self powered 4-20mA sensor on NI 9208

[Steffen01]

Hello,

I got a flow meter that generates 4-20mA signal by itself. I.e. does not need 24V connected. I want to connect it to a NI 9208. The NI 9208 takes 24V from an external supply and feeds it into the sensor, the sensor is powered by that and causes a 4-20mA current, see image below. So now I am worried if I connect 24V to that output of the flow meter, it will destroy that output. Yes I contacted the suppliers of the flow meter and of the NI 9208, no response.

So now, if I connect com to the + of the sensor output and - of the sensor output to AIO, then there should be a current flowing in that direction and I can measure it?

Any help appreciated.

Cheers,

Steffen

[AndyClegg]

There has to be a power supply somewhere in a 4-20mA current loop, and whilst that does not have to be connected to the Vsup and then passed to the device as per the 3-wire diagram you show, the power supply has to be in the loop as shown in the first figure in this:

https://www.ni.com/en-gb/innovations/white-papers/08/fundamentals--system-design--and-setup-for-the-...

Which is what is implied here - https://knowledge.ni.com/KnowledgeArticleDetails?id=kA00Z000000kIxISAU&l=en-GB

Get the sensor / power supply arrangement confirmed by the sensor manual or vendor, and see how it maps onto what is above.

[Steffen01 · Accepted solution]

the instrument supplies the power. I sorted it out. Connect positive to AIO and negative to COM, closes the loop. I tried with the lab supply first and then with the meter, works fine. I think there is a small resistance somewhere, so it did not short circuit.