Basics question of Inductor to powering high voltage lamp

NascentOxygen

I assumed that initial offset was [somehow] due to the power supply, but you say it isn't? Can you account for it? (The blue line marks zero, does it?)



Reversal occurs the instant that the power supply disconnects.
Only if you have one.

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sophiecentaur

How long has this thread been running? Yet you still use terms like "instant". Nothing is "instant" in electronics. The time involved depends upon the REAL values of all the components involved. Have you still not grasped that?

The Electrician

Have a look at the capture where there was no clamping device. The first voltage excursion, which occurs just after the disconnection from the power supply, is positive going. I thought you were referring to the polarity reversal that occurs at 2.5 divisions, during the ringing.



I'll look into your "blue line" question when I get back home in a day or so.

NascentOxygen

No, I wasn't referring to the unloaded ringing. I refer to the clamped waveform on which I added the blue notation: at the vertical rise the inductor voltage reverses and it stays reversed for the duration of that steady 120V* level. I expected the zener's voltage prior to that vertical rise would be of opposite polarity to what it is after that point, and I can't account for it not changing polarity at that point.
Fine, no hurry, I think this thread will still be going strong.

* is the oscilloscope indicating your "approx 100V" zener is close to 120V?

What does the 64V figure on the foot of the display indicate?

The Electrician

Yes.

It's the trigger level, corresponding to the orange arrow on the right side of the screen.

The Electrician

Concerning the offset you asked about in post #69. I connected the power supply to the inductor with a pair of alligator clip leads, and performed the disconnect by simply touching and then "un-touching" one of the alligator clips. When doing this, one gets a spark every time, and usually you don't get a clean ring waveform. I had to do it a number of times to get the waveform I posted. Even though I was able to get a single disconnect without any bounce, there was still an arc. The offset voltage you asked about is the voltage of the arc. The true disconnect doesn't happen until the arc stops, then the inductor current is truly interrupted.

This capture shows what happens with a current probe monitoring the current. The orange trace is the voltage across the inductor, and the blue trace is the current measured with a current probe clamped around the external inductor lead. Starting at the left edge of the screen until about 3.8 divisions, the current is ramping toward zero, the alligator clip having been disconnected and arcing is in progress. At 3.8 divisions, the ramp stops and the current suddenly jumps to zero and the ringing at the self resonance frequency commences. The oscillation results from energy exchange between the distributed capacitance of the inductor and the inductance itself. This oscillating current and voltage occurs completely internal to the inductor and no sign of it is seen in the blue trace which is the current in the external inductor lead.



This capture shows the current in a loop of the wire comprising the inductor. I pulled a single turn of the wire from the middle of the inductor and clamped the current probe around it. Now we can see oscillations in the current because we are sampling the current internal to the winding of the inductor.



To get a better result, I got a couple of 500 volt power FETs (IRF740s) and connected them source to source, with the two drains available to interrupt the inductor current. This arrangement is necessary to avoid the clamping effect of the body diode which would occur if only a single FET were used.

This capture shows the current ramping up as the FETs are turned on and the power supply charges the inductor. The FETs are turned off at the middle of the screen.



See the next post.

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The Electrician

In this capture we see at a shorter time scale the disconnecting of the power supply by turning off the FETs. The capacitance of the FETs is of the same order as the distributed capacitance of the inductor, and now we see quite a bit of oscillating current in the external lead of the inductor. The voltage oscillations (orange) are somewhat non-sinusoidal due to the nonlinear capacitance of the FETs.



Notice that there is no offset in the voltage as we approach turn-off from the left side of the screen. There is no arcing before the complete disconnect from the power supply.

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NascentOxygen

You've abandoned the zener diode in all of these?

The Electrician

I wouldn't have used the word "abandoned". I just didn't have it in place because I was demonstrating things that couldn't be seen with it in place.