13
8.5 Display backlights
The BA307C & BA308C indicators can be supplied
with one of two types of backlight. The loop
powered backlight produces green background
illumination enabling the display to be read at night
or in poor lighting conditions. No additional power
supply, IS interface or field wiring are required, but
the indicator voltage drop is increased.
Alternatively, the separately powered backlight has
a bright orange output which enhances daylight
viewing, but an additional IS interface and field
wiring are reequired.
8.5.1 Separately powered backlight
The separately powered backlight is segregated
from the measuring circuit and has been certified
as a separate intrinsically safe circuit, but it does
not comply with the requirements for simple
apparatus.
This backlight must be powered from the safe area
via a Zener barrier or a galvanic isolator as shown
in Fig 10. Any certified device may be used,
providing the output parameters do not exceed:
Uo
=
28V dc
Io
=
110mA
Po
=
0.77W
The EC-Type Examination Certificate specifies that
the maximum equivalent capacitance and
inductance between terminals 12 and 13 is:
Ci
=
0.045µF
Li
=
0.02mH
To determine the maximum permitted cable
parameters, these figures should be subtracted
from the maximum permitted cable capacitance
and inductance specified by the certificate for the
Zener barrier or galvanic isolator powering the
backlight.
Fig 10 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.
The BA307C backlight requires a minimum
supply voltage of 14V, and the larger
BA308C backlight a minimum supply voltage
18V. The backlight current can be calculated
from:
For a BA307C
Backlight current = Vsupply –14
End-to-end resistance of barrier
#
For a BA308C
Backlight current = 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.
