4/20mA measuring circuit as shown in Fig 16. In
normal operation the voltage and current applied to
terminals 1 and 13 of the BA324ND must not
exceed:
Ui
=
6V dc
Ii
=
30mA dc
Providing the maximum input current in normal
operation is less than 30mA, the maximum voltage
between the indicator terminals will automatically
be limited by the internal safety components.
Fig 16 Loop powered backlight
