10.4 Display backlight
The BA358E 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, intrinsic safety interface or
field wiring are required, but the Rate Totaliser
voltage drop is increased. When separately
powered the backlight is brighter, but an additional
intrinsic safety interface and field wiring are
required.
Fig 16 Terminals for optional backlight
10.4.1 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
The input intrinsic safety parameters of the
combined Rate Totaliser and backlight are the
same as for the Rate Totaliser alone. The EC-
Type Examination Certificate states that for
intrinsic safety considerations, under fault
conditions the output voltage, current and power of
the combined Rate Totaliser and backlight
terminals 12 & 3 will not exceed those specified by
clause 5.7 of EN 60079-11 for
simple apparatus,
w
hich simplifies system design and documentation.
Providing the increased voltage drop can be
tolerated the intrinsic safety and system design
described in sections 3 and 4 of this manual
remain valid with the backlight loop powered.
10.4.2 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 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
Any certified Zener barrier or galvanic isolator may
be used, providing the output parameters do not
exceed:
Uo
=
30V dc
Io
=
200mA
Po
=
0.84W
The following equivalent internal capacitance and
inductance between terminals 12 & 14 should be
subtracted from Co and Lo of the intrinsically safe
interface powering the backlight to provide the
maximum permissible cable parameters.
Ci
=
13nF
Li
=
8µH
36
