9.4 Display backlight
The BA558E loop powered Rate Totaliser 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 Rate Totaliser voltage drop is increased.
When separately powered the backlight is brighter,
but additional field wiring is required.
Fig 16 Terminals for optional backlight
9.4.1 Loop powering the backlight
The backlight is loop powered by connecting it in
series with the Rate Totaliser’s 4/20mA input as
shown in Fig 17, which increases the maximum
Rate Totaliser voltage drop from 1.2 to 5V.
Fig 17 Loop powered backlight
9.4.2 Separately powering the backlight
The optional backlight may also be powered from a
separate safe area power supply as shown in
Fig 18.
Fig 18 Separately powered backlight
When separately powered the backlight is a
current sink drawing a constant current when the
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
35mA
11V
33
