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Joule Thief Charger

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Let your dead battery give life to another! An open circuit Joule Thief can put out 50 or more volts. Enough to charge a AA or AAA Nicad or NiMH rechargeable battery.
 
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Step 1: Make the Charger

Picture of Make the Charger
Use this schematic to build a standard Joule Thief circuit with the added diode.

My joule thief uses twisted network wire passed through a small ferrite core. I use 6 turns of wire. You can find a core from a burned out compact fluorescent bulb. You can see how others have wound the coil and built the Joule Thief, since so many have done so. Just add a diode and LED in series with the charging battery. The LED is useful as a charge indicator.

A high-speed schottky diode would be the most efficient. The 1N4005 was handy at the time and works.

Step 4: Charger in action

The donor cell on the left has a voltage of 1.057 volts. This battery is charging the battery on the right.

Step 7: First 8 hours

Picture of First 8 hours
Top red trace is the cell being charged. The cell voltage has stabilized and is accepting a charge.

Bottom blue trace is the donor cell. Notice how the voltage is slowly dropping off. The life force of the donor cell is slowly slipping away.

The Joule thief output jumped between two voltages and is a bit steppy. Nothing is perfect.

Step 8: Last Chart

Picture of Last Chart
The final 5.7 hours before I stopped recording data.

The charged cell is still receiving a charge and the donor cell now has dropped to about .62 volts. The Joule Thief is still running.
kyooby28 days ago

Hi.Can I light up 12 leds(bright light not low) with this circuit? I sow somebody on youtube hooking up 53 leds lightning for about 15 hours.Can this be true or fake?

argha halder8 months ago
its impossible because leds will burn to ashes at 50 volts and even the best joule thief with 6 turns on coil wont give 50 volts.may be you put the wrong dial on the voltmetre to measure.but except it is a great instructable.
botronics (author)  argha halder8 months ago
No, No you are so wrong. You can place an led in series on high voltage circuits as long as the current is limited and not reverse biased. Its done all the time in led light bulbs that run on 120 vac. The diode in series protects the led from any reverse voltage and the joule thief current is limited. The meter clearly shows 50 volts. I kid you not.
The high volts if from the zener diode you're using right? I have some different zeners I've tested and a few of them output about 50 volts too.
oh then sorry .i thought it to be mistake .but now i understood and thanks for both the comment and instructable.
rodan1956imod5 months ago
The network cable used for networking has 4 pairs. Does it make any difference if I use one of the 2 pairs used in networking (serial data comm) as I observed in your Joule Thief you use the blue / white blue pair which is for telephone connections. This pair is loosely twisted as compared to the 2 pairs used for networking.
harmhero1 year ago
Strange, my Joule Thief works fine, white LED is burning, but I get the same output voltage (without load) as my battery, 1.5 volts. Doesn't make sense really because the LED is burning with the JT but not with the battery.
How many turns go BEFORE the 6 turns?
botronics (author)  blinkyblinky2 years ago
You wind the turns through the core as a pair. So both are 6 turns.
So technically If I wond 30 turns would it make any difference?
botronics (author)  blinkyblinky2 years ago
The turns is not critical for it to work. The frequency will usually go down with more turns. Experimentation is in order.
Thanks. On my digital meter, when I measured and checked for voltage I got 0.1 volts less than the battery had. What happened?
namzer04 years ago
how much is the current draw? when not charging/open circuit and when charging?
You cannot measure the current draw in the open circuit only the voltage.
I am very interested in Joule Thief's and i am curious how to place a variable ohm resistor into the circuit? please reply asap.
agatornz2 years ago
i like the idea of this for hiking - using a combination of nimh rechargables and alkalines for my electronics - it would be great to be able to charge the former - is there some way you can include a simple "charge level indicator" in the circuit so one would know how well charge the rechargeable is?
botronics (author)  agatornz2 years ago
Your better off using a solar cell for charging. You can attach one to your backpack. A picaxe mcu could be used as part of a circuit to measure amp-hours when charging.
twighahn3 years ago
can u put this in layman's terms?
ab3t3 years ago
What if the batteries have a voltage of 3.7 volts used? What should be changed?
botronics (author)  ab3t3 years ago
Nothing, the battery voltage will adjust itself. Just feed it electrons.
mikedoth4 years ago
Stupid question but why doesn't the LED blow from the high voltage alone?
because your using a 1k resistor as well
I'm still learning but will a 1k resistor drop 52.6 volts down to 1.5 to charge the battery? How about a voltage regulator?
botronics (author)  mikedoth4 years ago
No regulator is needed if you are charging a battery.  The voltage is high because it is an open circuit, but the current is low.  When the battery is connected, the voltage will drop to the voltage of the battery. Same thing with a LED connected to the output.  The voltage drop of the led will keep the voltage down. Think of the LED as a zener diode.
more like a constant current source? ;)
btw.. what are better options than the 2n2222? I need something that generates 40v @ 20uA (the more the better ;P) from anything around 1 to 3.4v. Anything in this range, but I don't know how to start..
knektek3 years ago
is this like a campain for charging up batteries. 'All dead batteries please save a ni-cd's life today and donate 500 milli amps each month'.
so hwne the battery is charge dose the led go out ?
i have tried this and well the charge is false the voltage is right but there is never any ampage to run anything i have tried charging then running down over and over and dose not seem to improve or work
CyborgGold3 years ago
you should have done this logging with the rechargeable battery starting out dead... this one just hovers at full charge... actually it appears to drop slightly in charge.
botronics (author)  CyborgGold3 years ago
I discharged the battery into a load till its voltage was .9 volts. That is the recommended lowest you can go without damage to the cell. When a battery just sits there open circuit, the voltage will rapidly rise to a higher level, which it did. During charge the voltage will only rise slowly. The battery was no way fully charged. It was only a test to see if the circuit would raise the voltage and allow current to flow in a charging direction. Given enough time and dead batteries, the battery will charge.
flying pie3 years ago
circuit is too small and can u add a solar panel to the "charge donor " so u can charge it without using up a battery to charge a diffrent battery
Are you serious? The circuit is too small? Adding a solar panel would defeat the purpose. You're using a next-to-dead battery to charge another battery.
botronics (author)  A good name3 years ago
Next-to-dead batteries still have a lot of joules left. I have nite lites that run for months using next-to-dead batteries. We are just transferring whats left to something more useful.
You know you don't need that diode... the led is acting as a diode. Light Emitting Diode. Just a tip.
botronics (author)  tinstructable3 years ago
There might be a negative going spike as the circuit oscillates that can harm the led. I could just use a diode and forget the led, but the led acts as a indicator.
What is the frequency range for these types of circuits?I'm curious because if the frequency is high enough a fast type diode could be a better choice. Also it will help in determining the size of the capacitor if one decided to filter the output.
botronics (author)  electrofreak4 years ago
The frequency is in hundreds of kilocycles. Watson's e blog has a lot of information on joule thief circuits.
lolzertank4 years ago
Wouldn't the led eat up a lot of power? Assuming that its a red led (2v), then the total load is 0.7v (diode) + 2v (led) + ~1.3v (battery) = 4v * (your charge current). Without it, it would be 2v * (your charge current) total, increasing the efficiency dramatically. Of course, since you're using dead batteries, this might not be important at all.
 Shottky diode shouldn't be .7V. Probably could use a germanium too. Either gives you capability of working with lower input voltage. personally, I'd get rid of the LED too (maybe have a pushbutton to see if it's working. You could also pass it through a capacitor to remove the DC component and the try a full wave rectifier. Might get better results....I'm not sure.
yourdiyguy4 years ago
I am not able to get this kind of voltage out of my JT what could i be doing wrong? My meter always shows the around the same voltage of the battery I am using. I am able to get the LED to light up. Could it be the type of meter im using?
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