Joule Thief

[Pirate88179]

From Wikipedia:

Power sources

Main article: LED power sources

The current–voltage characteristic of an LED is similar to other diodes, in that the current is dependent exponentially on the voltage (see Shockley diode equation). This means that a small change in voltage can cause a large change in current. If the maximum voltage rating is exceeded by a small amount, the current rating may be exceeded by a large amount, potentially damaging or destroying the LED. The typical solution is to use constant-current power supplies, or driving the LED at a voltage much below the maximum rating. Since most common power sources (batteries, mains) are constant-voltage sources, most LED fixtures must include a power converter, at least a current-limiting resistor. However, the high resistance of 3 V coin cells combined with the high differential resistance of nitride-based LEDs makes it possible to power such an LED from such a coin cell without an external resistor.[

They do not mention that you can use high voltage and high frequency to light LEDs easily.  Very little power at all and the voltage does not burn them up.  I have no idea why this works like it does, it is something we discovered way back and I am still working on it.

Bill

[SeaMonkey]

If it's also a fake, WTF do people do that?

Fair question which does deserve a stab at
an answer.

The world of youtube has become very
competitive and members will often do
whatever it takes to increase "hits" or views.

Thar's Gold in them thar hills!

That be my take on it...

I have no idea why this works like it does, it is something we discovered way back and I am still working on it.

MileHigh has addressed this in prior discussion
and explained how and why pulsed LED operation
is advantageous within reasonable limits.

Hopefully he'll do so here too.

[Farmhand]

Here's an alternative to the "HexLight" "Open Source" programmable torch.

It's called "Nextorch" - Mytorch. Here they are on special for less than half price.
http://www.batteryjunction.com/nextorch-mytorch-18650-r5.html

Product page.  Not open source though I don;t think.
http://www.nextorch.com/mytorch/en/main.html

Video some waveform and info towards the end.
http://www.nextorch.com/mytorch/en/gallery/myTorch.swf

They are available on the ebay as well. Similar price as the HEXLIGHT, the HEXLIGHT looks un-mountable though.
http://www.ebay.com.au/itm/NEXTORCH-P8A-Cree-XML-U2Torch-660-Lumens-Rechargable-Aust-Seller-Flashlight-/251376258065?pt=AU_Sport_Camping_Hiking_Lamps_Torches&hash=item3a87314c11&_uhb=1

..

[Farmhand]

I guess like regular diodes LED's could be made to be designed better for different uses like high frequency high brightness (strobe) on off that we can't perceive type use, or for high power constant current DC operation. 

These 7.4 volt LED's in these LED flashlights, be they Cree or otherwise seem to be very good. Can anyone give an evaluation of what they can take ?

..

[MileHigh]

Bill:

As you saw from the diode/LED IV plot, i.e.; current vs. voltage, a diode can't sustain high voltage across it because it would instantly burn up.  In conventional circuits you can think of a diode as a device that "clamps" the voltage across it to about 0.6 volts, or whatever forward voltage for the particular device.  That's called the "typical forward voltage."  As you can see from the IV plot, the actual forward voltage slowly increases as the current increases.  There are many different types of diodes and LEDs.  When you dissect an LED light bulb, they may be wired in series on the PCB.  So by the same token you add all of the clamp voltages in series.  But the designer of an LED power supply will forget about that all together and design the power supply for a given current output.  By looking at the maximum wattage of the power supply and the rated current output you can determine how many LEDs it can drive in series by adding up all of their voltage drops.  Suppose you know the power supply can drive nine particular LEDs in series.  So you can put between one and eight LEDs in series and hook it up to the current-output power supply and not even think about it.   In a sense there is nothing to measure or check as long as you know that you are not putting too many LEDs in series and overloading the power supply.  With a conventional voltage-based power supply it's the opposite, putting more LEDs in series will make the LEDs dimmer and then go out completely.  Like all current-source power supplies, it will shoot up to its maximum voltage output if you disconnect it whereas a voltage-source power supply does nothing, it just keeps outputting the same voltage.

So it's likely you never observed LEDs connected across across short pulses of very high voltage.  It's much more likely that you saw that high voltage before you actually connected the LEDs.  Then the voltage dropped to the forward voltage(s) of the LEDs when you connected them.

MileHigh