Table Of Contents

Connecting External Sensors

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    Last Modified: February 13, 2019

    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 1. Powering a 4-Wire External Sensor
    Figure 2. Powering a 3-Wire External 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-116xx 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-116xx 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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