Current Loop


Thread Starter



i designed and wired a 2-wire current loop where a 4-20mA signal was to be transmitted to a Yokogawa recorder and a Schneider mixed module plc 170 AMM 090 00. My challenge was with the plc module which could not pick up current signal within the loop but the recorder was fine. It is a single loop, and i ended up wiring across voltage terminals with a shunt 250 ohm and the module accepted the 1-5 volt signal.

My question is how come the current could not be read by the plc?
Like any analog situation, it could be anything under the sun. On any given day, I think I've seen it all and someone manages to do it wrong in yet a different fashion. Since my crystal ball is foggy, I'll just list a few of the things that I find in these circumstances

Wiring issue:
polarity, connection to wrong terminals, wrong card, missing shunt resistor, wrong value resistor, AI loop power bucking the loop power supply

PLC mechanics:
jumpers, DIP switches, rotary switches, terminal block not on all the way

PLC configuration:
type of input (voltage vs current), channel enable, channel range, current loop into an RTD/T/C/frequency card, PLC gets the signal OK, but the HMI isn't reading the PLC at all, or properly.

Common mode issue:
single ended PLC AI can't handle the common mode

Did you test the PLC AI with a current calibrator/source to see what it reads?
I expect you were trying to connect the recorder in series with the PLC and both have a ground reference on the current input.
The current will be grounded by the first instrument.

Most instruments have a 1-5 Volt input with a 250 Ohm in parallel to make it accept 4-20mA, run the 4-20 into the first instrument and connect the other in parallel as a 1-5 Volt input.