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 Ω 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 amplifiers power-supply Pin 3 (nominally 0.5 mA but may vary slightly) is not part of the sensing loop. It forms a negative offset in the measurement and is not a serious problem. To make this current nearly constant, a 3.3 V zener diode and an LED in series with the sensing resistor form a voltage drop of 4 to 4.5 V across pins 2 and 3 of the amplifier chip. The amplifier works well over 3 to 28 V, so this 4 to 4.5 V power-supply range presents no problems.
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| Figure 1. | A current-sensing circuit derives its power from the 4- to 20-mA current loop. | |
The output of the amplifier is linear from 350 to 1950 mV for 4 to 20 mA through the loop. The measurement meter at the output must not draw more than 5 µA from the output for 1% full-scale measurement accuracy. The LED shows visual intensity variation for changing current in the loop. Its main purpose is to raise the voltage by approximately 1 V across the sense resistor with respect to the power-supply return Pin 2 of the amplifier. This increased voltage gives better common-mode performance to the amplifier against common-mode noise in the sensing resistor and prevents the amplifier from saturating near the power-supply rails.
