djedesign1 (Customer) asked a question.
Need Isolated Analog Inputs for BRX, P1000, P2000
I find many, many 4-20mA sensors are grounded on the negative side (-). Essentially they are sinking devices. To use a Productivity, I have to use a P3000 with PS-04ADS Analog Input or use Signal Isolators. WIth BRX, Signal Isolators are the only option. Going to a P3000 over P1000/P2000 or adding Isolators increase cost and the panel size. I have the same issue with Hazardous Environments wherein common Intrinsically Safe Zener Barriers ground the negative side of the sensors.
Usually very competitive ADC product becomes costly!
See attached file as example
Adobe PDF Analog Sensor
Specifically, which flow sensor is that shown in your attached diagram?
The diagram is generic to show the Grounding on the Negative side of the sensors which makes using the current Analog modules useless without isolation (except for P3-04ADS).
The McCrometer FC101 Flow Register is and example of what a local municipal water system has standardized on.
As mentioned, Zener Barriers for Intrinsically Safe Sensors in Hazardous Environments most often ground the Negative side too
From the FC101 manual, it looks like you shouldn't have any problems.
If I was using only 1 sensor per Analog input module that would work.
Again, the FC101 is only and example. There are many sensors that the Negative Side is grounded. I need isolators or an Analog Input module that has the Common on the positive side. Similar to using an open collector on a sinking Discrete Input but Analog.
Another example: I have a customer that has hundreds of Pepperl + Fuchs Zener Barriers for sensors - see attached datasheet
Why could you not use multiple FC101 (for example) sensors on one analog input card?
The new BRX -3 analogs are not fully isolated by channel, but do have 2 isolation groups per module.
Look closer at the schematic. The Sensors are grounded to their cases on the Negative side I actually need the Common on the Positive to work without using Isolators.
If your schematic is accurate, you don't actually need isolation with the -3 modules. The -3 inputs support a -20ma to +20ma range. You should be able to connect your 24vdc supply to the module common, and the sensor inputs to channels on the same common. You'll get negative current, but that can easily be scaled to engineering units.
Thanks, I missed the -20mA capability in the specs
"The Sensors are grounded to their cases on the Negative side I actually need the Common on the Positive to work without using Isolators."
If the FC101 is typical of wiring that you see, according to the manual, the shield wire is grounded to the case not the "sensor" (whatever this means). But as BobO pointed out, with bi-polar inputs you could wire it up "backwards" if you want and apply correction logic in the plc.