Problems analogue signal 5

[flinana]

Hi,
I have the following problem. I am suppose to receive a 4-20 mA signal from a system called TELEFLO DIVA supplied by Air Liquide.
Problem arises that when i connect the two wires to our plc input card (configured as active) we read nothing. However wheni connect a 4-20 mA generator (as passive) and generate to the card it read ok. I belive the signal is given in parallel and not in series, could this be?

I attach the diagram that was given to me.

 

[amptramp]

flinana:

Are you saying you got the Teleflow output working? If so, what did you do to get the output working?

 

[roydm]

My guess is the teleflo transmitter output is referenced to something other than Negative.
It's quite common to find that especially in odd-ball transmitters, they don't bother to isolate the output from ground.
You could try this little test:
Put a 250 Ohm load on the Teleflo (active) and measure the voltage from ground to each terminal with the Teleflo at zero percent.
Make the same voltage measurement with your PLC as passive with your calibrator at zero percent, I think you will find the voltages to ground are different.
Please report back.
If you already have it working, let us know what you did.

Regards
Roy

 

[roydm]

Yes, just looking at one of the pics you posted the Teleflo output is referenced to +24
Your PLC input is referenced to ground
So assuming the Teleflo Power supply is grounded you will get 24/250 = 96 mA as soon as you connect Teleflo + to PLC +

Unless you can isolate the Teleflo power supply from ground a signal isolator is the only way forward