Thanks for your replies. I think i should make something clear to be understood.
Firstly we have already used a pot to get 1 , 2 or 3 ma by means of turning it and changing the probes of power supply for - currents.
But i want to make it programmatically controlled. therefore i need to use arduino with any board(i dont know which one or is it possible) to adjust the current with 28vd power supply.
it is not a problem that if voltage lowers when getting (for instance) 20ma from it.
i meant its initial value is 28v . of course if we get current from the curcuit , voltage will decrease .
To me , i would use a step motor to control pot but it should have more accuracy. So do you know any board or something like shield which is compatible with arduino to get the precise current value ?
To sum up , i will write an application and i will write 5ma or - 5ma in a textbox . then serialcommunication will start and arduino will control the board and finally i will have the current value that written in the textbox .
i hope everything is clear , if not i will try to explain my aims to do so.
Thank you.