Introduction: Joule Thief Low Voltage Battery Charger

Picture of Joule Thief Low Voltage Battery Charger

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.

Step 2: Materials

Picture of Materials

Here are the materials and tools that you will need to complete this project.


ferrite toroid core

Insulated Wire

NPN Transistor (2N2222, 2N3904, or similar)

0.01 microfarad Capacitor (capacitor code:103)

330 microfarad Capacitor

1 kohm resistor

6V Zener Diode


Printed Circuit Board or Perf Board

PC Board Screw Connector


Soldering Iron

Wire Strippers

Wire Cutters

Step 3: Background: How Does a Joule Thief Work?

Picture of Background: How Does a Joule Thief Work?

This circuit used in this project is a modified "Joule Thief." A Joule Thief is a self-oscillating voltage booster. It takes a steady low voltage signal and converts it into a series of high frequency pulses at a higher voltage.

Here is how a basic Joule Thief works step by step

1. Initially the transistor is off.

2. A small amount of electricity goes through the resistor and the first coil to the base of the transistor. This partially opens up the collector-emitter channel. Electricity is now able to travel through the second coil and through the collector-emitter channel of the transistor.

3. The increasing amount of electricity through the second coil generates a magnetic field that induces a greater amount of electricity in the first coil.

4. The induced electricity in the first coil goes into the base of the transistor and opens up the collector-emitter channel even more. This lets even more electricity travel through the second coil and through the collector-emitter channel of the transistor.

5. Steps 3 and 4 repeat in a feedback loop until the base of the transistor is saturated and the collector-emitter channel is fully open. The electricity traveling through the second coil and through the transistor are now at a maximum. There is a lot of energy built up in the magnetic field of the second coil.

6. Since the electricity in the second coil is no longer increasing, it stops inducing electricity in the first coil. This causes less electricity to go into the base of the transistor.

7. With less electricity going into the base of the transistor, the collector-emitter channel begins to close. This allows less electricity to travel through the second coil.

8. A drop in the amount of electricity in the second coil induces a negative amount of electricity in the first coil. This causes even less electricity to go into the base of the transistor.

9. Steps 7 and 8 repeat in a feedback loop until there is almost no electricity going through the transistor.

10. Part of the energy that was stored in the magnetic field of the second coil has drained out. However there is still a lot of energy stored up. This energy needs to go somewhere. This causes the voltage at the output of the coil to spike.

11. The built up electricity can't go through the transistor. So it has to go through the load (usually an LED). The voltage at the output of the coil builds up until it reaches a voltage where is can go through the load and be dissipated.

12. The built up energy goes through the load in a big spike. Once the energy is dissipated, the circuit is effectively reset and starts the whole process all over again. In a typical Joule Thief circuit this process happens 50,000 times per second.

Step 4: Modified Joule Thief That Can Act As a Battery Charger

Picture of Modified Joule Thief That Can Act As a Battery Charger

In order to make a battery charger, I made a few changes to the standard Joule Thief Design.

First I added a capacitor to the node between the resistor and the first coil. This helps to stabilize the output a little.

Then I added a zener diode to the base of the transistor. This helps to protect the transistor from being damaged by voltage spikes. The Emitter-Base junction is the weakest point of the transistor. Most small NPN transistors will have a maximum allowable Emitter-Base voltage of 6 volts or less. So I added a zener diode between the base and the collector of the transistor. The diode prevents the Emitter base junction from becoming reversed biased. It would

At the output of the second coil, I added a diode. This allows the output voltage to pass through but it prevents electricity from the battery draining back through the transistor.

The capacitors and the zener diode also help protect the transistor from high voltage spikes that can occur if the circuit is turned on without a load. The voltage of the second coil will jump up as much as it needs to in order to be discharged. If there is no load attached, the coil voltage can reach over 60 volts. This could quickly damage the transistor. The zener diode and the capacitors help to limit these voltage spike.

Step 5: Wind the Toroid

Picture of Wind the Toroid

The transformer in the circuit is made by winding wire around a ferrite toroid. These toroids can be purchased from electronics suppliers or they can be salvaged from old electronic equipment such as power supplies.

Take two pieces of thin insulated wire and wrap them around the toroid ten times. Be careful not to overlap any of the wires. Make the wires as evenly spaced as possible. To hold the wires in place while I was prototyping, I wrapped the toroid in tape.

Step 6: Prototype the The Circuit on a Breadboard

Picture of Prototype the The Circuit on a Breadboard

It is always a good idea to prototype any circuit on a breadboard before soldering it together. This lets you check to make sure that all of your components are working. It makes it easy to test the performance of the circuit and make any necessary changes.

Step 7: Solder the Circuit Together on a Circuit Board

Picture of Solder the Circuit Together on a Circuit Board

In order to make the finished circuit as small as possible, I soldered the pieces onto a perf board. This kind of circuit board lets you position the components anywhere on the board that you want and make custom connections between them on the bottom side. When I was done soldering, I trimmed the board around the components.

To make connections at the input and output of the circuit, I used PC board screw connectors. This lets you easily connect a variety of components to the input or the output. You can use whatever power source or battery you want to work with. To avoid mixing up the terminals, I recommend labeling the positive and negative terminals of each one and label them as either input or output.

Step 8: Select a Power Source

Picture of Select a Power Source

You can use any use any low voltage DC source to power the Joule Thief charger. You can use solar cells, hydrogen fuel cells, thermoelectric generators, or small DC generators. You could hook it up to a wind turbine. You could even setup a small generator connected to a hamster wheel. Use your imagination.

The source of the electricity doesn't matter. However, the input voltage will affect how high the output voltage can get. With the components that I used you will get the best performance from power sources that are between 0.9 volts and 2.0 volts (with a maximum at 1.50 volts). Below 0.9 volts, the circuit will have difficulty boosting the voltage to a high enough to effectively charge a battery. Above 2.0 volts the output voltage will can start to get to high and it will be limited by the zener diode that is protecting the transistor.

Step 9: Use You Jewel Thief Charger to Charge Batteries

Picture of Use You Jewel Thief Charger to Charge Batteries

This charger can easily charge battery packs up to 10 volts. However, keep in mind that in order to boost the voltage, the current drops significantly. So it will take a while to charge a battery. Because it charges at such a low current, it can't effectively charge devices like cell phones because they need a certain amount of current in order to switch to charging mode. The best kind of battery to charge is a simple battery pack made of either NiMH or NiCd batteries.


viktor.bozicevic.9 (author)2017-12-11

+DIY Hacks and How Tos Hi, what is the max. current output of this circuit given the minimum voltage drop on the input? Even if it's around 100 mA, could it be used for trickle charging of usb devices? For example, my Sony xperia mini phone draws around 500 mA at 5 V when normally charged.

I don't know the specifics of this device. But all joule thief circuits have very low current output.

Thanks for the reply!

mxx (author)2017-09-18

Excellent and useful instructable for us electronic noobs. Also the best explanation I've ever come across of how a Joule thief works. I'm going to try this with a small wind turbine. Thanks!

jompon2547 (author)2017-08-26

can i use anther transister?

profort (author)2017-07-11

can any of these parts be harvested from other machines?

I imagine almost all of them could.

tesladevices (author)2017-06-29

Would a capacitor charged to 1.3v run this joule thief?

It depends on the capacity of the capacitor. But not for very long.

atiq-urR (author)2016-08-09

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.
The output checked on the multimeter :-
First movement of magnet- 3.1v
Second movement- 2.7v
Third - 2.1v
So on..
Thank you so much in Advance.

205330 (author)2016-03-28

This is not working

mistrije (author)2053302016-05-18

a common mistake on this type of circuits: do you made connections accordingly with starts and ends of the windings?
points next of windings are starts, if you wire it "normally", exactly as you wind it up, this circuit will not work.
so, you need to cross terminals of one of the windings, and it will work! :)

205330 (author)2053302016-03-28

My Zener diode is a 5.6 volt one

MIGHTY BOMBER (author)2016-04-29

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.

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.

Thanks for prompt reply. Kindly give a try to these links: ,, and may be you can comeup with your new joule thief. Kindly use 2N2222 instead of 2N2222A. Once again thanks.

daphnem1 (author)2016-03-30

You mentioned in an earlier commeny that the current of this curcuit would be too low to switch phones into charging mode

Is there any way to increase the current so that it can charge a phone that needs 2100mah???

Great instructables by the way thank you for your time :)

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.

205330 (author)2016-03-28

This is not working.

that should be fine

205330 (author)2016-03-24

Is their a way I can contact you I have a lot of questons.

You can send me a message on Instructables.

Also my zener diode is 5.6 volts is that okay?

205330 (author)2016-03-26

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?

I think that the magnet wire might be your problem

What gauge is the wire that your are using?

As a general rule, I would say to use the biggest wire that you can fit onto your torroid without over lapping the wires

gmccusker made it! (author)2016-02-07

Also the first time I've etched a board.

ThothLoki (author)gmccusker2016-02-19

Awesome job!

gmccusker (author)2016-02-05

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.

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.

Adhiraj S (author)2016-01-26

Hi..First of all a great instructable.

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

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.

ultimatekey (author)2016-01-17

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?

This kind of joule thief could only charge a battery very slow. So it would be very ineffective at charging a large LIPO battery.

I have another question though. So how much does this affect the current?

The current would be very low.

fr3dd4 (author)2016-01-05

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.

BrooklynT (author)2016-01-01

could i use this to charge a portable phone charger? its input is 5v DC.

or would i need to put in a circuit to accumulate charge and put it out as DC?

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.

JamesV32 (author)2015-12-25

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.

ryawan (author)2015-11-02

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.

Ujwal Bhagat (author)2015-10-03

Mine not working plz help

What is it doing/not doing?

FaizulI (author)2015-10-02

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.
And is the zener diode necessary?
Please reply soon.

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.

toby7 (author)2015-02-05

what size wire and how many turns? Jeez!

About This Instructable




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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