I am looking for a way to control current in a circuit with voltage supply of +28V. The current is supposed to have discrete levels with a maximum value of up to 2 Amps. The discrete levels maybe a jump of 0.5 Amps or 0.25 Amps.
If power losses are an issue, you may consider using switching voltage regulator configured as constant current source .. see attached picture (values set for 2A operation) ..
By introducing for example digital resistor around IC2b you can vary the output current ..