13
Fig 8 Separately powered backlight
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 BA304D
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 BA304D 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
For guidance, ITS System Certificates Ex02E2031
and 2032 have been issued which list some of the
devices that may be used and define cable
parameters.
Fig 9 Loop-powered backlight
Connecting the backlight in series with the
indicator increases 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 BA304D to a horizontal or vertical pipe.
Detailed assembly instructions are supplied with
each kit.
BA392D Stainless steel bracket secured by two
worm drive hose clips. Will clamp to
any pipe with outside diameter between
60 and 80mm.
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.
