Arduino Forum

Using Arduino => Sensors => Topic started by: SMArd on Sep 15, 2013, 01:49 am

Title: 0...20 or 4...20mA current loop sensors
Post by: SMArd on Sep 15, 2013, 01:49 am
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?
Title: Re: 0...20 or 4...20mA current loop sensors
Post by: Nick_Pyner on Sep 15, 2013, 03:26 am

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
Title: Re: 0...20 or 4...20mA current loop sensors
Post by: dc42 on Sep 15, 2013, 10:59 am

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.
Title: Re: 0...20 or 4...20mA current loop sensors
Post by: retrolefty on Sep 15, 2013, 04:27 pm
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
Title: Re: 0...20 or 4...20mA current loop sensors
Post by: olof_n on Sep 16, 2013, 06:01 pm
Hi!

I used the schematic in this thread http://forum.arduino.cc/index.php/topic,19613.0.html (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