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Author Topic: 0...20 or 4...20mA current loop sensors  (Read 638 times)
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Is it possible to connect these standard industrial sensors with a current output to the Arduino?

5V/20mA = 250 Ohms. Will putting a 250Ohm resistor between VCC and GND convert the 20mA signal to 5V? Do I need an external 24V (or whatever) power supply? How? Why? Whatever?! It's kind of confusing, anyone care to explain?
« Last Edit: September 14, 2013, 08:04:50 pm by SMArd » Logged

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Is it possible to connect these standard industrial sensors with a current output to the Arduino?


Here is a 4-20ma interface

MCI Electronics of Santiago de Chile make a 4x 4-20ma shield with on-board clock


* 4-20 ma.png (6.26 KB, 686x486 - viewed 137 times.)
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Is it possible to connect these standard industrial sensors with a current output to the Arduino?

5V/20mA = 250 Ohms. Will putting a 250Ohm resistor between VCC and GND convert the 20mA signal to 5V? Do I need an external 24V (or whatever) power supply? How? Why? Whatever?! It's kind of confusing, anyone care to explain?

The answers to your questions are yes, yes but the 250 ohm resistor doesn't go between VCC and GND, yes, any way that works, because, ?, and probably.
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The two wire 4-20ma sensor (typically called a transmitter) will require a DC 'loop power supply' typically anything  from +12 to +48 vdc. This external power supply must have it's negative terminal also wired to an arduino ground pin. Then a 250 ohm resistor is wired between ground and the negative path to the external sensor's negative terminal and it's positive terminal wires to the external power supply's positive terminal. Then a wire from the ungrounded end of the resistor to a arduino analog input pin. The results will be a voltage reading of 1-5vdc representing 0% to 100% of the sensor's measurement range.

 Such 2 wire 4-20ma transmitters (sensors) are used extensively in petro-chemical industrial plants where this interfacing method allows sensor wiring runs of up to 1,000 feet or more without effecting the accuracy of the measurement. However such sensors are generally build to industrial strength requirements and are not hobbyist friendly priced.

Lefty
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Hi!

I used the schematic in this thread http://forum.arduino.cc/index.php/topic,19613.0.html and it worked fine.
The only thing I added was a 5.1v zener diode for extra protection.

/Olof
« Last Edit: September 16, 2013, 11:09:15 am by olof_n » Logged

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