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

lemming

I want to make my arduino mimic an industry standard 4-20ma sensor.

I understand that the load that it will drive (PLC, VSD, etc) are usually around 100 ohms.

Has anyone got any ideas on the circuit that would be required to do this?

winner10920

Are you talking about an analog or digital sensor? If its digital (on or off) than a simple npn transistor will do the job(or better yet optoisolator)
analog would be a little trickier but still doable

lemming

I'm talking analogue; e.g. pressure, water level, etc.

winner10920

You could use a pwm pin and a lowpass filter to drive a transistor, hardest part will be finding the right combination of base resistor and cap to give a sufficiently smooth yet responsive result, perhaps the best way would be a separate lowpass into an opamp as a buffer before the transistor to separate the base resistor from your lowpass filter
you may find you need to use a potentiometer to limit the base current as the hfe of a transistor can vary alot under various conditions and you may need to tune it to get your needed limits

retrolefty

#4
Nov 26, 2012, 02:34 am Last Edit: Nov 26, 2012, 02:46 am by retrolefty Reason: 1
Emulating a 4-20ma current loop transmitter sensor is a matter of implementing a programmable constant current output. This will cannot be done with a single transistor as it involves having and using a feedback reading of the actual loop current to set and control the loop current to the desired measurement output value desired based on the sensors primary measurement. There are special industrial ICs that help perform this function. Also the source of the 'loop voltage' used to generate the loop current will need to be defined as if at the transmitting end (emulating a 4 wire transmitter with active output) or from the receiving end (emulating a 2 wire transmitter, with passive output).

Lefty  

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