A 4- to 20-mA loop needs no external power source


Posted on Jul 9, 2012

The simple circuit in Figure 1 uses a low-current-drain MAX4073H amplifier to sense the current flowing through a 4- to 20-mA loop. The circuit senses the current through a 1 resistor with a fixed gain of 100 and uses no battery or dc power supply. The low current drain of the amplifier (0.5 mA) enables the circuit to tap its power from the 4- to 20-mA loop to power the amplifier chip. Note that the current flowing in the amplifier`s power-supply Pin 3 (nominally 0.5 mA but may vary slightly) is not part of the sensing loop.






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