Hello everyone, I am testing the waters out in this thread for my 1st post. I have a 4 to 20ma reading from a pressure sensor that is going to a controller out in the field. I also need to read this signal on the plc for monitoring purposes. What would be the best way to make this happen without affecting the signal going to the controller? For the PLC I would be using an allen bradley 1794-ie8. Thanks in advance for the help.
4 to 20ma readings
- Thread starter cranfob
- Start date
jaden
Member
You can put the PLC card in series, if the total resistance of the current loop is not too high. Applied voltage divided by total resistance must be 20 ma or more.
Mickey
Lifetime Supporting Member
Signal conditioner, isolate the 2 systems. Save yourself some grieve ( ground loops to name one)
See link for one example.
https://www.automationdirect.com/ad...rs/DC_Voltage,_Current_or_Potentiometer_Input
See link for one example.
https://www.automationdirect.com/ad...rs/DC_Voltage,_Current_or_Potentiometer_Input
danw
Lifetime Supporting Member
Putting two receivers in series is doable, but risks a ground loop. Parallelling the voltage drop across the controller's analog input has the same risk of a ground loop.
The only practical way to know whether a ground loop is created is to make the connection and see. If either signal has an offset when connecting tha 2nd signal, there's a ground loop. So try it and see.
The surefire method is to use a signal splitter. Google APD 4393 for an example. Signal splitters always isolate the input from the outputs to eliminate a ground loop.
The only practical way to know whether a ground loop is created is to make the connection and see. If either signal has an offset when connecting tha 2nd signal, there's a ground loop. So try it and see.
The surefire method is to use a signal splitter. Google APD 4393 for an example. Signal splitters always isolate the input from the outputs to eliminate a ground loop.
Bit_Bucket_07
Member
The advice that Mickey and danw are offering is good advice. That said, it's most likely that you're dealing with an instrument that has an isolated output and that seriesing the signal through both the analog input and the controller would probably work with no problems. Both loads are probably 250 ohms, and 500 ohms is probably not an overload for your transmitter.
mendonsy
Member
Does your PLC input module support a 1-5vdc input? If it does you can connect that in parallel with the existing circuit. That is essentially a 4-20ma input but without the added load resistor.
Tom Jenkins
Lifetime Supporting Member
Mother nature does not allow free lunches. You can't get something for nothing.
To get 1-5 VDC from 4-20 mA you need to use a 250 Ohm dropping resistor, which represents a 250 Ohm load on the circuit. And, as others have pointed out, this may not solve ground loop problems.
If the first load in the loop is isolated (not grounded) and the total load is within the transmitter capabilities then you can put the two in series. If not, or if you are uncertain, follow Mickey's advice.
mendonsy
Member
True enough Tom, but since he already has the dropping resistor in his existing circuit he can use that one and parallel a 1-5vdc analog input essentially for free as long as it is not grounded. If he did not have the existing dropping resistor then it wouldn't work.
No, it doesn't solve the ground loop issue if one exists. An isolated 4-20ma splitter is the only sure way to do that.
You can add the signal conditioners and such, but it would be much simpler to just run the signal through and isolated analog input card.
If you are using the same power supply for several field devices you could also use a non-isolated card using differential inputs.
If you are using the same power supply for several field devices you could also use a non-isolated card using differential inputs.
I couldn't agree more with the above poster.
Depending on whose conditioner you buy they are $120 to $150.
They save a lot of arguing, rechecking, etc.
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