Circuit Review

Wow missed that one. Completing the loop would be nice....

Wow missed that one. Completing the loop would be nice....

Tom,

Again, I am not sure that you can count on the second leg of the 4-20ma loop to be at ground potential.  You might end up shorting out a driver or introducing ground noise into the circuit.

Here is a link to some information on the risks of assuming a ground potential on the low side of the loop.

https://www.dataq.com/blog/data-acquisition/4-20-ma-current-loop-measurements/

Also, not all 4-20ma loop devices are actually supplying current.  There are passive devices that act as a variable resister, and the measuring circuit actually supplies the drive.

On the outside simple.  But potentially quite complex.  If you know the devices that you will be receiving from, then you can make some simplifications to the measuring circuit.

Gene

Tom,

Here is an even better link.  This shows how to differential sense the loop current.

https://www.sensorsmag.com/components/implementing-a-4-ma-to-20-ma-sensor-interface

Good luck!

Gene

Another useful source I found was instrumentation amplifier data sheets - the AMP04 from Analog Devices has a suggested 4-20mA receiver circuit that also shows some TVS protection diodes being used to protect the inputs, which could be useful in an industrial application?

A

Great info but I have a problem with the shunt resistors. My goal is to create something general to be used with most 4-20ma sensors.

Is this possible?

I checked out the AMP04 like the circuit but the -15 +15VDC supply becomes an issue.

I also found the RCV402 but there are expensive (old chip too). Anyone know of something newer and similar?

I though making a general 4-20 industrial loop would be easy…… well. Simple concept….. so many options….. lol

We sometimes use the Libelium 4-20ma device to connect to theses Siemens sensors. For sure the libelium board is not the complex.

https://www.google.com/search?q=libelium+4-20&client=firefox-b-ab&source=lnms&tbm=isch&sa=X&ved=0ahUKEwigmbOI2sHbAhVDlJAKHRdBCLQQ_AUICigB&biw=1093&bih=482#imgrc=cbzOIBxvMb2TTM:

Sensor:

https://w3.siemens.com/mcms/sensor-systems/en/process-instrumentation/level-measurement-with-level-measuring-instruments/continuous/ultrasonic/transducers/pages/echomax-xps.aspx

Control unit

  https://w3.siemens.com/mcms/sensor-systems/en/process-instrumentation/level-measurement-with-level-measuring-instruments/continuous/ultrasonic/controllers/Pages/MultiRanger-100-200.aspx

Tom,

In the article https://www.element14.com/community/external-link.jspa?url=https%3A%2F%2Fwww.sensorsmag.com%2Fcomponents%2Fimplementing-… , the circuit only uses a general purpose op-amp to perform the differential reading and the single ended power rail should work nicely with Arduino.  The shunt resistors are not that bad, it is just a matter of choosing as low a value as possible to get the voltage range you would like.  Going too high on the shunt resistor runs a risk of existing the drive or compliance voltage on the xmitter side (10K @ 20ma = 200V, which is most likely greater that the drive voltage, 5-28V).

Looking at the Libelium devices, they are using jumpers on the receiver to manage the possibility that the xmitter is un-powered, so you could likely do the same.  I have used any thing from 100Ω to 200Ω (2 to 4V max across the resistor).

Any generalize design will have a lot of variables when you are working with such an open protocol as 4-20mA loops.

Good luck!

Gene