Output Current Signal - Total Loop Resistance Vs Power Supply Voltage; Loop Resistance - Group I D-Guard; With Backlight Disabled - Loop At 22Ma; With Backlight Enabled - Loop At 22Ma - GasTech D-Guard2 Operator's Manual

18. APPENDIX A:

18.1. Output Current Signal - Total Loop Resistance vs Power Supply Voltage

The maximum total current loop load resistance that can be driven by D-Guard
the detector. This total resistance includes all aspects of an installation such as barrier resistance, cable resistance /
length and the value of the sense resistor used at any connected controller's input channel.
The operator is advised to account for all of these system resistances and provide a source of power supply that will
allow the detector to operate safely with the ability to drive the loop to 22mA (fault signalling).
Incorrect allowance for system resistances and power supply voltages may result in the detector being unable to
correctly signal the measured gas concentration.
The graphs below show the relationship between detector power supply voltage output and maximum loop resistance
for both Group I and Group II versions of D-Guard
can be configured prior to installation to use the display backlight or not. Disabling the backlight allows a wider range
of supply voltages and loop resistances.
18.1.1

Loop Resistance - Group I D-Guard

18.1.2 Loop Resistance - Group I D-Guard
2
. Note that there are two graphs for each type since the detector
2

With Backlight Disabled - loop at 22mA

2

With Backlight Enabled - loop at 22mA

56
D-Guard
2
depends on the voltage supply fed to
2