I would appreciate some help with this seemingly simple problem.

I was given an LED desk lamp that didn't work. The first thing I did
was test the transformer (Input 120 VAC .15A and output 12 VDC 500ma)
which seemed dead so I dug up transformer #2 (Input 120 VAC 10W and
output 12 VDC 500ma) which seemed to work fine until my wife yelled
for me an hour later. The lamp went out, there was a burning smell and
the transformer was very hot. Seems to be fried.

I had THOUGHT that since it was 12 VDC 500ma like the first, it would
be fine. I now know W=V*I which means my replacement is .083A and the
original was .15A. I do not understand why my replacement was not up
to the task.

Moving on, I found another transformer (Input 120 VAC 17W and output
12 VDC 800ma) which makes it 17W. Testing this one with a tester
showed almost no output but I know there are transformers that do not
produce output unless under load so I plugged it into the lamp and it
worked. It seemed a little dimmer than #2 and I noticed a slight
flicker.

After about 5 minutes, I smelled burning and smoke started coming out
from the circuit board where the LED array is attached but the
transformer was cool.

Now I'm confused and can't explain what is going wrong unless the lamp
is defective.

Any explanations would be appreciated!

Thanks,

Mike