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A "Joule Thief" is a simple voltage booster circuit. It can increase the voltage of power source by changing the constant low voltage signal into a series of rapid pulses at a higher voltage. You most commonly see this kind of circuit used to power LEDs with a "dead" battery. But there are many more potential applications for this kind of circuit.

In this project, I am going to show you how you can use a Joule Thief to charge batteries with low voltage power sources. Because the Joule Thief is able to boost the voltage of a signal, you are able to charge a battery with a power source whose output voltage is actually lower than the battery itself.

This lets you take advantage of low voltage power sources such as thermoelectric generators, small turbines and individual solar cells.

Step 1: Watch the Video

Here is a video walkthrough of the project.

Hi Thank you very much for helping everyone, i need to recharge a battery through home made generator in which we run the magnets in up and down motion between the coil, its like pedaling for generating electricity, and the output comes like 3.1v in first movement of the magnet through the coil and then it dereases rapidly to 0.could your great Joule thief circuit work in this type of output. <br>The output checked on the multimeter :-<br>First movement of magnet- 3.1v<br>Second movement- 2.7v<br>Third - 2.1v<br>So on..<br>Thank you so much in Advance.
This is not working
<p>a common mistake on this type of circuits: do you made connections accordingly with starts and ends of the windings?<br>points next of windings are starts, if you wire it &quot;normally&quot;, exactly as you wind it up, this circuit will not work.<br>so, you need to cross terminals of one of the windings, and it will work! :)</p>
My Zener diode is a 5.6 volt one
<p>What is voltage rating of 330 microfarad capacitor ? Where are positive and negative terminals of 330 microfarad capacitor connected ? What is the inner diameter and outer diameter of ferrite core ? How many Nickel-Cadmium cells can be charged at one time ? Thanks.</p>
<p>To be safe, you probably want the voltage rating of the capacitor to be at least 10V. The diagram in step 4 shows that the negative lead of the capacitor is connected to the negative output terminal which is also the common ground. the diameter of the core isn't critical as long as it is less than 1 inch. The more batteries that you want to charge, the longer it will take. So I recommend just charging one at a time.</p>
<p>Thanks for prompt reply. Kindly give a try to these links:https://www.youtube.com/watch?v=jKfB3p2UbtY ,https://www.youtube.com/watch?v=QP8PkgT1T9o, https://www.youtube.com/watch?v=7Mb1p5JE-mI and may be you can comeup with your new joule thief. Kindly use 2N2222 instead of 2N2222A. Once again thanks.</p>
You mentioned in an earlier commeny that the current of this curcuit would be too low to switch phones into charging mode <br><br>Is there any way to increase the current so that it can charge a phone that needs 2100mah???<br><br><br>Great instructables by the way thank you for your time :)
<p>There is no way to make a practical joule thief circuit that can produce 2100mah. A joule thief by its very nature is able to produce higher voltages because it is breaking the constant DC voltage into a series of pulses. This drastically reduces the current.</p>
This is not working.
<p>that should be fine</p>
<p>Is their a way I can contact you I have a lot of questons.</p>
You can send me a message on Instructables.
<p>Also my zener diode is 5.6 volts is that okay?</p>
<p>Can I use magnet wire on the toroid? I have all of the same components that are in the video but it still is not working. The capacitor does look different. Is this one okay?</p>
I think that the magnet wire might be your problem
<p>What gauge is the wire that your are using?</p>
<p>As a general rule, I would say to use the biggest wire that you can fit onto your torroid without over lapping the wires</p>
Also the first time I've etched a board.
<p>Awesome job!</p>
very nice. ill be messing around with this tomorrow. quick question. got 4 x 1.2V nimh batteries in series im going to charge. am I right in assuming that the batteries will handle been left in the charger permanently? I always hear you can leave nimh charging like that as long as the current is 10% of the battery Ah or less.ill be using a small solar cell.
<p>Generally speaking, when charging a NiMH battery, it is pretty safe to overcharge it a little as long as your current is less tan C/10 (capacity /10). The lower the current the better off you are. At C/20 (5% of the capacity) charge rate you can leave it charging indefinitely without causing much of a problem. A joule thief uses such low current that you could leave them in forever without problem. But because the current is so low, it will take a very long time to charge your batteries. </p>
<p>Hi..First of all a great instructable.</p><p>Now i wanted to know how could it be modify to drain energy for a 9 volt battery which now does not have enough energy to run a DC motor and then put it into another rechargeable 9 v battery </p>
<p>I don't think that the circuit should require any modification. But you do need to make sure that all the components are rated for at least 9 volts. </p>
<p>Hi. How do I modify this to charge Lipo batteries? The specs of the charge needed is 12V and 2.7A. Is this possible to do it here?</p>
<p>This kind of joule thief could only charge a battery very slow. So it would be very ineffective at charging a large LIPO battery.</p>
<p>I have another question though. So how much does this affect the current?</p>
<p>The current would be very low.</p>
I found the use of the thermoelectric generator to be interesting. If using a small input of less than .9 volts, is it possible to have a regulated voltage of 5 volts output? I built something similar but my output drops to 3 volts when trying to charge. Any ideas? Thx.
The output voltage will of the joule thief will always drop to the voltage of the battery when charging. Things that are directly connected always have to be at the same voltage. But this shouldn't prevent it from slowly charging a battery.
<p>could i use this to charge a portable phone charger? its input is 5v DC.</p><p>or would i need to put in a circuit to accumulate charge and put it out as DC?</p>
It would be possible as long as it doesn't require a minimum current in order to charge. Also keep in mind that it would charge very slowly.
can I use this to drain used batteries to charge a cell phone?
Probably not. Most phones require a lot of current to switch to charging mode. Joule thief circuits have a very low current output.
can i use this to power a 12v computer fan with a 3.7v 18650 or 5v powerbank? thanks
I don't think so. A joule thief only works for very low current applications. I don't think it could drive a fan.
Mine not working plz help
<p>What is it doing/not doing?</p>
I have seen that you have said that the ferrite toroid needs to be winded 10 times. But I already have a ferrite toroid winded 20 times for each winding with also enamel coating or magnet wire. <br>And is the zener diode necessary? <br>Please reply soon.
<p>The number of windings isn't critical. 10 is just the standard when you are making your own. Yours should work fine. The zener diode isn't 100% necessary. Just make sure that you always have a load connected. Otherwise the voltage can spike and damage the transistor.</p>
<p>what size wire and how many turns? Jeez!</p>
ten wraps of thin insulated wire
<p>so would 24 guage work, or would one need thinner?</p>
<p>0,5 square millimeters thick</p>
You need to wind it 10 turns. Use the biggest wire that you can and still be able to make ten turns around the torroid without overlapping the wires.
he said ten wraps around the torrid
Hi, <br>This is very interesting.... I will definitely try this. Can u please suggest me, what is the minimum current required for a &quot;joule thief&quot; circuit to boost the voltage of a 1.2 V source?<br>Thank you....
<p>This is very interesting. I will definitely make one for me. Can you please tell me what is the minimum voltage required from the power source to run this?</p>
That depends on what parts you are using but usually 1V is enough to make a joule thief work.
<p>can we use 3v zener diode</p>

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Bio: My name is Jason Poel Smith I am a Community Manager here at Instructables. In my free time, I am an Inventor, Maker, Hacker, Tinker ... More »
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