29
Note:
Direct access to the alarm setpoints is only
available when the menu is enabled - see section
9.3.12
9.4 Display backlight
The BA338C 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 15. Any certified Zener barrier may be
used, providing the output parameters do not
exceed:
Uo
=
28V dc
Io
=
159mA
Po
=
0.8W
Ci
=
40nF
Li
=
30µH
Alternatively a galvanic isolator may be used In
place of the barrier. For UK installations only the
galvanic isolators specified on the system
certificate should be used.
Fig 15 Backlight powered by Zener barrier
The display brilliance depends upon the current
flowing through the backlight. This 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
9.5 Pulse output
The BA338C may be supplied with an opto-
isolated solid state pulse output. The output is a
polarised current sink that closes for a
programmable duration each time the least
significant digit of the BA338C display changes. A
programmable divider enables the pulse output
frequency to be reduced by 10, 100, 1,000 or
10,000. See Fig 17.
Ron
=
60
Ω
+ 3V
Roff
=
1M
Isink max
=
10mA
The output pulse is non-synchronous and the
maximum pulse frequency is determined by the
programmed pulse width.
Max frequency
= _______1_______ Hz
2 x (pulse duration)
If the instrument power supply fails or is
disconnected, any untransmitted output pulses will
be lost.
Fig 16 Application of pulse output
