Picture of The Joule Thief
The Joule Thief. I found this device on accident. I was searching for a WORKING decision maker and found this on accident. It can power many LEDs.

The inevitable: I make an instructable on: the Joule Thief.

This is a great circuit to teach children about electronics especially inductors and coiling considerind it contains so few parts and is so easy to make...

Let's get started.

To get us started, I have a video.

By the way: I had forgotten to put the arrows for the LEDs...sorry.

Step 1: Materials, Materials, Materials

Picture of Materials, Materials, Materials
You will Need:
3 ultrabright white LEDS
1 "dead" battery (with 1 volt left.)
1 2N3904 Transistor
1 jumper
1 handmade inductor or two pre-made. 
1 Solderless breadboard
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DHeigh1 year ago

I'm trying to make a Dodecahedron (12 sided polyhedron) Christmas ornament, with each face having an LED lighting it up. How do I hook 12 LED's safely into this circuit. Do I still use the 1K resistor?

The 1k resistor is technically optional because all it does is limit the amount of current that can flow into the base of the transistor (it's a good idea/practice to include the 1k resistor however). When adding in LEDs, all you have to do is wire all of the LEDs in parallel. Keep in mind that the 2N3904 transistor can only supply a limited amount of current (0.2A) so if you plan on using a large number of LEDs, you may want to use a transistor that can supply more current like the 2SC1384 (1-1.5A) or a darlington transistor like the TIP110 (2-4A). You may also want to try high voltage power transistors that are commonly found in CFL light bulbs (I've never tried using them) like the KSE13003 (1.5-3A).

Okay, another crazy question: can the 2N3904 be used in parallel to increase current capacity? Since the transistor is voltage controlled, being in parallel won't affect their switching on (altho, differences in sensitivity might).

What do you think?

blinkyblinky (author)  DHeigh12 days ago

The answer to this question is actually more difficult than I thought it was; I didn't know you could do such a thing until I researched it just now. However, in doing so you may have to be quite careful because one transistor will often try to draw a greater amount of current which will cause it to heat up, causing it to draw more current, causing it to heat up even more, etc. In order to try and resolve this, you can connect them in parallel like you said, but at the emitters of each transistor put a 0.6-1 ohm resistor.

It is probably better to just use a single transistor that can handle a greater amount of current. However, if you are in a rush it is a viable solution, just risky.

Thank you.

Buying the high-current transistors is not possible right now, but I have plenty of 2N3904 in stock.

Yep, just put how ever many LED's in parallel and choose the right battery capacity.

kc9cmt1 year ago

Have not had any success at all in replicating this experiment. I suggest that you need more that a 1:1 wind on the toloid

Treknology3 years ago
As you were winding your enamel wire in a pair, how many turns did you actually put through the core?
blinkyblinky (author)  Treknology3 years ago
The large one with the Green wire I had put as many turns that would fit (About 30) The smaller one I had tried to do the same but it didn't work out because I ran out of wire. Oh well, guess I have to get more.
So does that mean that you ended up with 2 * 15 windings or 2 * 30?
blinkyblinky (author)  Treknology3 years ago
1==please explain what is the gauze of the wire?
2==what is the diameter for the ring that you use?
3==magnetic ring or ferric core ring?
I would never use a magnetic toroidal core for a Joule Thief. The output might blow a hole in the universe, or just not work the way you planned it to work.
The Large Hadron Collider created fears of punching a hole in the fabric of the Universe and it hasn't done so, well, not so far as we've noticed. Although I've never seen a toroidal magnet (the round ones from Microwave ovens are face-to-face), I don't think that on a domestic level it achieve much apart from stuffing up the rest of the circuit.
blinkyblinky (author)  recuat3 years ago
1. I think it is AWG 24.

2. I used a 1 inch (2.535cm) diameter ring.

3. "Ferrite" not "ferric" core ring.
keitre2 years ago
how much amps and voltage can it produce? can it survive if i will use a 9v battery not totally drained?
blinkyblinky (author)  keitre2 years ago
Well, if the coil is wound correctly and everything is right then I would say it could produce somewhere around 20 volts or more (on one battery)

what do you mean when you say, "can it survive if i will use a 9v battery not totally drained?"
i mean, will the joule thief will not be burned out or overheat with the higher voltage from a 9v battery that is not totally drained? how about the amperage, how much can it reach?
blinkyblinky (author)  keitre2 years ago
I'm not exactly sure of the amperage but I do know that a 9 volt battery should work.
A 9v battery generally has a maximum amperage of 550 mAh, so chances are you can't get much more than 550 mA out of a 9v. As with most conversion circuits, boosting the voltage will cut the delivered current somewhat, but LEDs typically need much less than 550 mA to function, usually in the 20-30 mA range.
blinkyblinky (author) 2 years ago
To Whom It May Concern: For those of you who are worried about me not putting in a resistor, I did not put one in in the breadboard version but I put one in the final version so as to not harm the transistor...I was powering a lot of LEDs in the breadboard version so I did not put one in. However, in the final version I did include a small resistor in the circuit.
deeppu2 years ago
can i use any transistor and regulator
blinkyblinky (author)  deeppu2 years ago
Stick to using a small transistor and not a power transistor (the one with the heat sink and is rather large) and as for the resistor, you may want to put one in, in the final version I did put a small resistor in because I wasn't powering as many LEDs at once. You can put any resistance in that you like, as long as you consider the amount of time you will be using it so as not to harm the transistor. As for the regulator, well, I am not sure what you are talking about.
reopoop2 years ago
can someone help me.. i want to power a joule thief using a cellphone charger out 5v ~ 400 or 600mA.. is it posible or means it is good? and what resistor should i use? a 1k resistor or 2.2k resistor? i am not on electronics so pls. explain simple.. . im just copying on what i saw on other people.
blinkyblinky (author)  reopoop2 years ago
Well as always, a resistor helps to limiting current it always helps. What is your application for the joule thief or is it for fun and educational purposes? 

The short answer is this: The 1K resistor should be better...but this is one of the most counter intuitive circuits that I have encountered. Also, the five volt supply shouldn't make much of a difference but you should always check the voltage before you connect anything to it by using a volt meter. Experimentation is your best bet here. What parts do you have access to? The best way would be to purchase a solderless breadboard like in the ible and a potentiometer (1K or 10K...it doesn't really matter. 1K is limited but is very fine tuned. 10K is more rough in the resistance but has more values to try out. You can try both if you like.) Slowly rotate the potentiometer's wiper (the knob part) and measure it on a volt meter and see what voltage is the highest and maybe mark where it is the highest. Put the volt meter in series with an LED or, put it in series with a small diode (1N914, 1N4148, etc.) and then measure the voltage to find the optimal amount. You have to be a little careful that you do not overestimate the power of your Joule Thief because as soon as it picks up a load (anything that draws power) there may be a voltage and current drop (the voltage you read may not be what is output to your circuit.)

Hope this helps...Once again, what is your application for this circuit?
Nawaz2 years ago
Not sure if anyone commented on it already but the purpose of the 1K resistor to the base of the transistor is to limit the current to that pin and thus protecting the component. Nice instructable by the way.
vitalzero2 years ago
Mine doesn't work, doesn't even light up 1 single led with 1.5v, the strange thing tho, is that it works with a 3v BUT with reversed polarity, I tried 2 toroids, one with 7 turns and other with 15 and the same. I also did a transistor tester (one from here) and they work. Days ago tho, I modified it but don't remember how, the problem was that with a 1.5v button cell battery it worked but started to dim over time, and if I used a 1.5 AA battery, it flashed turned it off and the transistor started to heat up really quickly :( why?
blinkyblinky (author)  vitalzero2 years ago
You may need to add a small resistor like 470 ohm or something.
Thanks for the help, the problem was the freaking transistor that was labelled CEB instead of EBC, another issue is that with a resistor lits up and then goes off, I removed it and put a diode instead and lits up all the time, but is not very bright :(
blinkyblinky (author)  vitalzero2 years ago
Diodes create a voltage drop, the 1N400x (x generally varies from 1-7) have about 0.6 volts and the 2N3904 generally has a 0.3 volt drop so it might just need to much voltage. I am not sure about 1N4148 and the 1N914 but it is less; but still has some.

What transistor are you using?
Well, the issue was the transistor, the first one was a SPS 13001 found in a CFL lamp, I changed if for another bigger transistor also found in CFL (looks like a MOSFET) and lit up really bright compared to the last one. Also, tried a H1BJ13002 and 13003 (mje or something) small transistor also found in CFL and it's also bright. Now the issue with the resistor still remains so, I have the toroid connected to the base of the transistor with a signal diode and works like a charm. Now, if I try any modifications like in other instructables ( adding caps in parallel before the led) doesn't work and If I add more leds in parallel, all of them get dim, so I suppose I need more winding in the toroid (currently 7). Nothing gets heat up.
blinkyblinky (author)  vitalzero2 years ago
I am glad it worked out for you as well as the fact you took apart a CFL and reused it!

However, those transistors are designed for high voltage applications (maybe not the first one because it is small, but generally, the bigger a transistor the more it takes to drive it (turn it on) so you might want to try 2N3904 or BC547. Those seem to work out the best.

You see, I kind of found this out the hard way because I tried 2N5551 and thought, since it drives high voltage appliances I think a Joule Thief is an OK application...I had already built the joule thief on a breadboard and was now soldering but I got my hands on a few of those transistors and so thought, "Why not?" That was a mistake. I had failed to look around and only afterward did I realize that 1.5 volts probably couldn't drive it. I ended up having to desolder quite a bit.

rabitt2 years ago
hey men on the pic it show 11 leds and you make a intrucc of 3 leds... how i conect more leds... 20 for example...tnks
blinkyblinky (author)  rabitt2 years ago
You can connect them in series or parallel. See here: http://www.youtube.com/watch?v=x2EuYqj_0Uk
vivdal2 years ago
Enameled wire works very well.
Why the comment?
donmatos3 years ago
A cada dia que passa, vocês, experts, conseguem diminuir o número de componentes destes circuitos. Eu torço para que consigam eliminar a bateria e passem a alimentá-lo através do toróide. Parabéns. Se eu construir um desses (eu sou leigo) comentarei a façanha.
wei29123 years ago
Might be a pull up/down resistor :D
blinkyblinky (author)  wei29123 years ago
can i know how is the circuit go on this transistor? is that using the same circuit? KSD5041, KSC2500, 2SD965, NTE11
blinkyblinky (author)  acmefixer3 years ago
My sentiments exactly.
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