26
For a BA328C
Backlight mA = Vsupply –18 _
End-to-end resistance of barrier
#
# or output resistance of galvanic isolator
Two indicator backlights may be powered in
parallel from a single 28V 300 Zener barrier or
galvanic isolator, but the display brilliance will be
reduced.
9.6.2 Loop powered backlight
This backlight may be connected in series with the
4/20mA measuring circuit so that, like the indicator,
it is loop powered. This eliminates the need for a
separate Zener barrier or galvanic isolator and
associated wiring for the backlight, thus
significantly reducing the installation cost.
As shown in Fig 16 the backlight is connected in
series with the indicator. Any Zener barrier or
galvanic isolator certified EEx ia IIC by an EEC
approved certification body may be used providing
output parameters do not exceed:
Uo
=
30V dc
Io
=
200mA
Po
=
0.85W
Fig 16 Loop powered backlight
Connecting the backlight in series with the
indicator will increase the total voltage drop in the
4/20mA measuring loop from 1.1V to 5.0V
