hi,
One way to get a controlled current source is by using a PMOSFET.
Choose the value of Rsense and RV1, such that the voltage drop across the Rsense is proportional to the current in the load resistor.
Rough example:
By setting the gate voltage using RV1 to turn on the FET and
say Rsense is 200R and Imax= 20mA then Vdp across Rsense = 4V.
Reducing the gate on gate voltage [wrt the +V] will reduce the current in Rsense until a balance is reached, it could be set for Imin = 4mA, so the Vdp across Rsense = 200R * .004 = 0.8V
The actual resistor values will depend upon which PMOSFET you choose.
This circuit is normally used as a Constant Current generator.
Do you follow OK.?