10
1.9.8 Optional display backlight
The BA364D can be supplied with LED
backlighting to improve display contrast when the
instrument is installed in a poorly illuminated
area. The backlight is segregated from the
measuring circuit and has been certified as a
separate intrinsically safe circuit
.
System
certificates for use with Zener barriers and
galvanic isolators have therefore been issued.
The backlight must be powered from the safe
area via a Zener barrier or a galvanic isolator as
shown in Fig 1.6. Any certified Zener barrier
may be used providing the output parameters do
not exceed:
Uo =
28V
Io
=
159mA
Po =
0.8W
The system certificate lists some of the galvanic
isolators which may be used.
The maximum equivalent capacitance and
inductance at the backlight terminals is:
Ci
= 40nF
Li
= 30µH
To determine the maximum permissible cable
parameters, these figures should be subtracted
from the maximum cable capacitance and
inductance specified by the barrier or galvanic
isolator powering the backlight. The system
certificates specify maximum cable parameters
for some devices.
Fig 1.6 Backlight powered by Zener barrier
