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from the Instrumentation department...
4-20mA loop problem
PLCs and related questions. topic
Posted by D Bhat on 2 September, 2005 - 12:56 pm
I am getting 28 mA from a valve positioner instead of the normal 4-20 mA position feed back!?

The loop contains an analog input module (A-B Flex IE8 module), an isolator (24VDC powered) and a valve positioner. The valve positoner output is ok (4mA for close position & 20 mA for open position) when checked through a mA simulator. But when the positioner output is connected to the PLC input through the isolator, the mA is around 28. The distance between the positioner in the field & the PLC panel is less than 200 meters. Can somebody throw some light on the causes?


Posted by Alejandro Flores on 3 September, 2005 - 11:54 am
Had something similar. In the end I changed the 24VDC power source to regulated power source and problem went away. I was getting high readings as you were. Maybe this. If you have a different regulated power source I recommend giving it a try.

Alejandro


Posted by D. Ogle on 3 September, 2005 - 11:55 am
Are the negative side of your 24 vdc power supplies earth and/or chassis grounded? I've had floating power supply grounds cause more problems than you can shake a stick at, including excessive loop currents as you are experiencing. Good luck!


Posted by Curt Wuollet on 3 September, 2005 - 12:24 pm
Google constant current compliance and look at your loop voltages.

Regards
cww


Posted by abdullah al-mutairy on 3 September, 2005 - 11:54 am
hi , how are you
i think you should first isolate the analog input module to check the 4-20mA loop without the PLC to be sure the valve positioner is ok and also the loop cable is ok after that if everything is ok then you will know that the analog input module (A-B Flex IE8 module) is the problem
with my best regards

Eng.abdullah al-mutairy
http://www.q8engineering.tk


Posted by ilker subasi on 9 September, 2005 - 9:53 pm
Check the voltage between the shield of the 4-20 mA cable and the negative terminal of the PLC. If you measure more than a few mVs use 3 way isolated 4-20/4-20 converter between positioner and PLC.

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