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4-20mA current loop with common ground

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jelezarov

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current_loop.png

I have this schematic for a 4-20mA current loop transmitter. My understanding is that the op-amp in my circuit compares its feedback from R4 with its input and adjusts its output so that the input voltage is equal to the voltage at R4. Then the Ohm's law takes care that exactly 4-20mA would flow when the input is 1-5V.

I need to transform the circuit in such way, so that the loop ("4-20MA_OUT" in my circuit) shares the ground (V-) instead of (V+), but can't think of a way for the described comparison to work because one does not know the resistance of the loop.
 

Hi,

do a forum search. There are several threads discussing this.

Klaus
 

I did already but maybe I am searching with the wrong criteria.
 

Thanks!

I searched with everything but the simplest criteria...
 

You can sense the current on the high side using a differential amp. Values of R7/8/9/10 must be well matched. The 4 mA offset resistor can be a trim-pot for better adjustment.

The total loop resistance can't be more than roughly 1.1k (including the sense resistor R1) using a 24 V supply.
 

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Hi E-design,
Just from curiosity but how much is the phase margin of the feedback system?
 

E-design said:
You can sense the current on the high side using a differential amp. Values of R7/8/9/10 must be well matched. The 4 mA offset resistor can be a trim-pot for better adjustment.

The total loop resistance can't be more than roughly 1.1k (including the sense resistor R1) using a 24 V supply.
Click to expand...

Thanks!

A couple of questions:
- The 5V reference means something like this one ?
- Does R1 control the current upper limit? I am trying to understand the circuit in general.

As a side one - what is the simulation software you are using - it looks interesting
 

frankrose said:
Hi E-design,
Just from curiosity but how much is the phase margin of the feedback system?
Click to expand...

I can't give you a number, since I have not checked it. I did check the squarewave (100 Hz) response because of the zener action in the circuit and I could just observe very low level damped-oscilations on the current waveform. I assume it must be better than 45 deg.

Placing a 220 pF capacitor across R2 eliminated most of this.

There should not be any problem with slow control loop inputs I think.

- - - Updated - - -

jelezarov said:
Thanks!

A couple of questions:
- The 5V reference means something like this one ?
- Does R1 control the current upper limit? I am trying to understand the circuit in general.

As a side one - what is the simulation software you are using - it looks interesting
Click to expand...

Any 5 V reference that is stable can be used. It does not need to provide any significant current.

The value of R1 was slightly reduced from your 250 ohm to compensate for the loading over it by the diff amp.

The soft ware is TINA 11 by designsoft. There is a student free version as well as a Texas Instruments version available on the web. I am not sure what are the limitation of the free versions. We use the full industrial version.
 

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I implemented this circuit in Tina and I got -25° for phase margin. I added the 220pF capacitor what you mentioned and it increased to -17°. Still oscillating.
Tina can use only gear and euler integration method, these supress oscillation under transient simulation. I am not sure I did the simulation well, I probably have got different model for LM358, but I am still suspicious that the 3 stage feedback system (2 OPAmp inverter + 1 PNP source follower) can be unstable even in typical condition without layout parasitics.
 
Last edited:

frankrose said:
I implemented this circuit in Tina and I got -25° for phase margin. I added the 220pF capacitor what you mentioned and it increased to -17°. Still oscillating.
Tina can use only gear and euler integration method, these supress oscillation under transient simulation. I am not sure I did the simulation well, I probably have got different model for LM358, but I am still suspicious that the 3 stage feedback system (2 OPAmp inverter + 1 PNP source follower) can be unstable even in typical condition without layout parasitics.
Click to expand...

Which model did you use? I actually made a similar circuit with opto-isolated coupling in the past (it is posted somewhere in a thread), which I expected to have stability problems, but performed very well up to a few kHz. The only real proof is building it up and test.
 

"LM358/MC" is the name what I have. And I have to disagree, if something doesn't work in simulation well we shouldn't believe it works after building. Reality usually worse than what we expect.
 

frankrose said:
" And I have to disagree, if something doesn't work in simulation well we shouldn't believe it works after building. Reality usually worse than what we expect.
Click to expand...

I never said that. I agree if there are obvious problems with the simulation results, then there is a good chance that you will have problems. If you have simulation results that seemed good with some models and show problems with others, then you build the circuit and verify. Simulations are only as good as the models. I am sure you have seen cases where simulations show no indication of problems, but the real circuit just doesn't perform on the bench.

Can you show your simulation results, and input test conditions? I would like to replicate it.
 

@E-design :
One more thing - as I am reading about ground connected load, it is almost always that the supply voltage has to be precise and stable. This is not the case here, because of the zener, right?
 

frankrose said:
"LM358/MC" is the name what I have.
Click to expand...

I looked at this Motorola model, and I could get it to go unstable under certain conditions. I updated the diagram with values that seems to give good margins with all five different models I tested using minimum and maximum loop resistances.

The circuit does not need to have a large bandwidth, so C3 takes care of that. R2 was increased and network C2/R12 was added to improve the phase margin.

- - - Updated - - -

jelezarov said:
@E-design :
One more thing - as I am reading about ground connected load, it is almost always that the supply voltage has to be precise and stable. This is not the case here, because of the zener, right?
Click to expand...

Small variations will not make a difference. Obviously if the supply dips too low, you won't get the required loop current with high loop resistances. Look at the plot below. I varied the supply between 22-26 V, and the loop current is still under control with a 820 ohm lower loop resistance. The zener is there just in case you use an opamp that can't swing high enough (near supply rail) on its output to turn off the transistor.
 

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Hi,

In a couple of hours I can show a stable - one Opamp solution.

Klaus
 

The basic design fault is to use the LM358 open loop. Why not use the most simple solution, a little bit of PI feedback around U1? I would target to 100 Hz up to 1 kHz loop bandwidth.
 

Yes, I think the instability comes from this design not the OPAmp itself. E-design, your last figure doesn't show the loop gain, the DC gain of the open-loop system should be around 100dB for this system. So the phase margin is not that what you show, and probably your simulation is wrong.
1 Opamp is the best, most straightforward solution, I also recommend that.
 

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