Michell Instruments Easidew PRO, User Manual

Page: 32 / 58

I.S. Field Display User’s Manual
26 97321 Issue 2.3, May 2016
OPERATION
3.4.2 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 Figure 14.
Hazardous Area Safe Area
4-20 mA
indicator
circuit
Backlight
supply
Zener barriers or
Galvanic Isolators
4-20 mA
ppm
V
Easidew PRO I.S. Field Display
P E
S
S
S
S
+
0
0
+24 V
9999
14 12
4-20 mA
S
S
1
2
3
4
3 4 6 5 2 1
Figure 14
Separately Powered Backlight
When separately powered, the backlight draws a constant current when the supply is
equal to or greater than the minimum speci fi ed voltage. Below this supply voltage the
backlight continues to function, but with reduced brilliance.
Current Minimum voltage
34.7 mA 11 V
Any certi fi ed Zener barrier or galvanic isolator may be used, providing the output
parameters do not exceed:
Uo = 30 V DC
lo = 200 mA
Po = 0.84 W
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 = 8 μ H
The separately powered backlight is a current sink which will draw a constant 35 mA
when the voltage between terminal 12 and 14 is greater than 11 V. If the supply voltage
is less than 11 V the backlight brilliance will be reduced.