Joule Thief 101

[MileHigh]

I looked at the current trace for the light bulb being lit at about 25:00 in the clip.  If you ask me, Lewin is getting whackadoo when he says, "a breakdown in Ohm's Law."  All that he is demonstrating is a resistance where the value is a function of time.  Or you could say that it is a resistance where the value is a function of the temperature of the element.

There are some issues relating to deductive logic when you look at things like this.  I remember once reading something where the person talked about scientists "discovering" the value of Pi.  When you look at it logically, there is nothing to discover.  For Ohm's Law, all that it states is that for a two-terminal device you will be able to measure the voltage across the device and the current though the device.  So how can you get a "breakdown" in Ohm's Law?  It's nothing more than an observation of voltage and current for a device under test and defining the term "resistance" as the voltage divided by the current.  Presuming that the voltage and current are always measurable, then there can be no such thing as a breakdown in Ohm's Law.

[Pirate88179]

Well thanks fellows.

I got confused when he showed the resistance jumping up (not linear) on his graph and then dropping down....and then rising up but not high enough to make Ohm's law correct.

Brad mentioned the heating up of the filament earlier.  So, this is not an anomaly where Ohm's law does not apply.  I wonder why he was showing this?  I mean, this was not the advanced class or anything....this was an MIT basic course.

I did not even have a chance to look at the one .99 was talking about earlier.  Why would he do that one also?

Bill

[poynt99]

That one is a lot more complicated than just Ohm's law.

Lewin left out a few important details on that one, resulting in misleading information.

Bottom line is Kirchhoff always holds, regardless of what your measuring tool is telling you.

[wattsup]

@all

If you really are interested in talking about curiosities, then follow this.....very simple test.

If anyone here has an OU bench, plug a simple 110 volts light bulb into the wall socket while scoping with both CHA and CHB scope probes (only)(DO NOT CONNECT THE GROUND CLIPS - EVEN BETTER REMOVE THE GROUND CLIPS) on each side of the bulb and both set at 50v/d and AC and seeing the waveforms that the scope produces. Post the waveforms here.

Then set the probe on the neutral line to 100mV/d and post that image as well while the other is still on 50v/d.

When that is done to advance even faster then do this.

Take any primary coil and use two cvr's one on each side of the primary then connect the cvr's to your pulse generator set at sine wave and then scope across each cvr, and post again the image of the two produced wave forms. Also you need to identify which channel is on the positive side and which is on the negative side as well as how the probe/grounds are connected. 

For this to be objective I need someone besides me to do this. Do this and we'll talk but only if you guys want to really talk while remaining objective and civil. So simple, right in our faces but who thinks to really look that far back to basics.

In a previous post, I asked you guys 5 simple questions and all of you could only run away from them. That's understandable because those 5 questions being 5 of a good 50 are only scratching the surface of the reasoning of our effects.

So, who will post those images? It all starts by asking the right questions.

wattsup

[MileHigh]

Wattsup:

I think that many people around here would agree with me when I say that the concept of voltage, what it means, and how you interpret it and work with it is mostly an enigma for you.  So until you have mastered the concept of voltage, you are at a disadvantage when trying to do bench experiments.  If you are going to beat me up for saying that because it isn't politically correct, go ahead.

I will take a stab at your first questions in general terms.  I believe that in the US and Canada, the third-prong ground line is connected to the neutral line at the breaker panel.  Then that ground-neutral connection at the breaker panel is connected to a solid and nearby earth ground.  The big step-down transformer up on a telephone pole also connects it's center tap to a solid earth ground.  Please anybody correct me if I am wrong.

You did not mention if the scope is isolated or not, why is that?

If the scope is grounded (not isolated) then the two bare probes will pick up the hot and neutral potentials.  Naturally this assumes that no device plugged into the power line is misbehaving and there is no current flowing in the ground line.

That begs some questions for you to ponder:  What if between you and the breaker panel there is a coffee maker plugged into the line and you turn it on?    What if there is a coffee maker further down the line and you turn it on?   What will you see on the scope display for the two channels when the coffee maker is ON?

If the scope is isolated (not grounded) then the two bare probes will be floating and yet in all likelihood you will still see some waveforms on the scope display.  Before even discussing the waveforms you see, there is a fundamental problem with the test in this case, what is it?  Moving on, why will you see those waveforms, and what will they be?  How do you explain those waveforms?  What can you say about the impedance of the waveforms that you are looking at?  What about putting the coffee maker between you and the breaker panel and putting the coffee maker further down the line?

Keep in mind that I never did a lot of the "crazy tests" you see discussed around here.  Even though way back when I had a lot of bench experience, I never had any logical reason to do them.

MileHigh