Alternative circuit for obsolete product
In order to determine if a replacement is suitable, you need to specify the accuracy and performance you need.

This would work -- Note the connection of the 500 ohm to the 100 mohm R -- you need to wire it that way to eliminate the effect of wiring resistance on the accuracy (it's called a Kelvin connection).

Need to choose a FET that meets your voltage, current and power requirements. 







simulate this circuit &ndash; Schematic created using CircuitLab