BEKA BA324D, Руководство пользователя

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The backlight must be powered from the safe area
via a Zener barrier or galvanic isolator as shown in
Fig 15. Any certified device may be
used, roviding
the output parameters do not exceed:
Uo = 28V dc
Io = 159mA
Po = 0.8W
The EC-Type Examination Certificate specifies the
maximum equivalent capacitance and inductance
between terminals 12 and 13:
Ci = 40nF
Li = 20µH
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.
For guidance, ITS System Certificates Ex99E2006
and 2007 have been issued which list some of the
devices which may be used.
.
Fig 15 Separately powered backlight
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.
9.5.2 Loop powered backlight
This backlight is connected in series with the
4/20mA measuring circuit so that like the BA324D
indicator it is powered by the 4/20mA signal. This
eliminates the need for a separate Zener barrier or
galvanic isolator and associated wiring for the
backlight, thus significantly reducing the installation
costs.
Fig 16 shows the BA324D terminal wiring. 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 which may be used.
Fig 16 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.
9.6 External switches
For applications requiring frequent access to the
programme menus the BA324D can be supplied
with an external membrane keypad. These
switches, which maintain the IP66 integrity, allow
all functions to be controlled without removing
either of the enclosure covers.