mA signal to voltage input accuracy

[rta53]

Mellis, Yes, and I thought that Rob was proposing to use voltage transmitters instead of the present current transmitters. I was wrong. I see now that he was just debating whether to use the same current signals on a voltage or current input.

The thing I have been wondering is why were his 4-20mA transmitters connected to the voltage inputs in the first place. The logical thing would have been to use the current inputs. Unless of course the current inputs were already being used, but that is not what it sounded like from his original post.

 

[Brian123]

The thing I have been wondering is why were his 4-20mA transmitters connected to the voltage inputs in the first place. The logical thing would have been to use the current inputs. Unless of course the current inputs were already being used, but that is not what it sounded like from his original post.

One benefit to connecting a 4-20mA signal across a voltage input with a resistor is simple verification of the signal. You can simply put a voltmeter across the resistor to see what the loop is doing. With a current input, you would typically have to break the loop to insert the meter.

Brian

 

[mellis]

One benefit to connecting a 4-20mA signal across a voltage input with a resistor is simple verification of the signal. You can simply put a voltmeter across the resistor to see what the loop is doing. With a current input, you would typically have to break the loop to insert the meter.

There really is no functional difference in this case. You will measure the same voltage across the standard current input. In this case, since it uses a 500 Ohm resistor, you'd read 2-10vdc for a 4-20ma signal. The only difference is the resistor is internal, or it's on the terminals.

 

[mellis]

You do not need to use a precision resistor.

What you need is knowing exactly what the resistance is.

I hear you, but I will stick by the recommendation for a precision resistor. Not only will you not need to apply a special calibration factor to make it read correctly, precision resistors are simply higher quality parts. Stability over time, and temp coefficient are better.

 

[Brian123]

There really is no functional difference in this case. You will measure the same voltage across the standard current input. In this case, since it uses a 500 Ohm resistor, you'd read 2-10vdc for a 4-20ma signal. The only difference is the resistor is internal, or it's on the terminals.

True, provided you know the resistance value the instrument is using; and it's not an "active" input that was mentioned in another thread. I have never seen one myself, but I suppose it could be possible.

I hear you, but I will stick by the recommendation for a precision resistor. Not only will you not need to apply a special calibration factor to make it read correctly, precision resistors are simply higher quality parts. Stability over time, and temp coefficient are better.

Agreed.

Brian