Circuit uses a potentiometer to control the analog input to a PLC or other control device. This will regulate the current between 4-20mA and allow the user to adjust the input setting within the system.
The components have the following functions:
R1kOhm - Prevents the pot from shorting the input when wound all the way down. Is used to form the combined 6kOhm resistance required for 4mA.
R5kOhm Pot - varies the resistance to the input between 4-20mA. When wound all the way down and 'switched out' the input resistance is fixed by the 1kOhm resistor.
R250Ohm - Is the given input resistance of the 1769-IF4 current input (Analog input device I am using for Bradley PLC).
To build your own current regulator, remember Ohms law.
V=I*R
Therefore, for the low current setting (4mA)
R = V/I = 24/0.004 = 6kOhm
For high current setting (20mA)
R= 24/0.020 = 1.2KOhm ~ 1kOhm
The above math tells us that for current settings between 4-20mA, we need a variable resistance between 1-6kOhm.
