LED voltage question - Electronic Circuits Projects Diagrams Free

unseen wombat · 13th June 2008, 07:57 PM

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 · 13th June 2008, 08:33 PM

Joule Thief - Google Search

unseen wombat · 13th June 2008, 09:06 PM

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 · 13th June 2008, 10:08 PM

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

Hero999 · 13th June 2008, 11:36 PM

Make a Joule thief.

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

DigiTan · 14th June 2008, 12:15 AM

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 · 14th June 2008, 01:21 AM

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 · 14th June 2008, 05:49 AM

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 · 14th June 2008, 12:12 PM

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 · 27th June 2008, 10:39 PM

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 · 27th June 2008, 11:46 PM

Quote:

Originally Posted by 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?

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 · 28th June 2008, 12:23 AM

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 · 30th June 2008, 10:09 PM

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 · 30th June 2008, 10:14 PM

Quote:

Originally Posted by 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?

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 · 1st July 2008, 09:05 AM

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.

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13th June 2008, 07:57 PM	(permalink)
unseen wombat	LED voltage question 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? Last edited by unseen wombat; 13th June 2008 at 08:00 PM.

13th June 2008, 08:33 PM	(permalink)
Hero999	Joule Thief - Google Search __________________ I also post at the following sites: http://www.stop-microsoft.org http://www.heated-debates.com Screen name: Aloone_Jonez

13th June 2008, 09:06 PM	(permalink)
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?

13th June 2008, 10:08 PM	(permalink)
mneary	There are many designs called Joule Thief. Some have been put into very small spaces. Check several of the listed web pages.

13th June 2008, 11:36 PM	(permalink)
Hero999	Make a Joule thief. I wouldn't advise building some of the other projects on his site though. Make an tingle-tron. __________________ I also post at the following sites: http://www.stop-microsoft.org http://www.heated-debates.com Screen name: Aloone_Jonez

14th June 2008, 12:15 AM	(permalink)
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? __________________ The most-hated young engineer in America. * AVR Projects * Robot War: TI * About

14th June 2008, 01:21 AM	(permalink)
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. __________________ Uncle $crooge

14th June 2008, 05:49 AM	(permalink)
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. __________________ The most-hated young engineer in America. * AVR Projects * Robot War: TI * About Last edited by DigiTan; 14th June 2008 at 05:50 AM.

14th June 2008, 12:12 PM	(permalink)
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. __________________ I also post at the following sites: http://www.stop-microsoft.org http://www.heated-debates.com Screen name: Aloone_Jonez Last edited by Hero999; 14th June 2008 at 12:13 PM.

27th June 2008, 10:39 PM	(permalink)
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? __________________ The most-hated young engineer in America. * AVR Projects * Robot War: TI * About

28th June 2008, 12:23 AM	(permalink)
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. __________________ Uncle $crooge

30th June 2008, 10:09 PM	(permalink)
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? __________________ The most-hated young engineer in America. * AVR Projects * Robot War: TI * About

1st July 2008, 09:05 AM	(permalink)
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. __________________ The most-hated young engineer in America. * AVR Projects * Robot War: TI * About