Interfacing 0-5VDC Analog Sensor with 0-20mA Analog Input Modem 3

[dubc4]

Hi All,

I have a SCADA I/O modem that has analog input for 0-20mA industrial style sensors. The datasheet talks about a current loop with externally supplied power... see page 27/28 here:
Link

I have a bunch of low cost 0-5VDC sensors here that I would like to interface with this modem. After doing some searching around I had considered trying to build a circuit using op amps etc to achieve this, but quite honestly I'm strapped for time. Instead, I started searching for some kind of module to do "just work" and do the conversion. I found this:

Link

Seems to fit the bill, 0-5V in, 24V supply (which I will have nearby), with 4-20mA out... but it is only a single Iout ... the modem requires a current "loop" .. two wires... maybe a stupid question but what am I missing here? It also has adjustment pots and no datasheet so that isn't very helpful.

Alternatively, I also found this four channel 4-20mA transmitter

[URL unfurl="true"]https://store.ncd.io/product/ad5696-4-channel-4-20ma-16-bit-current-loop-transmitter-with-iot-interface/[/url]

Seems like I can monitor my sensors with Arduino and use I2C with this board to output a 4-20mA signal. In the "drawings" tab of this product, it shows how the external power (ex:24VDC) would be put in the loop with the device being controlled. Maybe this is a better approach?

I'm hoping someone can provide me with some guidance on how to interface these things. I'm a mechanical engineer so please be nice

Thank you!

 

[xnuke]

Using the first proposed product, connect the Current output to IN2+ and GND to IN2-. Repeat for the other analog input. Note that the product you propose may not be listed by a testing lab, and therefore may not be accepted for installation by an inspector or testing lab doing field evaluation.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[dubc4]

Thanks for the quick reply!

Also, just for my own understanding... I've read that the purpose of the 4-20mA loop was to avoid the voltage drop / noise associated with trying to run a voltage signal over long distances. Considering that a sensor sending the 4-20mA signal may be very far away from the receiver, this would not require the sensor and the receiver to share the same power supply or GND reference, correct? In my application they can (and will) share the same 24VDC supply, but theoretically this is not a requirement?

Also for my own knowledge and education, the second solution that I proposed... would that have also worked? In the diagram from the product website it shows the transmitter connected to some kind of a pressure sensor / transducer. The external voltage supply has the + side of the supply connected to the sensor and the - side connected to the transmitter. The signal line goes from transmitter to sensor. Maybe a silly question but if I wanted to create a current "loop" isn't this connection backwards if the transmitter is trying to source current from the supply and push it through the sensor/receiver?

Thank you!

Edit: Side note, I am not monitoring anything critical to safety or sensitive in nature. Even a fairly wide margin of error with the values being monitored is acceptable. This is not a precision system.

 

[IRstuff]

The input circuit doesn't get affected by the wiring resistance, since it's measuring a current. That said, the proximal and distal ends should share a common ground reference to protect the circuitry from getting zapped during power-on transients and to ensure that common-mode doesn't saturate the circuitry.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[dubc4]

I see.

So from my understanding, ground "here" and ground "there" are different. How would a common ground be achieved with two devices a great distance apart in a circuit like this? In addition to the signal wire, would you have to run a long ground wire as well so that both devices share this same reference?

 

[danw2]

> this would not require the sensor and the receiver to share the same power supply or GND reference, correct?
Hopefully, yes.

The category of instrumentation that generates/regulates a 4-20mA loop current and uses an external power supply is a "2 wire loop powered" instrument. The instrument itself uses up to 3.5mA of the current to run its own electronics, and regulates the remaining loop current to be proportional to whatever its measurement task is (flow, pressure, temp, pH); hence the term 'loop powered' because there are not separate wires for a power supply. Industrial grade 2 wire loop powered circuits are not grounded, they are floating or isolated, although their housings have provision for a protective earth ground.

Devices that generate 0-20mA and 4-20 analog outputs from devices like a PLC, analyzer, controller or DCS are typically (not always) '4 wire' 4-20mA outputs, being powered from the devices power supply and share a common connection with that power supply. One can encounter ground loop problems when the ground potential at both the analog output end and the analog input end are not the same when connecting to 4 wire devices. That module you linked to is similar to 4-20mA isolators used to eliminate the ground loop problem encountered with 4-20mA signals. The key word is 'isolation'.

The word isolation in the statement "Current Module Isolation Transmitter Signal Converter Module 0-5V to 4-20mA" tells you that the circuits do not share a common ground.

>The external voltage supply has the + side of the supply connected to the sensor and the - side connected to the transmitter.

Sounds like typical loop current wiring.
There are always three components in a loop: DC power supply (p/s), sensor/transmitter, receiver (analog input).
DC p/s (+) to xmtr (+)
xmtr (-) to receiver AI (+)
receiver AI (-) to DC p/s (-)

It's just the way the electrons flow.

 

[IRstuff]

The signals don't have to share a ground, but unless you are using an isolation transformer, the grounding matters. Just imagine if your current loop powered up and was 1 kV higher than the current loop receiver chip, what do you think will happen? You get fried green base-emitter junctions; BTDT.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm