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

lemming


What exactly are you trying to emulate or achieve and in what enviroment?


It probably doesn't matter what I'm trying to measure as long as I can output the industry standard 4-20ma.
There are a few scenarios which I would like to use it for, however the one at hand is for pressure measurement as previously stated, into the aforementioned VSD (or VFD as some call them).

Its starting to sound like the complexity of the circuit may be a bit beyond me, but I may play around with this setup:

The base of a transistor connected (through a resistor) to a PWM pin of the Arduino. A resistor in the circuit collector->load(vsd)->power-> emitter to measure the current flowing through the load. The voltage across this resistor is fed into an analogue input of the Arduino so that it can read the current flowing through the load and regulate the voltage going out the PWM pin and into the base of the transistor; thereby completing the feedback loop.

This may be a bit simplistic but I will see how it goes. Any views on improvement?


dhenry

The typical implementation is a current mirror + a current amplifier: the input voltage is used to generate a current into one leg of the mirror, and the current amplifier amplifies the mirrored current in another leg. It can be either a current source or a current sink (less common). Fairly simple.

A opamp - based solution would be a simple opamp buffered by a current amplifier, typically as a current sink.

winner10920

So you are talking about a three wire or two wire sensor? Id assume three to keep it simple?

retrolefty


So you are talking about a three wire or two wire sensor? Id assume three to keep it simple?


That is the thing. The OP stated he would like to have the arduino send a 4-20ma measurement signal to a PLC like system. The majority of PLC interface via two terminals to the remote 2 wire transmitter and the loop voltage is coming from one of the two 'field terminals' of the PLC. So my contention is that it will be a difficult interface circuit to design. If however the PLC is supplying just a passive 250 ohm ground based resistor to the 2 field wires then it would be somewhat easier but still not simple.

I've noticed that no one has supplied an example circuit for a arduino to PLC current loop interface, so that would back my opinion that is not as simple a task that a few have stated it is.

Lefty

be80be

Here look like a workable idea

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