The Active-to-Active Conflict
In this loop, the field flow transmitter is powered from an external source, so it actively drives its 4-20 mA output. The motor controller's analog input is also powered from its controller, meaning that input is designed to source loop power for a passive (two-wire) device. Connecting an active transmitter output directly to a powered input puts two current-driving stages in series with no defined reference, so the measurement will be wrong or the loop will not regulate at all.
The fix is a signal isolator (often called a barrier in this context) between the transmitter and the controller. The isolator accepts the active 4-20 mA signal on its input and presents a passive input to the motor controller, so the controller's loop power drives the isolated side while the isolator repeats the measured current.
Isolator Options
Two Pepperl+Fuchs units address this exact configuration:
| Model | Type | Auxiliary Power | When to Use |
|---|---|---|---|
| P&F KFD2-CD-Ex1.32 | Powered signal isolator/barrier | Requires a 24 VDC supply | When panel 24 VDC is available; Ex-type barrier suits hazardous-area installations |
| P&F KFD0-CC-1 | Loop-powered signal converter | None required | When no spare 24 VDC supply exists and the isolator can be powered from the loop itself |
The two units solve the same wiring problem; the deciding factor is whether you can dedicate a 24 VDC supply to the barrier.
Selection and Wiring Decision Path
One ambiguity in the original question is the word "barrier": if the transmitter sits in a hazardous (classified) area, you need a barrier rated for that area, not a plain isolator. If the installation is safe-area only, either isolator above is sufficient on signal grounds alone. Decide in this order:
- Confirm whether the field transmitter is in a hazardous area. If yes, use the Ex-rated barrier (KFD2-CD-Ex1.32) and provide its 24 VDC supply.
- If safe-area and 24 VDC is available at the panel, use the powered isolator.
- If no 24 VDC is available, use the loop-powered KFD0-CC-1.
- Wire the active transmitter output to the isolator input, and wire the isolator output to the motor controller input so the controller supplies loop power to the isolated side.
Verify by forcing 4 mA and 20 mA at the transmitter (or driving the process to known endpoints) and confirming the controller reads the matching values with no offset or saturation.
FAQ
Why can't I connect a powered 4-20 mA transmitter directly to a powered analog input?
Both devices actively drive current, so the loop has two sources in series and no defined reference. A signal isolator breaks this by accepting the active signal and presenting a passive input to the controller.
What isolator do I use between an active transmitter and a powered controller input?
Use a Pepperl+Fuchs KFD2-CD-Ex1.32 if a 24 VDC supply is available, or the loop-powered KFD0-CC-1 if you have no auxiliary supply.
Does the KFD0-CC-1 need a 24 VDC power supply?
No. The KFD0-CC-1 is a loop-powered signal converter and requires no separate 24 VDC supply, unlike the KFD2-CD-Ex1.32, which does require one.
