Lets try this again. Here is the diagram with idea like Igor suggested except I put in a 20::1 divide. I picked PWM filter components that presume a relatively low PWM frequency. For a higher frequency PWM the capacitor could possibly be smaller. (This all depends upon how fast you want the selectable current level to slew when it gets changed). I added a polyfuse to the design to protect for short circuits to ground on the output. I would still recommend a very temperature stable 2.5 ohm resistor and the use of the 12 volt supply. Compliance as Raghunathan discussed is an issue. If you use two low of applied source voltage then you severely limit how much voltage can be allowed to drop across the load (i.e. limiting how high the load impedance can be). If the load voltage drop gets higher than the compliance of the circuit then the full set current will not be able to flow through the load loop.



Michael Karas