A 4- to 20-mA loop needs no external power source
Edited by Bill Travis and Anne Watson Swager
Shyam Tiwari, Sensors Private Ltd, Gwalior, India -- EDN, September 13, 2001
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 1Ω 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. It forms a negative offset in the measurement and is not a serious problem. To make this current nearly constant, a 3.3V zener diode and an LED in series with the sensing resistor form a voltage drop of 4 to 4.5V across pins 2 and 3 of the amplifier chip. The amplifier works well over 3 to 28V, so this 4 to 4.5V power-supply range presents no problems.
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 1V 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.
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