Loop-Powered Instrumentation Question....

Just a quick question for you guys. When you talk about "Loop-Powered" for instrumentation, will this term implies that the voltage require to power the instrument in question will be supply by the analog card that the instrument is connected too. The analog card I'm using is 1746-NIO4I.

No the 1746-NIO4I does not supply power to the loop. You must use an external power supply. See picture below.

Mickey thank you for your reply. Can you point me to where you got that picture. I was trying to print it and it wouldn't come up correctly. How can I determine the specs of the power supply I need. The instrument is a Vaisala HMT120.

See links below the the manuals on your Vaiala HMT120 and the manual for the AB cards Pg 3-14.

http://www.instrumart.com/products/...transmitters?gclid=CJyPrff0xLACFaUZQgodWlcvVQ

http://literature.rockwellautomation.com/idc/groups/literature/documents/um/1746-um005_-en-p.pdf

JoseM said:

Just a quick question for you guys. When you talk about "Loop-Powered" for instrumentation, will this term implies that the voltage require to power the instrument in question will be supply by the analog card that the instrument is connected too. The analog card I'm using is 1746-NIO4I.

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Loop powered usually means the device is passive, will not supply 4-20ma on its own.
There can be great confusion and many wiring mistakes on PLC analog input cards for these type of devices.

I have seen analog input cards that supply the loop power on one terminal, and the return from the instrument on an other terminal. These cards provide the path to the DC common to complete the circuit.

Other cards assume the loop power is provided externally from the card and the return from the instrument is connected to the card input and the card input common must be connected to the DC common to complete the circuit.

JoseM said:

How can I determine the specs of the power supply I need.

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The signal for a 2 wire loop powered instruments always maxes out at 20.0mA. Although the signal maxes out at 20.0mA, various fault mode/upscale signalling runs at greater than 20.0mA, from 21.5mA up to about 22.5mA

So, I usually figure 25mA per 2 wire device for a 24Vdc power supply.

25mA is 0.6W at 24Vdc. That's almost any 24V DC power supply.
The smaller commercial power supplies are typically 15W (650mA) or 30W (1,300mA).

It's a whole different situation for 3 and 4 wire devices. Some take a large amount of power and their specs need to be checked for current draw.

Thank you gentlemen, you guys being a great help.