13
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
8.4.2 Loop powered backlight
This backlight may be connected in series with the
4/20mA measuring circuit so that like the BA304C
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 costs.
As shown in Fig 9 the loop powered backlight may
be connected in series with the BA304C indicator.
Any Zener barrier or galvanic isolator certified EEx
ia IIC by an EEC approved certification body may
be used, providing the output parameters do not
exceed:
Uo
=
30V dc
Io
=
200mA
Po
=
0.85W
Fig 9 Loop-powered backlight
Connecting the backlight in series with the
indicator will increase the total voltage drop in the
4/20mA loop from 1.1V to 5.0V
8.5 Pipe mounting kits
Two pipe mounting kits are available for securing
the BA304C to a horizontal or vertical pipe.
BA392C
Stainless steel bracket secured by
two worm drive hose clips. For 60 to
80mm outside diameter pipes.
BA393
Heavy duty stainless steel bracket
secured by a single 'V' bolt. Will
clamp to any pipe with an outside
diameter between 40 and 80mm.
8.6 Panel mounting kit
The BA394 stainless steel panel mounting kit will
secure a BA304C indicator into a panel aperture,
but it does not seal the joint between the indicator
enclosure and the instrument panel