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8.4 Display backlight
The BA507E, BA527E, BA508E and BA528E loop
powered indicators can be supplied with a factory
fitted backlight that may be loop or separately
powered.
When loop powered the backlight produces green
background illumination enabling the display to be
read at night or in poor lighting conditions. No
additional power supply or field wiring are required,
but the indicator voltage drop is increased. When
separately powered the backlight is brighter, but
an additional power supply and field wiring are
required.
Fig 16 Terminals for optional backlight
8.4.1 Loop powering the backlight
The backlight is loop powered by connecting it in
series with the indicator 4/20mA input as shown in
Fig 17, which increases the maximum indicator
voltage drop from 1.2 to 5V.
Fig 17 Loop powered backlight
8.4.2 Separately powering the backlight
The optional backlight may also be powered from a
separate safe area power supply via an intrinsically
safe interface as shown in Fig 18.
Fig 18 Separately powered backlight
When separately powered the backlight draws a
constant current when supply is equal to or greater
the minimum specified voltage. Below this supply
voltage the backlight continues to function but with
reduced brilliance.
Current Minimum voltage
BA507E & BA527E
22.5mA
9.0V
BA508E & BA528E
34.7mA
11.0V
