Joule Thief

[Pirate88179]

We are seeing more AC power point mains outlets available with single or twin USB sockets incorporated...good idea. No doubt you guys over the pond have them also?

By the way how are you Bill haven't looked in for a while.

I am fine, how are you doing?  It has been a while.  It seems the days are flying by much faster now...ha ha. 


I have not seen what you have described with those outlets around here but, that does not mean anything.  It could be they are incorporated into new construction, of which I have not had any reason to see.  I will look around to see if they are available at the home supply stores next time I go.  It would be a good idea if they did have them.  I wonder how much noise they have in them from the transformation of the AC and also, being surrounded by the ac?


Take care,


Bill

[Pirate88179]

Here is a Joule Thief issue that has been in the back of my mind for a long time.  See the attached portion of a Magluvin scope capture of his Joule Thief collector-emitter voltage.  Of course the collector-emitter voltage is the voltage across the LED.  Naturally there is an unknown current waveform associated with the voltage waveform that will have a similar decaying waveform shape.

So the issue is this:  You can clearly see the declining voltage across the LED.  So how bright is the LED as a function of the voltage?  What if the LED is bright for the upper half and very dim for the lower half?  I don't know if this is true but it merits an investigation.

If if the lower half light output from the LED is very dim then presumably it does not contribute to the visible light output of the LED to the naked eye and there is a lot of "wasted energy" in the inductor discharge.  If that's true it is arguably an "issue" and it suggests you can build a better mousetrap Joule Thief.

So, what if, for the sake of argument, that half of the energy in the coil discharge is essentially wasted because it barely lights the LED?

MileHigh

MH:


I would think that it would depend upon the values associated with those points.  In other words, and I just read this around here somewhere in the past few days, the basic JT can put out 40 volts but it does not fry the led because it can handle the high freq. HV pulse and responds by lighting brightly.  So, if the lower point you indicate has the voltage dropping to 4 volts, 1/10 of the original value, that led will still burn brightly no?


I will try to find where I read this as it confirmed what I had been saying about the HV high freq JT's and leds for many years now.  It is something we found out during our experiments and playing here on this topic.


Maybe I did not understand what you were meaning and went off in another direction...if so, I am sorry and will look at this again once I get home.


Bill

[MileHigh]

Bill:

I think you read about the "40 volts" in one of my copy-pasted descriptions of how a Joule Thief works.  That is somewhat of a misnomer because the coil never outputs 40 volts when the LED load is connected.  It's the same old issue - the discharging coil is a current source and not a voltage source but mentioning that in any kind of magazine or online article, apparently even an article written for "hobbyist keeners" would throw too many people off and cause more confused people than enlightened people.

The real test to check how the LED responds to different voltages would actually be to slowly change the current through the LED and then measure the resulting voltage across the LED and compare that to the voltages you see in the scope shot.  It might be quite a surprise if the lower half of the voltage curve was barely lighting the LED.  My gut feel is telling me that "barely lighting" is an exaggeration but the proof is in the testing pudding.

I am willing to bet you that this investigation was never done in all 1126 pages and six year's worth of this thread.  Not bad for a guy leaning back in his Laz-Z-Boy eh?

MileHigh

P.S.:  And the follow-up to that is if it truly is a problem, what can be done to alleviate it?

[Pirate88179]

Bill:

I think you read about the "40 volts" in one of my copy-pasted descriptions of how a Joule Thief works.  That is somewhat of a misnomer because the coil never outputs 40 volts when the LED load is connected.  It's the same old issue - the discharging coil is a current source and not a voltage source but mentioning that in any kind of magazine or online article, apparently even an article written for "hobbyist keeners" would throw too many people off and cause more confused people than enlightened people.

The real test to check how the LED responds to different voltages would actually be to slowly change the current through the LED and then measure the resulting voltage across the LED and compare that to the voltages you see in the scope shot.  It might be quite a surprise if the lower half of the voltage curve was barely lighting the LED.  My gut feel is telling me that "barely lighting" is an exaggeration but the proof is in the testing pudding.

I am willing to bet you that this investigation was never done in all 1126 pages and six year's worth of this thread.  Not bad for a guy leaning back in his Laz-Z-Boy eh?

MileHigh

P.S.:  And the follow-up to that is if it truly is a problem, what can be done to alleviate it?

I think you are correct about where I had read that.

Waay back Gadgetmall was using a ceramic cap on the base of his transistor.  My thoughts are, that if indeed it is wasting energy by having a moment of low voltage to the led then what about using a decent sized supercap, say 1 F, in series with the led?  This way, the voltage would never drop as the cap would absorb all of the pulses, both high and low.  Sort of a smoothing operation but not exactly the same.

Does this make any sense?

Bill

[MileHigh]

Putting a cap in series with the LED doesn't make any sense.  But we all know that putting a cap in parallel with a weak battery as an example can keep the voltage steady when the load momentarily increases as compared to the weak battery alone which would crap out.  So you are in the ballpark in the sense that somehow a smoothing cap could be part of the solution.