Isolation of 4-20ma inputs on non-isolated input card


Thread Starter


Working with 504 SLC non-isolated analog input
cards. Have several inputs 4-20ma that are "current-sourcing" What is the correct setup for conditioning these inputs. Looking at signal isolators for these field inputs, but there are several types. Help!

alan whitfield

Hi, alan whitfield, contract plc and scada engineer responding.

I understand your problem. have experienced it myself. It can be fixed. depends on which input card you are using. if using 1746ni8, you can mix loop powered and differential (current sourcing) inputs if you connect loop powered on the first group of four and differential on the second group of four. It becomes a problem if you have five or more of one type and less than four of the other.

you can call me on 01779 664 677 ext 2181 (Bristol)if you wish to talk about it. It is easy to convert loop powered to differential using a loop powered isolator, thus making all the inputs differential. it is not so easy to convert differential to to loop powered.

Richard O'Donnell

By "Current-Sourcing" I am assuming you have 2-wire transmitters (aka loop powered): The Input to the actual i/o card is just a shunt resistor.

(1) you are "in luck" if the above is correct since your total loop impedance is really limited by your loop power supply (providing some options)

(1.1) The simplest solution is a loop-powered isolator 4/20 in and 4/20 out ... just be certain that this isolator can drive 4/20 ma through the input resistor on the I/O card (you can measure this - should be 500 ohms or less).

I was an Engineer for AGM Electronics (Tucson, AZ). They used to offer loop powered isolators that were very economical and carried a seven(7) year warranty. Their Telephone No. is (520) 722-1000 /Model I am thinking of is a 4000-24 (keep in mind that was over 10 years

Ps. I will just mention briefly ... I can understand the need for last-minute changes: My last plant start-up I was having ground loop problems and was tempted to throw some isolators in the input loops ... after a little circuit tracing, I discovered that the Electrical Contractor had grounded all the (-) sides of the loop supplies... also, some of the electricians assumed black was (hot/+) for DC ... !!!!

The loop supply should have a xfrmr in it and therefore, be isolated ... make sure your Shield connections are not terminated on both ends (only a suggestion)
You might want to look at Spectrum Controls input modules. Spectrum is an A-B partner that makes 1746 series input modules for the SLC line and includes an isolated analog input module. This would eliminate the need for separate isolators. I'm not affiliated with Spectrum. I've simply designed systems with their modules which have performed well.

Andrew Cullen

I've had the same problem when both the transmitter and i/o card need to be the current source.

I've used Pheonix Contact Configurable Passive Isolator p/n MCR-CLP-UI-I-4.

as well as Action Instruments Q501, 1or2 channel, 4-20 mA.

both are din rail mounted, and configurable for other input ranges.

Chimalkar, Sumeet

I had a similar problem while connecting a 4-20mA output from a third party PLC to a 4-20mA input channel on a Flex I/O module. I solved it by
installing MTL isolators in the circuit.

Have Fun.

Sumeet Chimalkar
Design Engineer-Instruments & Control
Jacobs H&G Pvt. Limited
Jacobs House, Kondivita, Andheri (E), Mumbai, India - 400059
Phone: (9122) 8208075 EXT 318 Facsimile: (9122) 8208295