I have a wall wart, whose output is rated 9VDC @ 500mA.
I am measuring 13.7 VDC unloaded. I put a 10k resistor across the output, expecting to see the voltage stabilize. It did - to 13.7 VDC. What am I doing wrong?

 

You are not doing anything wrong. You take a 500 ma wall wart and load it at 1.4 ma. (.3% of rated current) and get very little change in voltage. That is what should happen. Try 50% of rated and see what happens.

 

That must be one of the older linear W.W.'s which rely on the rated current to bring the voltage down, the vast majority now are SMPS and output a constant voltage level.
Max.

 

Thanks guys. Good to know, especially since the planned load is around 100mA. I guess I need a different wall wart.

 

Or you can add an LM7809 and two or three caps on the 12.7V output and run your project just fine.