Kip, Good suggestion - except that it's on *all* the time. It's not reversed off-is-on and on-is-off.

No Eye Dear - yes, very good...

Reminds me of the LASER safety sign: "DO NOT LOOK INTO THE BEAM with your remaining good eye"

Irving
Thank you - the diagram is really useful.

The FET I'm using is an FDN359AN - a fiddly surface mount thing.
The Vgs max is +-20v, which looks fine.
The gate threshold for switch-on is 3v max, 1.6v typical.

The circuit I've used is *very* similar to the one you show - An LM317 rigged as a current regulator.
What I don't have though, is the diode into the Gate of the FET, or the resistor down to ground.

*Numpty question*
Are they important?

(I'm not sure if this changes anything, but I ought to admit that it's not a straightforward BOB it's connected to. It's a CRH Electronics 'System 3+' driver board. Reading the instruction manual, it says that the output I'm connecting to the FET is 'buffered and capable of driving up to 30mA')

Keith.