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Joule thief? Answered

Is there a joule thief capable of powering a 3 watt LED at full brightness? Or just over 300ma output average?

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acmefixer (author)2010-12-11

Nowadays it's cheaper to buy a JT for high power LEDs. At a little more than two dollars each, it's not worth making your own. Here's the one I bought from dealextreme.com. They have other versions, too.

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steveastrouk (author)2010-05-27

Power in = power out - losses.

If the source can supply the power, there is no intrinsic reason why a joule thief can't feed the led BUT IT WILL STILL NEED A CURRENT LIMIT>

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.Unknown. (author)steveastrouk2010-05-27

So far, the joule thief with the highest output I've built was the 2 transistor super charged joule thief from quantstuff (http://quantsuff.com/LED2.htm), but that hasn't even been able to power a 1 watt LED at full brightness.

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steveastrouk (author).Unknown.2010-05-27

So what's the maximum current you can pull from the power supply you tried it with. What voltage did the LED need to turn on ? 

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.Unknown. (author)steveastrouk2010-05-28

Maximum current I can get drawn from a AA battery is somwhere around .9A. The LED needs slightly over 3 V to light up.

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steveastrouk (author).Unknown.2010-05-29

So, if you can pull 1A from a 1.5V battery, you have 1.5W MAXIMUM available. Now you are trying to drive a 3W LED.

What's wrong with this picture ????

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.Unknown. (author)steveastrouk2010-05-29

I know, but what I'm trying to ask is whether there is another joule thief that draws more current and is able to? I think I've reached near the absolute maximum I have for this circuit.

  I have a luxeon MXDL 3 watt 1 AAA torch that can get a 3 watt LED up to over half of its maximum. I happened to open it up one time, but I can't make sense of it's circuit (boost converter?) It has a transistor, a diode, and a capacitor (all surface mount) and an inductor

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steveastrouk (author).Unknown.2010-05-29

Am I missing something here ? 

You CAN'T extract more power from the battery than it can supply, which is what you are asking to do. The battery has a property called internal resistance which limits the output current into a short circuit.

The only way to do what you want is to use different batteries with lower internal resistance.

Steve

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.Unknown. (author)steveastrouk2010-05-29

Oh, sorry, I thought you meant the maximum current  possible drawn by the circuit, not the max current that the battery can supply. I tried putting a 10 ohm resistor across the battery and measured the current. Turns out that the current drawn by the resistor is 4.7 something amps.

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steveastrouk (author).Unknown.2010-05-30

Switching power supplies critically rely on the inductor design to work well or at all, that, and the transistor needs to be driven hard into saturation, so that maximum voltage is applied to the inductor, that also means that the transistor needs to have a VERY low Vcesat. LIke I said at the outset, there's no intrinsic reason why it shouldn't work, now we know that the battery can handle the current you need.

Looking at quantsuff's site explains what you need to know very well.

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