LED voltage question

[unseen wombat]

I have one of those mini LED flashlights. It has a white LED, which has a turn on voltage of 3 something volts, right? Yet the flashlight runs on a single AAA battery, which only supplies 1.5 volts, right?

How is this possible? What kind of black magic is going on inside my flashlight? And, can I exploit it to power several LED's in my circuits without having to use a 9V or 12V battery?

 

[Hero999]

Joule Thief - Google Search

 

[unseen wombat]

Thanks man! You're awesome.

Is there any way to make it smaller? Like is there an IC I could buy? Also, isn't the toroid just acting as an inductor? Could I use an inductor instead?

 

[mneary]

There are many designs called Joule Thief. Some have been put into very small spaces. Check several of the listed web pages.

 

[Hero999]

Make a Joule thief.

I wouldn't advise building some of the other projects on his site though.
Make an tingle-tron.

 

[DigiTan]

What limits the output voltage range on these? I mean, if you started putting LEDs in series, could this also provide 6V, 9V, 12V and so on?

 

[audioguru]

The Joule Thief has a very small output current so the single LED is lighted but it is not bright.
There are powerful voltage stepup iCs that are made for very bright flashlights.

 

[DigiTan]

Yeah, I've been using Maxim-IC step-up converters. But I mean, can I also get 18V or more from a joule thief? If these are capable of x12 multiplication or more, I'd like to know how they do it.

 

[Hero999]

It depends on the Q of the core, I'd guess you can get 10V to 15V maximum and only at a very low current.

The efficiency will also fall as the output voltage rises.

 

[DigiTan]

Okay, something about this is jacked up because I tried this with a 10mm white LED and not absolutely nothing. Even the tiny red ones barely got any illumination and I tired two transformers. Will just any toridal transformer work as long as it has 1:1 coupling?

 

[Torben]

Okay, something about this is jacked up because I tried this with a 10mm white LED and not absolutely nothing. Even the tiny red ones barely got any illumination and I tired two transformers. Will just any toridal transformer work as long as it has 1:1 coupling?

I built a couple of these: 1.5 VOLT WHITE AND UV LED DRIVE CIRCUIT (scroll down to "A Circuit Enhancement") and they worked great. It's just a variation on the Joule Thief idea. I used the 20-tap-20 windings on generic small ferrite toroids using 30 gauge magnet wire as suggested. One fully charged NiMH AA cell ran a 3V white LED for just about 7 days straight before it dropped to about 1/2 brightness.


Torben

Edit: I found a photo of the thing. The tape is just to hold the 30 gauge wire and AA cell in place; the big plastic spike was just over the LED for giggles.

 

[audioguru]

The Joule Thief operates the LED at a low current so it is not as bright as it can be.
Its brightness might be bright enough for what you want.

 

[DigiTan]

By the way, when people are building this: is anyone using breadboards? Normally I won't solder this kind of thing until I've seen it function on a breadboard first. But the the board's stray capacitance is in the way...well...does this also work on breadboards?

 

[Torben]

By the way, when people are building this: is anyone using breadboards? Normally I won't solder this kind of thing until I've seen it function on a breadboard first. But the the board's stray capacitance is in the way...well...does this also work on breadboards?

Mine works just fine on the breadboard. The only component I found that was all that important was the transistor, and it was perfectly happy with 4401, 3904, 2222, and so on, so even that wasn't terribly important.

Again, the one I built wasn't exactly the Joule Thief, but as far as I know it's essentially the same idea.


Torben

 

[DigiTan]

Good call one the modified version. That one is working for me and everything seems to work at max brightness. I used a 2N3904, a 1N4148, and a ceramic 0.1uF capacitor in parallel with different LED variations. My 1watt XLamp was bright enough to provide reading light to a 10x10x10ft room. That's the brightest I've seen it since I bought it over a year ago.

 

[DigiTan]

By chance, has anyone had any luck powering logic with this? Anything like glue logic, 555's, or microcontrollers?

 

[unseen wombat]

I tried to power a 556 with it, but no luck. I was only getting 3 volts out of the joule thief though, so if you have a better toroid, then you might very well be able to do it.

I saw your site, and I'm trying to learn AVR too! Sh*t is hard! Your stuff is amazing though. I like the PS2 controller project. I'll be happy if I can just get some LED's to flash.

 

[audioguru]

The minimum supply voltage of an ordinary 556 is 4.5V and its maximum supply current is 6mA without a load. A Joule Thief probably won't drive it.

The minimum supply voltage for a Cmos 555 is 1.5V and its current is almost zero. A Joule thief will drive it perfectly.