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I connect my arduino to a flowmeter sensor, with multimeter i got i=12miliamp v=4.2volts (without any resistor). As the sensor output is 4-20miliamps, and arduino limit is 5volts, how should I add a resistor so that I could capture all?

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"Two wire 4-20ma current loops" require an external DC power supply to provide the loop voltage, 24vdc is the most common supply voltage used but 12vdc will work for most 'transmitters' using that method. So wire the +12vdc to the sensor's positive lead and wire the sensor's negative lead to a 250 ohm resistor and the other end of the resistor wires to the 12 vdc negative terminal. Now add a wire from the 12vdc negative terminal to an arduino ground pin and finally add a wire from the non-grounded end of the 250 ohm resistor to an arduino analog input pin. The sensor will cause the arduino to see a 1 to 5vdc measurement value corresponding to a sensor 0% to 100% flow value.

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 For powering the arduino I am using the normal power adapter.
So my wire will come from the sensor output positive, negative and ground
Thank you for the reply.
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5V / 0.02A = 250Ω

> > >
current loop current : voltage across resistor, to Arduino Analog In pin
but
something makes me want to buffer that (LM358)
[Maybe it's those infrared hot dogs I ate?]
« Last Edit: March 24, 2013, 10:07:38 pm by Runaway Pancake » Logged

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Here are some basic stuff from National Instruments (English)

http://www.ni.com/white-paper/6940/en

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I think it is a 4 wire, and powered by transmitter ...so how? thanks
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