Inductive Dc Solenoid load which generates back e.m.f while switching 'off ' action of solenoid. Freewheeling diode across and near to load is shortest path for this generated back E.M.F. Fast de-magnetization with this diode.
Normally it is working for 24v supply line of 4-20ma Transmitter. lm317 acting as unidirectional solid state fuse. Minimum voltage drop across circuit is >3v irrespective of output current. Adjust terminal floating on output load.