Joule Thief

[MileHigh]

Bill:

I can envision what you are talking about.  There is a technical explanation for that that could apply here.  You can excite various components at high frequencies, not necessarily high voltages, and see very fast pulse trains across them.  The AC impedance of the physical part itself comes into play.  So it's almost a "fake" voltage.  You see voltage spikes but that's due to the inductance of the wire in the part.  The actual diode itself will have an AC impedance.  So the LEDs light up, and voltage spikes are kind of "sprinkled on top" of the current flow through the LEDs.  So the assumption is that this process can take place at high voltages also.

I am not challenging your observations about the apparent brightness, just saying that there is a voltage "fuzz" on top of the signal that is actually powering the LEDs.

You would probably need a proper ultra low inductance current sensing resistor to see the actual current flowing through the circuit.

With respect to the question of what waveform makes the LED appear to be bright, there are other issues at play like the longevity of the LEDs.  It may be that a "high spikey" current going through the LED makes it appear to be brighter, but at the expense of the premature shortening of the life of the LED.  I am just speculating of course.  You have to assume that there are diminishing returns as you increase the current flow, and of course eventually heat dissipation becomes a problem.  It makes me curious about what the current waveform output looks like for a "real" LED power supply used for architectural lighting.  There are fixed and dimmable versions.  Will there be tell-tale inductive discharge spikes or will it look like DC?  What about a dimmable LED power supply?  I cheated and peeked at a chip.  It fancier than and better than my suggestion for the previous chip that Farmhand pointed out.  They do variable PWM, pulse width modulation, of a continuous fixed-frequency control signal that goes to the transistor base input.  So there is always a current pulse stream going to the LED, and the size of the pulses is controlled by the PWM signal.  That makes sense, whereas my suggestion for Farmhannd's chip was a bad kludge.

What's interesting is that there is a good chance that the inductor has continuous current flowing through it.  It either goes into the LED or goes into the transistor.  When it goes into the transistor the coil is being energized.  So the LED sees a full-blown pulse train that looks like a square wave going from no current (like "ground") to a certain level.  The LED never sees the current/voltage decay to zero.  That's because below a certain current level, the LED is useless and not doing its job.  So to be more efficient they avoid the decay to zero alltogether.

Even though I am talking about current all the time here, in this case you can look at the voltage across your LED on your scope.  You should (I hope) see a nice pulse train with sharp rise and fall times and a slightly slanted "hat."  That's telling you that there is a sharp on-off current pulse going through the LED at sufficient current level to light it up and no current is being "wasted" with the useless decay to zero.

The key to all of this is to use a relatively small inductor, not sure if there will be a core, and switch that sucker at a relatively high frequency, perhaps between 30 KHz and 60 KHz.  It's small and cheap and does the job.

MileHigh

P.S.:  You would have to be worried about your scope here and make sure that it's not grounded if you were going to peek at the waveform across the LED if in any way the LED was deriving its power from the mains.  Disconnect the third prong.  I assume that all scopes have their own isolation transformers for their main AC power input.  Honestly, I would still buy a real isolation transformer to keep handy all the time.

[Magluvin]

Found Data sheets Mags see last post. -..

I see.     Both indicate 3.3 to 3.9v.  Which is typical.    8000v max short pulse. Ive been blinded with science!!

Mags

[Farmhand]

Yeah wow ESD withstand voltage (HBM per Mil-Std-883D) - 8000 volts, maybe some step towards protection from energy weapons for mil-spec stuff, emp maybe.

Who would have thunk LED's could be so interesting and output so much light ? Lordy.

My little 501b LED torch almost outperforms one of those 10 million candle power Spotlights with a 7 amp hour battery in it.  Lot's easier to carry for sure, kilo's difference.

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[Farmhand]

Hi Pirate, I like those LED light globes. Cree seems to be on the cutting edge of LED tech.

Do they really make better light, or a more natural light allowing us to see more natural colors ?


One thing that annoys me is that most lights make white- LED's or yellow-incandescent or some other non natural good light level illumination, regular fluro's flicker at 50Hz.

Another thing is heat and toxicity, they say they contain no mercury, and going by the efficiency for illumination I am guessing they have almost eliminated most of the heat losses, so the light is only a light and not a heater as well. That's a big point to me. And no mercury, awesome.

I'ma gonna try to get some from the ebay.  I like lights, lights are part of the very core of what is is to be human, to control light artificially. 

Cheers

Curiosity killed the cat because it caused him to get in the way of a curious human.

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[Pirate88179]

Farmhand:

Yes, these led bulbs are great!  The color is good and the light is very good when run on the mains.  I am in the process of swapping my last surviving cfls (most of them burned out in about 3 months) to these bulbs.

I believe they even offer a version that has a coating on the inside of the glass to better represent "true" colors.  There is a bit of heat when using these on the mains power.  They have provided a nice heat sink but, I have added even more using an adult beverage (beer can) cut and mounted to the base.  This may not be needed but, it gave me something to do.

There is no heat at all when using them on my modified flash circuits....zip, zero nada.  Which I like.  Milehigh has provided a possible explanation as to why I see what I see when using high voltage/high frequency on these types of leds.  I really appreciate that and I need to re-read his post before responding.  This is what I like about this site, I can observe something and posit why I think what I see is happening.  Then, the smart guys that know a lot of stuff can jump in with their opinion.  This is how I learn.

Try a few of these bulbs....like $10 each at Home Depot (Here in the US)  See what you think.  I think you will like them.  I do gut the inside driver boards from them when running them from the JT circuits.  I am just using the leds themselves.  The best light for the buck I have found so far....but, there is a lot of work going on in this area and it will get even better.

I too am fascinated with light for some reason.

Bill

PS  I attached a photo of my heat sink design.  Again, probably not really needed but, when run on the mains, it can't hurt right?