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Topic: 4-20ma output from Arduino (Read 8 times) previous topic - next topic

be80be

I no how it works Have you ever used one I worked where this was used to control mixing valves  read fluid temperature sensors

Quote
Analog current loops are used where a device must be monitored or controlled remotely over a pair of conductors. Only one current level can be present at any time.

Given its analog nature, current loops are easier to understand and debug than more complicated digital fieldbuses, requiring only a handheld digital multimeter in most situations. Using fieldbuses and solving related problems usually requires much more education and understanding than required by simple current loop systems.

Additional digital communication to the device can be added to current loop using HART Protocol. Digital process buses such as FOUNDATION Fieldbus and Profibus may replace analog current loops.


retrolefty


I no how it works Have you ever used one I worked where this was used to control mixing valves  read fluid temperature sensors

Quote
Analog current loops are used where a device must be monitored or controlled remotely over a pair of conductors. Only one current level can be present at any time.

Given its analog nature, current loops are easier to understand and debug than more complicated digital fieldbuses, requiring only a handheld digital multimeter in most situations. Using fieldbuses and solving related problems usually requires much more education and understanding than required by simple current loop systems.

Additional digital communication to the device can be added to current loop using HART Protocol. Digital process buses such as FOUNDATION Fieldbus and Profibus may replace analog current loops.




Yes I worked in a oil refinery for 29 years as an instrumentation analyst. We had thousands and thousands of current loops. Most used the true analog 4.-20ma current loops. Some use the Honeywell 'smart meters' in DE mode where they did drive the current loop in true digital serial data mode rather then analog mode. And then there were the Rosemont transmitters (that developed the original HART digital protocol) that could use the current loop as analog but at the same time superimpose a tiny FSK ac signal onto the loop so that digital communications could be had between the remote field transmitting device and the control house controller device while still sending the analog measurement value on the same 4-20ma current loop.

So what was your point about an old IBM PC that could send serial data with 4-20ma current loops. Did you really mean that is the same method this thread has been dealing with?

Lefty

be80be

This is my point the Op said he wanted to make a transmitter with the arduino  for
"pressure, water level"  I was just pointing out that there is more that could be done then read the steps on a resistor purely analogue.   

dhenry

A digital pot will work as well, in a current source programmed by a resistor.

Fairly easy to implement, as the two other approaches indicated earlier.

This thing is well understood and time-tested, even for a junior engineer.

jbiasutti

If you know the resistance of the receiver at the other end then current output is the same as voltage output.You said that the system had to drive 100ohm.

so use 4ma = 0.4V, 20mA = 2V.

Use a 100ohm resistor hooked between the arduino PWM output and put a 100microfarad capacitor from the output of the resistor to ground.

Hook the output line up to one of the analog inputs of the Arduino and use the PWM output to drive the voltage up and down. You could use a control algorithm to trim the voltage.

If you want a better circuit you could even put a second resistor (make them both 50ohm) after the first and hook the second analog input after this, so that you could directly measure the current via the voltage drop across the resistor, this would also make the circuit a bit safer as the capacitor would not be able to discharge instantly if you short circuit the output wires.

The only difficulty is that you will then find yourself using 1 PWM output and 2 analog inputs per output to the PLC, so you can only have 2 PLC outputs from the Arduino.

If you need more outputs you can of course by a DAC on a chip simiar to the AD5412. These appear to be able to be daisy chained similar to shift registers so you can have unlimited 4-20mA outputs using serial outputs.

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