Greetings List:

I am looking for an inexpensive way to convert a 4 - 20mA signal from a pressure transducer to a 0 to 5 Vdc signal so that I can scope the waveform
with a digital scope. The people selling the transducer tell me that all I need to do is put a resister in series with the output and then just measure the voltage across the resister. What I am wanting to know is how accuracy and linearization effected. Does anyone have a better solution or some simple calculations I should make to ensure that I will get the best
results.

Thanks In Advance:

Stephen Bennett
Kawasaki Robotics (USA), Inc.

 

Stephen:

You should be able to use a 250 ohm precision resistor to read 1-5 Vdc (V=I*R). This will not affect the accuracy or linearization of the
transmitter. To check you can do a five point calibration on the test instrument.

David Eidsor
Instrument and Control Technician

 

They are right. As long as you do not over drive the capabilitys of the transmitter with too much loop resistance, adding a 250 ohm resisitor in the
loop not change the loop accuracy at all. The 250 Ohm resistor will give you a 1-5VDC signal for a 4-20mA loop signal.

But you probbaly dont have to bother. Most analog input cards have a fixed impedence dropping the 4-20 mA signal across a fixed resistor and than measuring the voltage across it for the analog value. You should be able to
read directly across the analog input. Probably won't be 1-5 VDC, but you can turn the gain up on your scope.

Bob Peterson

 

Well, first of all - you won't get a more accurate result by anything else - since that is what all other do internally

But you will however get 1-5V, not 0-5V

/Johan Bengtsson

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You could actually use a 312.5 ohm resistor and an instrumentation Op-Amp toground reference the signal to 0-5 volts. The critical specification for the Op-Amp would be common mode input voltage range. The 4-20 ma signal transmitter acts as a constant current source with fairly high compliance.
The compliance voltage would determine the cmivr needed. this could be done inexpensively if you could either insure that the resistor operates
near ground or the very good inexpensive opamps have adequate cmivr. In the simplest case, this could be done with garden variety components. If this is the only load driven by the transmitter, grounding one side of the resistor would give you 1-6 VDC and the 0-5 could be obtained by
subtracting a 1.00 V precision reference.
If you are interested, I could draw a schematic.

Regards

cww

 

There is a small solution to convert current loop to voltage. I'd use an operational amplifier based solution. OP 07 will be a cheap solution, but you must add a simetric power suply. It's easy to find the schematics in many books on electronics.The accuracy will be good, and the calculations not very difficult.

Francesc J. Garcia Rabella
Politechnical University of Catalonia