4-20ma Control Loop

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Thread Starter

devgaur405

I am facing a problem with a 4-20ma control loop. There is a FCV (Flow control valve) which gives (sourcing) 4-20ma signal for its position feedback. This is to be wired to a analog input module of a PLC. The module has 8 no. of isolated channels each having 2 terminals +ve and -ve. There are 24V DC at these two terminals. Now there is a situation that both the transmitter and receiver has 24V DC. How to wire them to get correct current?
 
Use a 4-20 mA signal isolator/signal conditioner designed for just this purpose. Many manufacturers make such isolators. Some cost as little as USD75.00--some even less (though I suspect one for this application will cost a little more).

Write back to let us know what you choose and how it works!
 
How do you power the position transmitter? Position transmitter needs a separate voltage, which it takes from the DC voltage available in the AI channel of DCS/PLC. Typically a position transmitter needs 8-30VDC supply in order to power it. 4-20mA feedback is also transmitted through the same channel to the control system.
 
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The position transmitter is embedded in the flow control valve that is powered by 3 phase AC supply. The terminals where the transmitter sources 4-20ma signal has voltages around 20V in Open circuit condition and having galvanic isolation. On the other hand the Analog input channel of the PLC is also galvanically isolated 24V DC. Thus both transmitter and receiver has separate power.
 
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Alan Balcombe

You'll need an Output Loop Powered isolator.
Companies like Weidmuller will have suitable products.
 
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Thanks for your reply.

I have chosen another model of P&F that is KCD2-STC-Ex1 for our system.
this is same as yours but in this the output is configurable as source or sink type. Purchasing is in progress.

Thanks again to all of you.
 
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Hello friends!

The isolator module KCD2-STC-Ex1 is working fine for our system. Thank you all for your valuable suggestions.

Thanks a lot.
 
devgaur405,

Thanks very much for the feedback. "Feedback is the most important contribution!"(c) here at control.com. Because many people read these threads--both now, and in the future--feedback lets people know what works and what doesn't.

So, thanks again!
 
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