Here's a handy little current source I built a few months ago. It's quite useful, as it lets me measure DC resistances to 0.1 mΩ with my DMM and 1 μΩ with my bench multimeter. The batteries should last for thousands of measurements.

 

Interesting application. I have more need for portable stable voltage sources, than current ones.

I take it the casing is custom made, right? Neat job!

 

So you solved the stability issue. What did the trick?

 

I first built one using an LM317, but it dissipates lots of power when run at 12 volts and is a bit unstable even with the heat sink I put on it (but it still works fine for a 1 A current for a few seconds). The voltage ref/op amp/MOSFET design is a better one, I think, and the original stability problems with the prototypes were a) the prototype board, b) longer wires, and c) crappy Radio Shack sand resistors.

The part I like best is not needing a heat sink for the MOSFET.