The point where a sensor takes a measurement is, at times, many yards away from the point of control. One method of transmitting information from sensor to control point still in use after 60 years is the 4-to-20-mA current loop.
The loop is a series circuit that contains three items: a power supply, a sensor/transmitter, and a display or controller/receiver. A single wire connects the power supply to the sensor, sensor to controller, and controller back to the power supply. While the power supply provides voltage to the loop, the current flow through the loop is controlled by the sensor/transmitter. The sensor/transmitter can be a combination of a separate sensor and transmitter module, or the two components may reside in the same enclosure.
The output from the sensor is converted into an analog current ranging from 4 mA, representing “zero” or the minimum value, up to 20 mA which represents the highest reading. The 4-mA zero was chosen for two reasons. First, it sends a minimum current through the loop so devices that need little power can just operate from loop current. This eliminates the need for a separate power supply. Second, low current serves to indicate a bad loop circuit. For example, a current that drops to 0 mA usually indicates a broken loop wire or dead loop-power supply.
A series circuit is used as a means of utilizing Kirchoff’s Law: current entering a point equals that leaving it. Of course, a series circuit has only one entrance and exit per point, so its current remains the same throughout. This isn’t true of voltage-based controls where wire resistance creates voltage drops along the path. There is a voltage drop along the current loop as well, but it generally doesn’t factor into system operation.
However, the loop designer must know those voltage drops to provide enough head room for the instrumentation power supply. As an example, a manufacturer says its 4-to-20-mA pressure sensor needs a minimum of 12 V to operate, and will take up to 30 V. The sensor connects to a display/recorder system that, according to specifications, has an internal loop resistance of 249 Ω. Multiply the 249 Ω times the maximum loop current, 20 mA, to get 4.98 V. Add that to the 12 V needed by the sensor to get approximately 17 V. An 18-V power supply should work, and it does — until the readings get close to full scale. Then the calibration goes way off. We’ll find out what happened in a future Sensor Sense.
Edited by Robert Repas
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