Claimed OU circuit of Rosemary Ainslie

[MileHigh]

Aaron:

About your latest clip showing the oscillation:

The first and fundamental error you are making is that you are triggering on the falling edge of the spike across the shunt resistor.  We all know that the nature of the spike has a natural variability, and then that changes even more when you change the gate trimpot.  All that you had to do for a more or less rock-steady display on your scope was to use the second channel to trigger on the 555 timer waveform.  Something so simple that I would have done it that way without thinking, it would have been a reflex - a no-brainer.

My first recommendation to you would be to do the whole clip over again triggering on the 555 timer signal.

Before going on any further, I am not a fan of low-end devices like these with a very limited pixel resolution.  The A/D sampling rate is being massaged down to the pixel resolution of the display.  There must be some sort of software layer in there that is repackaging the data for display, and most likely trying to make a "one size fits all" compromise display when you are looking at noisy or aperiodic data.  Just like a digital multimeter, you can assume that the software algorithm has limits and gets "challenged" fairly easily.  Then you can't forget the biggie: humungous aliasing effects when you try to display a large number of periodic waveforms on the display at one time, or for any high frequency transitions, like ringing.  To overcome this limitation you have to keep on turning up the timebase to take a peek at what the "spikes" really look like, and then go back to your "working" time base.  I hope that you are taking notes Aaron.

At 0:50 Aaron changes the gate resistance, and like magic the frequency of the waveform appears to drop considerably.  What gives Aaron?  I don't think that you were mistakenly playing with the 555 trimpots, so it looks to me that somehow the scope already has "scrambled brains" and everything on the display is now suspect.

Here is the first one that comes to my mind:  The spikes have gotten much thinner because Aaron changed the gate resistance, and the notorious aliasing and software layer effects are coming into play.  You can assume that the spikes are still being digitized by the scope and put into the memory buffer.  Then the software looks at the spikes and decides that they are "too thin" to merit displaying yet another single-pixel with spike on the display for the current time base.  However, some of the real spikes from the shunt resistor "make the grade" and are just wide enough for the software to decide to display them.  Of course we "all" know that the width of the spikes are subject to some variability.

Therefore Aaron, at 0:50 what you describe as "oscillation" is really just display aliasing effects coupled with your ridiculous choice of a trigger.  Note that the trigger is only valid for a spike that is wide enough to make the grade, meaning the triggering itself is also intermittent under these conditions.

I am only at 50 seconds but I think that I will post this already.

So what do you think Aaron?  Do you agree with what I am saying, or is what I am saying simply not credible because I am not credible?

MileHigh

[TinselKoala]

It is not "laughable" to display an RMS reading in this case, especially across the heating element/ shunt resistor.

Are you suggesting that only DC power will heat it, and spikes of both polarities won't?

Or are you just attacking anything you can get away with?

Of course it is. First, the "heating element" and the "shunt resistor" are two different components. The voltage drop across the shunt is the input CURRENT measurement; this is a squarish pulse with a very short, comparatively, ringdown at the end. RMS voltage makes no sense here. You want an AVERAGE voltage for the power calculation here.
Across the load, the "heating element" as you call it: you will be seeing a small AC signal riding on top of a relatively large DC offset here. Again, displaying "RMS Voltage" is not a useful measure. The important parameter here is the peak spike voltage and the peak to peak spike range--NOT including the DC offset.

[TinselKoala]

@MileHigh:

I just realized why Aaron, Ainslie et al. seem to prefer these digital toy oscilloscopes: it is because they do not understand how to make quantitative measurements using an analog scope.

All those little numbers around the dials, the graticle display, doing the math...it's just too much for them, so they need to see the digital numbers right in front of them. And they trust digital displays implicitly. After all, they are digital, how could they possibly be wrong?

[MileHigh]

Aaron:

Another posting to back up what I just said in my previous posting.  Call this the "smoking gun" posting.

Let's talk about the picture on the right side in your posting #891, the one you have marked-up with the diagonal red lines.

Notice that the distance between the main spike in the center of the display to the main spike to the left of that is slightly less than two divisions.  Notice that the distance between the main spike in the center of the display to the main spike to the right of that is slightly more than two divisions.  It doesn't jive, you know that the waveform is regular and periodic, so this should not happen.  In this case we are "forgetting" that there appears to be a "magic drop" in frequency.

I can't explain everything that is going on in that waveform but there is no point to explaining it anyway if you think it contains suspect data.

With respect to your "increasing oscillation" and the diagonal red lines that show the envelope of the increasing amplitude:  In all likelyhood these are aliasing effects again.  There is a "beat frequency" or a "Moiré pattern" between the real scope sampling and the input signal that affects what data is being digitized into the memory buffer, or between the memory buffer and the display-layer software and the LCD display itself that is creating the ILLUSION of increasing oscillation amplitude.  In reality the "oscillation" is almost certainly at a more or less fixed amplitude.

I have a feeling that the "oscillations" are just the ring-downs from the negative spikes of the shunt current waveform that "make the grade" and manage to get displayed on the LCD display.  We can't forget the "magic drop" in frequency now, can we?  However, I can't explain the other missing parts of the shunt waveform when the MOSFET is on.  Perhaps there is a higher noise level on these shunt waveforms and the software filters them out completely because it decides not to display very high frequency noise as a rectangular blob of all-black pixels?  Just a guess.

With all of that baggage from the first 50 seconds and your posted picture, I doubt it is worth commenting on the rest of the clip but I might.

Remember what Hoppy said, "Competency to carry out the testing is another issue", something like that.

MileHigh

PS:  So I looked at the clip a second and last time and the whole clip is a triggering and display aliasing fiasco.  All that you had to do was trigger on the 555 signal, and then display the shunt current waveform for various gate resistance settings.  For each gate resistance setting you should have showed us the waveform at a few different time bases.  The most important time base setting would have been the one that shows two full pulse waveforms.  The seond most important time base setting would have been the zoom-ins on the ring-downs.  If you have an X10 setting on the time base, you could have inspected a full sweep from one pulse to the next to hunt for interesting stuff.

My conclusion by making inferences based on the extremely poor views of the waveform that you showed us is that there is no or very minimal oscillaton going on here.  However, the MOSFET itself is definately not starting to oscillate in the sense that I think you were meaning.  The only thing that is happening is that the nature of the current waveform signal changes as you change the gate resistance, the amplitude and look of the ring-downs change, etc.  This should be something that everybody is familiar with by now.

[qiman]

It is not "laughable" to display an RMS reading in this case, especially across the heating element/ shunt resistor.

Are you suggesting that only DC power will heat it, and spikes of both polarities won't?

Or are you just attacking anything you can get away with?

Exactly. It is an automatic setting that will show both AC and DC readings at the same time so pick the numbers you want depending on what you're measuring?

The load has alternating current plain and simple.