Connecting External Sensors
You can power a 3- or 4-wire external sensor with an output voltage range of 5 V to 24 V by
connection through the sensor power (SP) and ground (GND) terminals on the FieldDAQ
device. The output voltage level is configurable using a 10-bit resolution for maximum
flexibility. Use NI software to configure a sensor to turn on prior to the start of an acquisition.
Figure 13. Powering a 4-Wire External Sensor
GND
3
AI–
2
SP
1
AI+
4
Powered
Sensor
Figure 14. Powering a 3-Wire External Sensor
GND
3
AI–
2
SP
1
AI+
4
Powered
Sensor
The FieldDAQ device goes into a current limited voltage fold-back mode to protect the sensor
power excitation supply from short circuits or higher current than those listed in the
FD-11601
Specifications
. In this mode, the output voltage folds back to a lower value than the set
voltage, the overcurrent status is set in NI-DAQmx, and the voltage input LED lights red.
Removing the overload condition automatically triggers the output to return to the desired set
voltage, the overcurrent status is cleared in NI-DAQmx, and the voltage input LED returns to
normal conditions.
The FieldDAQ device also has an open circuit detection mode to indicate whether a sensor is
connected. For any sensor power output voltage, an open circuit is detected when the
minimum output current for sensor detection is not drawn, as listed in the
FD-11601
Specifications
. When an open circuit is detected, the Open Channel status is set in NI-DAQmx
and the voltage input LED turns off.
If a connected sensor draws a total current below the maximum specifications values, the
voltage input LED lights green.
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FD-11601 User Guide