How to Make a Joule Thief





Introduction: How to Make a Joule Thief

A Joule Thief (JT) is a step-up voltage transformer based on the working mode of a PWM (Pulse Width Modulation), it produce a oscillation in a inductor with the help of a transistor (2N3904, 2N2222, ...) then the output of the inductor is your new voltage. The result is that you can light a white, blue or even a ultraviolet LED with a single 1.5V cell (AA, AAA or any type of cell, i have used a watch cell too). This "voltage booster" circuit is not very efficient but it can use batterys that are near depleted. I whud not power up a microcontroler are anything that whud be disturbed by the voltage and power oscillation of the output, there are some dedicated step-up voltage boosters like the IC from Maxim with a 90% efficiency. With a bigger inductor and another transistor it is possible to get some very high voltage, i have see on the web some JT that can light up a neon tube from a 12v bike lead acid battery.

Step 1: Tools

Solder iron, Dremel, Hot Glue, Cutter or Exacto Knive and ... a pair of hands .

Step 2: Parts

- 1 toroid made from ferrite (it can be buyed or recovered from a old PC mainboard or a PC power supply)

- 1 meter of enamel coated copper wire with a diameter of 0.5mm (i got my from a old transfomer)

- 1 transistor (any NPN transistor like 2N3904, 2N2222, ... read the datasheets) i will use a 2N3904

- 1 resistor 1kohm brown-black-red (a 1k variable resistor whud make it easy to "tune" the JT for lower voltages)

- 1 pcb of 1cm X 1cm is the smallest (my is 1cm x 2,5cm for the 2 screws to fix it, and is recovered from junk)

Step 3: Winding the Toroid (transformer)

The circuit runs at up to 30 to 50,000 cycles per second, so the transformer needs to be made from a material suitable for those frequencies. Fortunately, small ferrite beads used for electrical noise suppression etc are readily available and work well for this task.

To wind this, you take the thin enamel coated copper wire and fold it in half. Now, pass the folded end through the core most of the way so that the two free ends stick out about 40mm from the core. Glue these to the core side-by-side and let the glue set. Hot melt glue makes this simple and fast, but make sure you only use a tiny dot of glue. Now, wind 20 turns onto the ferrite core in a bifilar fassion (well, you have to really as you are using two-core wire!) and once finished, spread the turns evenly around the core and glue the other end (the folded end) of the wire to the core to stop it unwinding. The best way to do this is to put a tiny squirt of hot melt glue into the hole of the ferrit core. This stops the windings moving or unwinding. Try to keep the turns as tight as possible against the core, but small gaps between the wire and the core are ok. Now you need to cut the folded end of the wire so that you have two separate windings on the core. Trim the wire ends
to about 20mm in length and remove the enamel coating. Once the wire ends are stripped, use a
multimeter to find the two ends of each winding. They become windings A and B. Wire ends A1 and B1 start at the same place on the transformer, as do A2 and B2.
Another way to wind this .
2N3904 Datasheet (pdf)

Step 4: Observations

1. My JT is made for the purpose of light a single white LED, thats why i have experimented with different sizes of the toroid and different types of wire until i got the smallest battery drain, 17mA. (my first toroid i made got a healthy 45mA).

2. JT can drive multiple LED in series, puting them in parallel is a bad idea, it will only light a small number of them and the drain will increase unnecessary.

3. Read the datasheet of the transistor you use and do not go over the max power and intensity specified there or it will burn the transistor.

4. Adding a ceramic capacitor (104 or smaller) in parallel will drop the LED drain and it can then light up more LEDs. Putting a small ceramic capacitor in parallel with the resistor will make it work with lower input voltages. Lowering the resistor value from 1kohm to less can make it work with lower voltages too. Be aware not to go over the max values after you lower the resistor.

5. The pictures are made with a infrared camera (is the only i have that has a acceptable macro mode) and that is why the colors are strange.

6. Efficiency calculation : see the picture ... (i hate math :P )



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    18 Discussions

    It is said that you got your toroid from a dead cfl. Have you tried smaller toroids? Like half the size of your cfl toroid? (Just thinking ahead for a peoject where space is at a premium.)

    1 reply

    I had a even smaller one, the outer diameter is 5mm, but it is a pain in the aXX to wind it. :D it works well, the small one, i even used just a fer-rite rod and it worked too.

    A 555 ic, can do that mutch better for that, hope you dont belive the "overunity bla bla" if you drive a bigger load the power drain is bigger on the battery. A JT can drive a cfl easy if it is made for that purpose. Some ppl used the flyback transformer as inductor for a jt, but you need a better transistor for that, not the usual 2n3904 or 2n2222.

    There are some experiments, with a ferrite bar, nai, wood and even air core, but the efficency will be dramaticaly decreased ...

    what i mean is to completely removed the toroid and boost the current with just the transistor

    Ohh you meen a 1 transistor amp, that is not the same as a JT, in JT's the transistor and the inductor are used to make a oscilation, and the resultants "spikes" are used to light up the LED. The gain of a single transistor amp is not tha hight as a JT. Dont mix audio amps with PWM.

    As long as it is a diode it is ok, if you want to feed the voltage to somthing else, keep in mind that the voltage it is not DC and it has some big freqvency, i dont recomand to try to power somting that can be damaged easly

    if the voltage is not in DC, is it in AC? My idea is to connect the joule thief to my created earth battery to charge a cell phone that use voltage under 3volt. Can it be done? Please help me.... thanx

    Hi,I'm new to the electronics thing and I was wandering if I could use a LM317 transistor instead of the 2N3904, I live in S.A. and cant realy find a 2N3904. Can someone please direct me to site from which I can learn and understand more about electronics,tnx

    3 replies

    A LM317 is not a transistor, it is a 3-Terminal Adjustable Regulator ( datasheet :  ) but i used some outher transistors too, like 2N2222 or BC547C, even a unknown transistor i recovered, it whas fit in a heat sink and i cudent get his name (i used my multimeter to get HFE value and pinout). If you have not found any of the transistors then just experiment with things you can recover from electro junk :) , read the datasheets to know the pinout of them, easy way put on google the name of the transistor folowed by the word "datasheet" (ex. "BC109 datasheet").

    Watson im a big fan of your blog, i find really usefull the material you post on there. I use the 2cm external diameter cores that i got from AT and ATX PC sorces and from CRT monitors, i got my hands on a big number of them i "cleande" them of usefull things b4 sending them to recycle. The wire i use is standard emaeled and the size is around 1/3 of a mm (sorry i used only a ruler, dont have better measurement devices). The telophon wire is not really good for JT, in my view, it jst take too mutch space when winding the toroid. If you have a email or msn i whud enjoy to talk more about electronics with you (send me a pm with contact info if you wanna)

    You are absolutely right aobut not going over the maximal voltage of the transistor, thats the same as going over the maximal amperage of the transistor. Shure you can add up leds in series and in parallel, heven as combo. I dont understand why ppl need to light up 20 leds, you can make enough light with max 3 ultra bright leds too. The ideea of a JT, in my own view, is to light up a led with with somthing thats is SMALLER that 2-3 AA or AAA batterys in a holder and some leds. I like to have a small device that helps up and that use "dead" batterys.

    In my new instructable (hope i will upload all stuff this week) i gonna show ppl how to have a JT with a battery drain of 2.60 mA.

    How would a C9013 transistor do in this? I'd like to see a more comprehensive list of transistors that would work, just for those of us whose sources for components are broken childrens' toys.

    2 replies

    C9013 (100Mhz) i think it whud work ( datasheet here : if it dont oscilate then try to lower the resistor value at the base of the transistor or add a small capacitor (a ceramic capacitor like 104 ... as a thumb rule is better get a biger Mhz value and try to get a transistor with the voltage range you need

    The transistors i listed are very cheap the 2N2222 (250Mhz) in plastic package is realy cheap, and the 2N3094 (300 Mhz)