Fig 16 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.
10.3.2 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 17.
Fig 17 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
35
