Joule Thief 101

[tinman]

Does it? A pure resistance doesn't change its resistance value with frequency.If it is how you say "we know it does", kindly demonstrate it or link to an example please.

The skin effect can cause the effective resistance to increase with frequency. The highest current density is at the outer circumference of the conducting wire,and decreases as we near the center of that conducting wire. As we increase in frequency,the skin depth decreases,and so the effective resistance rises.

I have a feeling as to what your answer may be to this,and if you do answer as such,then i will explain as to how my claim of increasing resistance is valid to the question of a change in impedance.

Your question provoked me to think about modifying the impedance of a coil or transformer, so I tried to engage you to think about it too, hence the questions. I thought we'd explore the concept in tandem.

The reason i asked the question,is because there may be some simple way of checking to see if i can get the impedance to change. The question and investigation into this stems from MHs comment in relation to my last video. He seems to think that the drop in input power,and the increase in output power is the result in the change of impedance of the coil. As far as i am aware,everything that is needed in order for the impedance to change,is not there in the circuits operation. There is no change in the phase relationship of primary EMF and current,there is no change in frequency,there is no change in the output load(which is a pure resistive load)and there for there is no change in effective resistance or reactance.

This is the reason for the question,as from what i know,there can be no change in impedance if one or all of the above do not change. So i asked you in case you new of something else that can change the impedance of a coil.


Brad

[tinman]

.......It's clear that "reactance" changes with frequency.  The easiest and most straight forward means for manipulating the inductance of the primary winding is by placing a load across the secondary.  The trick is finding a load for the secondary which varies with frequency, the net effect being that each time the current flows in the secondary, its in such a direction that it "augments" the supply, while at the same time, it drops the inductance of the primary, allowing for more current to be drawn by the primary.


The majority want consumption to drop, I am not a member of that school of logic.


Regards

Why would anyone think that a true pure resistance changes with frequency is beyond me

I did quote effective resistance--poynt quoted pure resistance.
The effective resistance will change in relation to frequency. This is up in the higher frequency ranges where skin effect starts to reduce the effective resistance due to the decrease of skin depth.

Brad

[poynt99]

The skin effect can cause the effective resistance to increase with frequency. The highest current density is at the outer circumference of the conducting wire,and decreases as we near the center of that conducting wire. As we increase in frequency,the skin depth decreases,and so the effective resistance rises.

Skin effect can certainly play into circuit currents, but since we are dealing with low frequency sine waves here, skin effect can largely be ignored in this case. Skin effect usually pertains to metalic conductors, and I am uncertain as to the effect in resistors themselves. I'll defer to PW on that as he may know the answer.

This is the reason for the question,as from what i know,there can be no change in impedance if one or all of the above do not change. So i asked you in case you new of something else that can change the impedance of a coil.


Brad

I'm willing to explore the possibilities of dynamic impedance changes, but at the moment can't see how it might be accomplished in a simple air core transformer, other than bringing ferromagnetic material near it.

[tinman]

You are in an area where I have little to no interest.  I have come to understand that when one desires to operate at frequencies that high, its best to let the tank circuit or circuits generate them for you.  To the best of my knowledge, "pure" resistors don't change their values, and I find that this is the only measure of safety one has, if they could change like reactances change, we would be in a world of hurt.  Ideally, the circuits we should be engineering, have no DC resistance.  The resistance aspect should be governed by a reactance. 


Effects like the skin effect owe at least part of their existence to the mechanism of opposition to change, being informed of this, we have what we need to eliminate it (the skin effect).  That which must be sacrificed is painful for the majority. Do it and reap the reward. 


It's about perspective, to me anyway......


Regards

To the best of my knowledge, "pure" resistors don't change their values

That is correct,but an inductor is not a pure resistor.


Brad

[tinman]

Skin effect usually pertains to metalic conductors, and I am uncertain as to the effect in resistors themselves. I'll defer to PW on that as he may know the answer.

Skin effect can certainly play into circuit currents, but since we are dealing with low frequency sine waves here, skin effect can largely be ignored in this case.

The exact answer i thought you would post--and this is good.
We can now remove any change in effective resistance from the equation

I'm willing to explore the possibilities of dynamic impedance changes, but at the moment can't see how it might be accomplished in a simple air core transformer, other than bringing ferromagnetic material near it.

What would need to change if a ferromagnetic material was bought close to the air core transformer in order for the impedance to change--E.G,inductance?

Im not sure if you have seen my last video on the experiment,but maybe if you watch it,you will understand as to what it is i am referring to. You may relate this to another experiment,where we did not quite see eye to eye.

So in the video,i do test the difference in inductance with and without the magnet near the coil. I also place a large chunk of laminated steel core inside the air core cavity,so as i can get some sort of reference later on as to how much difference raising the inductance by a large amount makes on the P/in=P/out results. I think it is safe to say that the inductance increase is so small with the magnet in play,that it is not the cause of the effect.

Have a good hard think about it Poynt--about what drives the magnet oscillator,and how it could be possible to get the results shown. I will add that(although mentioned in the video),the input RMS value is kept at a constant throughout the test by the voltage regulation function of the FG--as can be seen in the scope shots below,where Ch1 is the voltage across the primary coil,and CH2 is the current flowing into the coil,measured by way of voltage drop across a 3 ohm CVR.

https://www.youtube.com/watch?v=mlb79xSh93w

Brad