Inductor test bench charging 'scope trace.

[dr pepper]

Please help me understand this.
I have a test board for inductors and transformers so I can see at what charge time and current the inductor starts to saturate, the board just connects the inductor to the supply then disharges it into a diode, the voltage accross a current sense resistor in series with the supply is plotted on my 'scope, the fet gate drive is the x trigger.

I've been trying some larger new and salvaged cores and I've been getting some different readings, the charge current doesnt show up on the 'scope, the disharge does but not the charge.
Heres some bad pics of my 'scope, I have tried various combinations of time on/off for the fet gate drive, nothing seems to change, I thought at first the charge slope was just too slow for the 'scope sweep.

First pic, a 330uH test choke (on board the 'test bench'), second a larger choke that doesnt show the charge slope, and third the 'test bench. The lower trace is the gate drive for the fet, high being fet on:

 

[FvM]

Are you asking: "Why can't I trigger the oscilloscope correctly"?
Hard to say without knowing the capabilities of your oscilloscope.

 

[dr pepper]

I tried the dartron 'scope and my 100mhz phillips, both are similar.
I think the scope is triggering ok you can see the gate drive for the fet ok, and the off time for this is 20x the on so you cant see it unless the 'scope triggers.

What I dont get is why dont I get the charge current ramp on some inductors, its almost like on some inductors the switching of the mosfet is reversed, I get a ramp up after the fet is switched off as though the on/off has reversed.

- - - Updated - - -

Thanks Fvm, you were along the right lines, intentionally or not.

For some odd reason my 'scope was triggering ok up to a certian current, above that the 'scope messed up.

The current sense resistor was in the + line, my supply was being pulled down at high current and the 'scope ground was conected to power supply +, so ground as far as the 'scope was concerned was all over the place.

I have moved the sense resistor to the ground leg of the inductor, one end of it is grounded, so now 'scope ground is power supply ground, and everything works fine.

 

[dr pepper]

Funny how everything makes sense once you've sorted it.

 

[FvM]

I must confess, I nearly forgot that an analog scope needs a signal generator with a pretrigger to completely display the triggering edge (except for fast oscillopes with internal delay line).

 

[dr pepper]

Thats something I didnt think of either, my philips does have a delay line, it wouldnt need 2 signals.