Hi mic3, no i get a 20v + from a controller,in opperation it goes to a air
transducer that gives me the 4 to 20 mA signal out. the controller displays
that signal as air pressure. i am building a simulator to test the controllers
out of opperation. therefore i nead to vary the 4 to 20 mA to get different
pressure readings.

 

Here is a circuit which does what you would like:

http://www.allaboutcircuits.com/vol_3/chpt_8/7.html