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[solved] 4-20mA floating output (line) - can it be done w/o actually galvanic isolation ?

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Grossel

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At a facility, I came over some sensory equipment (ref. device A) that feed 4-20mA to a HMI device (ref. device B). During fault location, I couldn't miss noticing that a 4-20mA output from device A, two terminal feeding into a wire with 2 strands to another room - that measuring voltage between ground to any of those two strands would lead to a zero volt reading, whilst voltage on terminals are according to actual readings (there was never an actually electrical fault anyway).

This tells me that the 4-20mA loop is somehow isolated from ground, maybe due to use of optical isolators (the equipment is like 20 years old and schematic doesn't exist in public).

This leads me to the following question. Let say I've ±12V and ground (rails not isolated from ground) to start with. How can I construct an op-amp based circuit that have 2 channel output - constant current - but in addition to a regular constant-current output, when there is a load (presumably resistive) on the output, each wire should act as isolated from ground (not mention if the output are lifted close to rail voltage - i.e. say it never get lifted beyond ±10V ?

I assume in real world, this is acomplished by using smtp combined with optical isolators for feedback, but I'm curious if this can be accomplished without smtp/optocouplers ?

I've looked at some of the circuits on this page for ideas, but I can't see that the outputs are true isolated from ground.
 
Not sure what you are asking, but normally the sensor at the end of a 4-20mA current loop is isolated (not grounded), however the receiver that both generates the power for the sensor and detects the signal over the two wires does not need to be isolated from ground.
The purpose is to prevent any ground loops between the sensor and the receiver that could corrupt the signal.
In a factory setting there can be a significant difference between the ground voltage at the sensor end and that at the receiver end.

None of that needs optical isolation (the 4-20mA loop was invented in the 50's, well before the opto isolator).
 
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Not sure what you are asking, but normally the sensor at the end of a 4-20mA current loop is isolated (not grounded), however the receiver that both generates the power for the sensor and detects the signal over the two wires does not need to be isolated from ground.
The purpose is to prevent any ground loops between the sensor and the receiver that could corrupt the signal.
In a factory setting there can be a significant difference between the ground voltage at the sensor end and that at the receiver end.

None of that needs optical isolation (the 4-20mA loop was invented in the 50's, well before the opto isolator).
Not strictly true - you had opto-isolaters back then as well, using neon bulbs and LDR's :D
 
Not sure what you are asking
Ok - I'll try another approach.

There is a black box (many of my teacher over the years I went to school like to use that term) that has the following connections:
  • ±12V input, 0V grounded so you'll measure 12V from ground.
  • Input - Let's just say for the example that is variable voltage measured from ground (form some sencor).
  • Output: A pair of terminals that provide 4-20mA based on Input voltage/current/resistance (mud hole trap).

The following rules must applies
  • GND must be related to Vcc/Ncc - as if two battery cells in series with center point at gnd.
  • No opto coupler nor smtp allowed in order to gain true isolation between ground and output.
  • The 4-20mA output pairs must behave in such a way that when the current loop is closed (i.e. a resistive load in receiving end of wire pair) - the wire must operate as if it was isolated from ground. Only exeption is if the voltage get shifted towards Vcc or Ncc, and also some capacitance to ground are accepted.

My question is simply this - given the basic description above, is it possible to construct such an output stage, you think? I've thinking on something involving a voltage follower from one of the output terminals, but I cannot assemble enough peace in my mind to get the necessary kick in order to come up with a working solution myself.

This is either because I'm not given it enough time for a idea to mature - the issue is I haven't found a place to start, but I also have hard time grasping the idea that I'm the only one ever asking for this. Or maybe I'd find peace if somebody can confirm that such a thing cannot exist, but then I also need to know why.

Also I put in a mud hole trap warning, because whilst important, it's not important in order to cave out a plan on how a rudimentary output stage should look like. I.e. I don't care about about nifty details and amplification tuning - I'm only interested in ideas of output stage.
 
Sounds like what you want is a floating current source where you can
ground or leave floating the return leg of the Rsense at receiver end ?

Or just do a galvanic isolated loop, where source side is grounded but
receive side uses a differential receiver to detect V or Rsense. Eg. neither
source nor return wire is grounded, always isolated. Note its not perfect
galvanic isolation but close, and the diff amp can tolerate a wide, but not
infinite, CM at receive side.


Regards, Dana.
 
Sounds like what you want is a floating current source where you can
ground or leave floating the return leg of the Rsense at receiver end ?
Yes, you put the wording right.
 
It could be that the transmitter circuitry (the source of the 4-20mA current) is just not tied to ground.

I don't know of any requirement that low voltage circuitry must be grounded.
 
It could be that the transmitter circuitry (the source of the 4-20mA current) is just not tied to ground.
Yes - I'm aware that is probably one of the actual explanations in real life. But for the question it's irrelevant, because the question is if it is possible to construct an output stage that act as if it was isolated from ground, despite it's power supply are not isolated from ground and no smps or opto coupler is being used.
 
Possible solutions discussed here :


One can imagine a V controlled current source using an optical or wireless link
to control it (to achieve the isolation).

Using such a link control something like this (photo resistor to control the current) :

Current_Regulator_2C2D7.gif



Regards, Dana.
 
Note if you used fiber or wireless link would be relatively easy
to add a cal routine to the link in which host side sweeps the
current source and measures accuracy of current issued, and
use that as a table on host side to correct current non linearity
and errors when it issues current.

Regards, Dana.
 
Ah, thanks for the paper. Omg - turns out it's dead simple to achieve the goal after all, only thing one need is dual set/mirrored current source (sink and drain) and tune those to try keep voltage (somewhere) around middle. Why didn't I just think about that in the first place? :eek:

Thank you guys :D
 
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