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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.

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You can use an opamp and a potentiometer like this:

schematic

simulate this circuit – Schematic created using CircuitLab

Whatever voltage you set by adjusting R2 will cause the opamp to source current to produce the same voltage drop across R1.

Since 5/250 = 0.020, this circuit should be good for a 0 - 20 mA range. You can cap the bottom end at 4 mA if you add a series resistor to the potentiometer. I will leave calculating that to the reader.

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