I am designing at the time a process in which we will use a PLC to control some valves and mass flow controllers. The PLC we have has an input module for 0-20 mA. One of our sensors (flow measurement) gives a signal between 0 and 10 V.
Is it possible to convert that signal from voltage to current only including a 500 ohm resistor (Using Ohm's law)?
The datasheet of the analog input module says it has 100 ohm resistance, so maybe I should use a 400 ohm resistor?
You're on the right track but forgot something.
simulate this circuit – Schematic created using CircuitLab
To convert 10 V to 20 mA the total resistance required is 500 Ω. You need 400 Ω in series with your input.
You also need to check that the 0 - 10 V driver can source 20 mA (or can drive 500 Ω).
Your best bet, if I'm understanding your understanding correctly, is your last...add a 400 ohm resistor, to combine with the internal 100 ohms to keep the current between 0-20mA.
In the future (or if this is the wrong answer) it will be better if you can attach links to the specifications of the devices used. For instance, if 20mA is your target maximum, 400 ohms is fine. If 20mA is your Absolute Maximum Rating, you might want to add some safety margin.
simulate this circuit – Schematic created using CircuitLab
Look, the point of 4-20mA is that it doesn't depend on wire length, so you need to ensure current, no matter what resistance is there. The way to do it is by a closed loop amplifier that measures this current and has the 0-10V signal as it's input.
The circuit there is a very simple sketch, I'm too lazy to do the full design for you :)
