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A current sensor is a device that detects electric current in a wire and generates a signal proportional to that current. The generated signal could be analog voltage or current or a digital output.
Hall effect current sensor with internal integrated circuit amplifier. 8 mm opening. Zero current output voltage is midway between the supply voltages that maintain a 4 to 8-volt differential. The non-zero current response is proportional to the voltage supplied and is linear to 60 amperes for this particular (25 A) device.
They utilize a "current-sense resistor" to convert the load current in the power rail to a small voltage, which is then amplified by the current-sense amplifiers. The currents in the power rail can be in the range of 1 A to 20 A, requiring the current-sense resistor to be a resistor typically in the range of 1 to 100 mΩ.
The current sinking scheme creates a fail safe situation. Should a control wire be cut or the controller fail, the lights will illuminate. [contradictory] Commonly, the 10 V control voltage is supplied through a resistor. Control is achieved (and current is sunk) by connecting a variable resistor between the control voltage terminal and ground.
The circuit would normally be over a telephone line or similar wiring. Control function currents come from the remote control console at the dispatch office end of a circuit. In two-way radio use, an idle circuit would normally have no current present. In two-way radio use, radio manufacturers use different currents for specific functions ...
The control signal must be coupled to the controlled circuit in a way which provides galvanic isolation between the two circuits. Many SSRs use optical coupling. The control voltage energizes an internal LED which illuminates and switches on a photo-sensitive diode (photo-voltaic); the diode current turns on a back-to-back thyristor ( TRIAC ...
Some sensors require external voltage or current source of excitation, These sensors are called active sensors. (E.g. a temperature sensor like a thermistor & RTD, a pressure sensor (piezo-resistive and capacitive), etc.). The stability and precision of the excitation signal directly relates to the sensor accuracy and stability.
The voltage follower is loaded by a constant (current sensing) resistor acting as a simple current-to-voltage converter connected in the feedback loop. The external load of this current source is connected somewhere in the path of the current supplying the current sensing resistor but out of the feedback loop.