Unusual TP4 Waveform

[Qiaozhi]

Hello Qiaozhi,

I have been looking for a critical damping for this problem, but the curve as red circles remain, after I change the wire emails, the curve becomes normal.

although coil is underdamped but if you use a good wire,result not like the red circle

underdamped for good wire result like this ;
[ATTACH]39482[/ATTACH]

Wahyu.

Even on your "good" result, the preamp output is still ringing. When the MOSFET turns off, you should see the preamp go into saturation (i.e. hit the power rail) and stay there for awhile before displaying an exponential curve that reduces to 0V. In your image, the preamp saturates in one direction, then swings into saturation to the other rail before the exponential decay starts.

Can you post the details of your coil: wire size, diameter of loop, inductance and resistance? If you could also remove the damping resistor, and then measure the period of oscillation on the scope, that would be very useful information.