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Calculating Supply Voltage for a Loop Powered Sensor

A common application for Digi’s Connect Sensor is connecting to a 4/20mA two wire sensor; also known as a loop powered sensor. In this configuration the sensor is powered from the current loop and is connected to the Connect Sensor per the wiring diagram below.
User-added image
The power output from the connect sensor is configurable by the user and that the configured voltage will be reflected between Power and Analog Ground in the schematic above. The voltage across the terminals of the sensor (between Power and Analog Input in the graphic above) will fluctuate depending on the loop current. This is because the variable loop current through the resistance inside the Connect Sensor will change the voltage between the Analog Input and ground. This is expected behavior for a 4/20mA two wire interface.

When users are determining what value to set the output power to in this scenario, they must calculate the range of voltages to determine if the voltage setting is sufficient. The voltage across the sensor terminals (Power and Analog Input) will always be less than the configured voltage.

The nominal internal resistance of the Connect Sensor is 375 ohms so an example calculation of the range of voltage across the terminals

Analog Output Voltage set to 24V
Sensor Terminal Voltage (Power to Analog Input)
Max: 24V – (4mA *375ohms) = 22.5V
Min: 24V - (20mA *375ohms) = 16.5V
Last updated: Mar 07, 2019

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