I am working on a project where I have a current source (analog output from a PLC) controlling a VFD. Based on the position of the dipswitch, the VFD will accept either a current or voltage input. The dipswitch is set to voltage by default, so I used a 500 ohm resistor to convert the 4 - 20mA signal into 2 - 10V. Very straightforward.
Now, what if I set the dipswitch to current and try to control the VFD with voltage, say with a potentiometer? But I just can't seem to build a circuit to convert the voltage into current.