Control Valve Input 1

[learner08]

Hi!

Simple question for you guys but one which I am not too sure of and need you help....

For example a control valve with 4-20mA input, and a load resistance of 300ohm, how do we use the 300 ohm load resistance information...

It must be for selection of controllers but I am not too sure how to use this info.

Hope my question makes sense to you.

 

[ScottyUK]

Your loop has a 4-20mA current source which drives a defined current into an unknown resistance. The source has an upper limit on how much voltage it can deliver to drive the current around the loop, so at some point the loop resistance becomes too high for the current source to operate correctly with. Typically the limit is 20V or so, occasionally a bit higher. If your I/P converter has an effective resistance of 300[Ω] then at 20mA it will require at least 6V at the converter to drive it. From this result it is possible to size cables to ensure that total volt-drop around the loop from the source (PLC, DCS, etc) to the I/P converter is within the working range of the source.


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Sometimes I wake up Grumpy.
Other times I just let her sleep!

 

[JLSeagull]

The significance of the device was greater before digital controls when it was necessary to series devices in the 4-20 mA loop. Typically you only have a transmitter and the I/O module in a loop.

With analog devices you might have needed a 4-20 mA alarm module for the high alarm, another alarm module, perhaps a summer if you needed to add two signals, include an analog meter at the transmitter and an intrinsic safety barrier. Then the loop doesn't work.