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Microcontroller Efficiently Converts Sensor Voltage to Current (.PDF Download)

Oct. 18, 2017
Microcontroller Efficiently Converts Sensor Voltage to Current (.PDF Download)

When taking measurements from analog sensors located at long distances, voltage drops and noise can affect precision of the readings. Therefore, a voltage-to-current (V/I) module is needed to transmit a current in order to obtain accurate readings. The reason is well-known: According to Kirchhoff’s current law (KCL), the sum of currents entering a node must be equal to the current leaving that node; thus, the sensor’s cable length is irrelevant when transmitting current and further, the low impedance of a current loop minimizes noise pickup in contrast to a voltage signal.

1. This circuit, based on a PIC microcontroller with a few added components, converts a sensor-based voltage into a 4- to 20-mA current loop. Shown is the circuit as originally drawn on a whiteboard.

The circuit in Figure 1 will deliver an output current from 4 to 20 mA for an input voltage (Vin) from 0 to 5 V.  PNP transistor Q1 (2N2907) is configured as a constant-current source;  its emitter current and resistor value (Re) are defined by the equation:

where Vcc = 5 V, and Vbe = 0.7 V.