Hi BradtheRad,
Votage at the inductor is gradaully decay as it suppose to be. Before that, there is a short interval of rise time of the signal before it start to decay. This rising time was puzzled me. As mentioned, My theroy right now is the rise time of the signal seem to be from the capcitance on the scope probe.
Lower resistance also provide lesser rise time.
---------- Post added at 09:47 ---------- Previous post was at 09:27 ----------
Forgot to mentioned that I've also measured the time constant with different resistor instead of using oscilloscope to calculate the rise time and it seem to prove the theory.
With 1 Kohm resistor, I measured about 15.4 ns, this give about 15.4 pf cap.
With 4.7Kohm resistor, I measured about 69 ns, this give about 14.7pf cap.
Reading the probe label, it said 15 pf max which look about right.