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Old December 7th, 2017, 03:41 AM   #1
graygnads
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Off-scale 4-20mA reading when connected to PLC

Hi,
I'm a noob after some help if possible.
I have been using a Click PLC with a 0-20mA analog input module, quite happily, for some time now. I have been using 2 and 3 wire transmitters with no issue. The problem I am now experiencing is with a 4 wire transmitter that is powered from a supply separate to the PLC. I measure a steady 4mA, in the transmitter loop output, with a multi-meter but when I connect the loop output to the PLC (between channel input and 0V) the PLC reads over-scale i.e. status monitor shows maxed out at 20mA.
This has me confused - does anyone have any advice they can offer?
Is this anything to do with input impedance and could I need an additional resistor?
I would be greatly appreciative of any assistance.
Cheers,
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Old December 7th, 2017, 04:03 AM   #2
mbartoli
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Not familiar with the Click, but does it have two or three terminals on the input card? If only two, it may be limited to sourcing the loop power; check your documentation. If that is the case, you may have to use a loop isolator between the transmitter and the input. If there are three terminals, usually one is signal input, one is +DC supply and one is signal ground. If that is the case, use the Signal input and ground terminals. Since you are getting an over-scale indication, the field-powered loop is adding to the input card loop power and driving it off-scale high. Eliminating the two loop powers will correct this.
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Old December 7th, 2017, 05:38 AM   #3
BryanG
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Common problem with 0-20mA or 4-20mA devices that have non-isolated inputs. You either need to use a loop isolator as already suggested or you need to be able to tie the 0V of both supplies together.
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Old December 7th, 2017, 07:40 AM   #4
rpoet
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Since you are already using AutomationDirect products:

https://www.automationdirect.com/adc...er_Input/FC-33

Phoenix Contact and many other manufacturers make ma-loop isolaters.

-rpoet
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Old December 7th, 2017, 09:42 AM   #5
shooter
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make sure the connection is correct, as it can be that the mA is controlled by the returning signal.
If this is gnded in two places, it will not be controlled.
Is a resistor present to measure the voltage?
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