Looking to take a 4 to 20ma control signal down by one ma. I assume I would put a resistor in line, however I would like to know what size resistor. The end result is the control signal at 4ma would reduce to 3ma, if it is 16ma it would be 15ma etc. Any help is appreciated.
The receiver for a 4 to 20mA control signal is usually a resistor of known value, and it is the voltage across that resistor that is measured. So changing the resistor value will give a proportional change, rather than an offset.
You could add a 1mA negative constant current source, and that would reduce the measured current by 1mA. But that would mean zero current you would read -1mA. Is that acceptable?