Our valve position actuator had a pt100 4-20mA transmitter. now the new actuator's 4-20mA feed back unit is not reading on our circuit.
makers advised to fix a passive isolator between new actuator and the existing system. but the reading is valve close shows 4 mA and open shows 5.05 mA instead of 20mA.

The path is follows:

From 24vdc power supply +ve to the +ve of 4-20mA transmitter and -ve of the transmitter to +ve of Analogue display (shows 0-100pct valve open position) and its -ve to a diode/resistor module (+ve) and the -ve of diode is connected to a AI Analogue input card (Wago 750-468 (4IANA1). The 24v power supply -ve is connected to AI input cards common negative.

where exactly the passive isolator can fix so as to get the reading on input card? If only the analogue meter is connected to only to the new 4-20mA transmitter then the reading is correct valve close 4mA and open 20 mA and meter shows 0 to 100pct.

Please advise.

 

I suspect your problem is a ground loop caused by the powered digital display. If you can substitute a 'loop powered' digital display for a powered display, you might get a working circuit without an isolator, because a loop powered digital display is 'floating'.

To use an isolator, your present circuit which is one large series loop becomes 2 separate circuits where both circuits are connected to the isolator, but one circuit is on the input side and the other is on the output side.

What comes in on the isolator's input is reproduced on the isolator's output, but without an electrical connection between the input and output.

One circuit consists of the DC power supply, transmitter, display and isolator input.

The other circuit has the isolator output, diode/resistor, analog input.

 

Thanks David.

There are 8 Actuators in the plant and it divided into two set on two 4 channel AI input card (Wago 750-468). and this wago card is 0-10v signal input type single ended. and Negative is common for all 8 channel. (which is the negative from 24v power supply).

Now I have to change only one actuator and the replacement is new type. other actuators produce 4-20mA signal from a pt-100 transmitter and the power supply is common for all these 8 actuator transmitters. Then it is routed to a diode/resistor module where the voltage drop on resistor is the input of wago AI card. (I think) as that card need 0-10v signal, as valve position.

The new actuator 4-20mA signal transmitter is a EPT-C card and using a different 24v power supply. This negative and existing system power supply negative have a voltage -potential diff, so obviously i cannot link both negative together. I have to convert the new EPT-C card out put to 0-10 volt and able to bridge to the existing system...isolating both negatives? If need I can mail a circuit diagram of existing system.

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