4-20 mA loop querry (from non-electrical engineer)

[phamgiatoanbk]

Hi experts,

I'm sorry if this querry is too basic but I'm not an electrical engineer and will need some clarification regarding the 4-20 mA loop.
So, we have an Interface that is normally providing 24 VDC power to our sensor and get back 4-20 mA signal from this sensor thru a 2 wire loop.
Now we have a third party sensor, which has its own power supply and sending 4-20 mA signal. And we need to connect our interface with this third party sensor.
We are just wondering if this is feasible? Is the power supply from our interface and the third party sensor will interfere with each other, and interfere on the 4-20 mA signal?

Thanks for your clarfication.
Toan,

 

[waross]

The answer will be in your data sheets. The answer is:
"It depends."
If the third party sensor is providing power to the loop it may not be good.
Some devices need auxiliary power but do not power the loop. That would be OK.
Some devices may be configurable to either power the loop or not power the loop.
RTFI

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LiteYear]

Take a look at this thread:

http://www.eng-tips.com/viewthread.cfm?qid=369099

Note danw2's 1 Aug 14 7:51 post.

See how in the 2-wire scenario (your original configuration) the measuring device (denoted by a dotted line) provides the loop power. In the 4-wire scenario (your latter configuration) the source loop power is not obvious. As waross says, either your sensor is providing the power or it is expecting your "interface" to. You need to make sure only one of them is providing the loop power.

 

[danw2]

Sometimes a field device is a powered, active 4-20mA output (which is what yours sounds like).

Sometimes the receiver expects not only a signal, but it expects to provide power to a passive 4-20mA field device and there's no option for turning off the power at the receiver.

When that happens, there's a conflict in power sources that can be resolved with a stand alone "loop isolator".

The diagram below shows a loop isolator whose 4-20mA input is an active current signal (powered by the field device) on terminals 1 and 2

The isolator's 4-20mA output on terminals 5 and 6 is passive, requiring external loop power, either from a separate DC power supply or from the receiver that is sourcing DC power from its analog input.

The diagram has a misprint, the first channel is a current input (I, not U as printed); the 2nd channel is correctly labeled as a U voltage input; a DIP switch selects either input 1 or input 2.

The diagram is a Phoenix Contact isolator, but dozens of vendors provide functionally equivalent isolators.