Optional Backlights; Separately Powered Backlight; Loop Powered Backlight - BEKA BA405C Manual

Any galvanic isolator certified EEx ia IIC by an
EEC approved body with output parameters equal
to or less than the maximum BA405C certified
input parameters may be used.
The maximum permitted cable parameters for the
loop should be calculated by subtracting Ci & Li for
the BA405C from the cable parameters specified
on the galvanic isolator certificate.
When designing the loop it is necessary to
establish that the supply voltage between terminals
1 & 3 of the BA405C set point station always
exceeds the minimum operating voltage shown in
section 4.1
4.3

Optional Backlights

The BA405C set point station can be supplied with
two different backlights.
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
minimum operating voltage drop of the set point
station is increased to 11.1V.
Alternatively, the separately powered backlight has
a bright orange output which enhances daylight
viewing, but it requires an additional IS interface
and field wiring.

4.3.1 Separately powered backlight

This backlight is segregated from the measuring
circuit and has been certified as a separate
intrinsically safe circuit.
The maximum input safety parameters are
Maximum input
Ui
Ii
Pi
Equivalent capacitance and inductance
Ci
Li
Any Zener barrier or isolator certified EEx ia IIC by
an EEC approved body which has output
parameters equal to, or less than, the maximum
input parameters specified above may be used.
Most installations would use a 28V, 93mA 0.65W
(28V 300Ω) device as shown in Fig 4.
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.
significantly reduced until the current falls below
20mA.
The loop powered
= 28V dc
= 159mA dc
= 0.8W
= 30nF
= 0.01mH
Brilliance will not be
6
Fig 4 Separately powered backlight
The separately powered backlight requires a
minimum supply voltage of 14V, The backlight
current can be calculated from:
Backlight current =
End-to-end resistance of barrier
# Or output resistance of galvanic isolator
Two BA405C separately powered backlights may
be powered in parallel from a single 28V 300Ω
Zener barrier or galvanic isolator, but the display
brilliance will be reduced.

4.3.2 Loop powered backlight

The loop powered backlight is connected in series
with, and powered by, the 4/20mA current from the
set point station This eliminates the need for a
separate Zener barrier or galvanic isolator and for
additional wiring, thus significantly reducing the
installation cost.
Fig 5 Loop powered backlight
Fig 5 shows how the loop powered backlight
should be connected in series with the set point
station. Any Zener barrier or galvanic isolator
certified EEx ia IIC by an EEC approved
certification body may be used providing the output
safety parameters do not exceed:
Vsupply –14
#