8.4 Display backlight
The BA507E-SS and the BA527E-SS 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 17 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 18, which increases the maximum indicator
voltage drop from 1.2 to 5V.
Fig 18 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 19.
Fig 19 Separately powered backlight
When separately powered the backlight draws a
constant 22mA current when the supply is equal to
or greater the minimum specified voltage of 9V.
Below this supply voltage the backlight continues
to function but with reduced brilliance.
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