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Topic: 4-20mA controlling (Read 3837 times) previous topic - next topic

allanhurst

#15
Jul 14, 2018, 08:45 pm Last Edit: Jul 16, 2018, 10:34 am by allanhurst
Quote
Are you saying that 4mA offset with a fixed resistor is different from 4mA generated from PWM.
Both using the same Arduino VCC.
Please explain.
Leo..
No - just saying that the simple opamp circuit does not contribute to this error.

edit - because of the high PSRR of the opamp /edit

And we still don't know what error is tolerable in the OP's application - any suggestions ,Wosche?
 

Allan

Wosche

So sorry to the late response.

First of all, thanks to you all for the discussion and the tips :) Hoping to be able to contribute the same to others in the forum :)

I don't supply the Arduino via USB, just because i would have to adjust the output voltage to minimize the error it would make.

The idea with the simple circuit and adjusting my output to 1-5V seems to be the easiest way of getting what I want.
As far as I have seen the error would be up to 3% right? I have the opportunity to geht the restistors with quite an accurat value, although I think i would try to calibrate it

allanhurst

#17
Jul 16, 2018, 10:25 am Last Edit: Jul 16, 2018, 10:32 am by allanhurst
Hi Wosche..

yeah -that's the way I'd go. Simple and probably good enough. Cal for better accuracy.

If you decide to calibrate you don't need to buy more accurate resistors - the cal process deals with this.



ReverseEMF:

Quote
Allenhurst described how to add calibration trimmers.  But, I wouldn't do that on the emitter resistor -- it will heat up, slightly, which may skew the resistance, and thus, the output current.  Also, thermal expansion and contraction could nudge the wiper on the trimmer, knocking it out of calibration.
Maybe. Even at 20mA, the resistor only dissipates 40mW total and the pot at 25 ohms setting 10mW  - not much. But adjusting the input divider is also perfectly viable. There's a hundred ways to skin this cat.

Allan

Wosche

If I do the 4-20mA conversion, I will share it here.
It would be quite a helpful thread, because I didn't find anything similar helpful here in the forum :)

Thank to all :)

allanhurst

You're not the first to have problems here - but it's not that hard.

The other end - the 'receiver' is a bit more complex, but not too bad.

This stuff has been around since the 70's( or before?)  and is a standard throughout many industries.

Allan

ReverseEMF

#20
Jul 18, 2018, 05:48 pm Last Edit: Jul 18, 2018, 05:50 pm by ReverseEMF
If you decide to calibrate you don't need to buy more accurate resistors - the cal process deals with this.
...but, be sure to use stable resistors -- for both the fixed and variable.  I.e. ones whose resistance will vary little with temperature (i.e low enough Temperature Coefficient).  Also, pay attention to Minimum and Maximum Operating Temperature, if that will make a difference in your application.
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ReverseEMF

Maybe. Even at 20mA, the resistor only dissipates 40mW total and the pot at 25 ohms setting 10mW  - not much. But adjusting the input divider is also perfectly viable. There's a hundred ways to skin this cat.
True, it's not much power, and my concern is informed by my experience with thermistors.  Even small current through a thermistor can heat it and skew the reading.  But, maybe I'm overreacting ;) 
Still, better safe than sorry, and since the job can be done on the lower current side, doesn't that make more sense?  
"It's a big galaxy, Mr. Scott"

Please DON'T PM me regarding what should be part of the Public Conversation -- Let it ALL hang out!!
Unless, of course, it's to notify me of a mistake.

allanhurst

Whatever.

A typical metal film resistor has a tempco of <100 ppm/C .A cermet preset a bit worse. A typical NTC thermistor has 3-4%/C. Different ballgame.

If the OP's happy with 0.5% resolution it's not significant. Either way would do.

Just don't put a thermistor in by mistake!

Allan.

andyj992000

Sorry to resurrect an old post, but I have too have been using an AD694 for the 4-20mA output and an Adafruit MCP4725 DAC with a 0-5V output. The AD694 datasheet is a bit vague around using input spans other than the standard 0-2 and 0-10V. I've been scratching my head for a while but I think I can answer the OP's initial question.

Basically its as simple as treating the buffer amp as a non-inverting amp - in the 0-10V span mode pins 1 (FB) and 2 (sig-) are connected therefore the buffer amp is a simple voltage follower. If you wish to use a different input span connect a resistor between pins 1 & 2, and a resistor between pin 2 and ground. The ratio between the resistors governs the gain. A gain of 2 will enable 0-5V input to equate to 4-20mA output.

My first post on here so I hope this helps

JohnRob

I wouldn't worry about the resistors,  if the OP needs something more stable then they should look at the PWM output voltage.  Without going through a lot of numbers I believe it is the largest source of error in this implementation.




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