BEKA BA334D, Manual

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9.3 Display backlight
The BA334D 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
9.4 Pulse output
The BA334D 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 BA334D 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