Tried to build a simple curve tracer today. It does work. Used a broken wall wart, three terminal and four diodes. After I built I tried a SPICE model to draw the diagram and run a simulation. Here is the drawing.
D5 is the device under test. The scope X-Y leads go across R2 and D5 with the ground in the center.Here are some screen shots of diodes being tested.
The wall wart I used was rated 15 Volts. You could go a little higher or lower but 15V gave good results for diodes, LED and transistor junctions.