Simple Miniature Joule Thief




Introduction: Simple Miniature Joule Thief

Hello, this is my first instructable and I'm going to show you how you can make a very small joule thief circuit that you can fit anywhere and can be used to power LEDs or any other device with nearly dead batteries. This simple circuit works from voltages as low as 0,5V.

Step 1: Parts List


soldering iron + solder

wire cutters

lighter (removing enamel from wire)

+ razor blade or hobby knife(removing enamel too)


ferrite toroid(broken CFL, motherboard...)

2N3904 transistor

magnet wire(I've used 0,5mm)

insulated copper wire(10 cm length is enough)

240Ω small 0,25W resistor

LED(I've used 10mm red, optional)

If you don't have 2N3904, then you can use 2N2222 or just any common NPN. For the base resistor 240Ω possible substitutes are any value between 200Ω-1000Ω

I'll be using molypermalloy powder core


Outside Diameter : 12.5mm

Inside Diameter : 7.5mm

Height : 5mm

Step 2: Winding Toroid

Start by taking two similar pieces of magnet wire. Normal insulated wire can be used too but takes more space and is less effective. You will need to know which wire is which when you finish winding. You can use magnet wire with different colors(like me) or you can use an ohmmeter. When you have the wire then you can begin winding.

1) Twist both ends of the wires (makes winding easier)

2) Wind the first turn (hardest part)

3) Continue by winding more turns.

4) Wind 8-12 turns.

5) Check that the wires are not crossing and every turn is close to the core.

6) After winding you will have 4 wires coming from the toroid as shown.

7) Take the two inner ones and strip the of insulation (or enamel) make sure they are different color and twist them or solder them together to form a center tap.

After you complete these steps, then you can proceed to testing.

Step 3: Schematic

In this schematic you can see that the circuit is really simple. I won't be explaining the working principle of the circuit here because the Internet is full of all sorts of videos and texts that can explain it better than me.

Joule thief explanation

Step 4: Testing Toroid on a Breadboard

After winding I recommend to test everything on a breadboard. Then you can just leave the joule thief like this on a breadboard or you can make a PCB for it or you can proceed to the next step where I will be showing how to stuff components into the toroid.

Step 5: Making It More Smaller

There isn't much to say here. Just stuff in all of the components into the toroid to make it smaller and thus usable in a small battery powered devices like for example bike light. You can do it your way or follow my instructions in images. Every time I do this i do it a little different.

Step 6: Try It Out

Solder some wires to your new miniature joule thief and you are done. You can also use hot glue to secure the components. Feel free to experiment with coils or even try to make it smaller or fit in a more complicated circuit made from SMD components.

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    6 Discussions


    5 years ago on Introduction

    With all these circuits, as soon as you add 10n to the join of the resistor and feedback and the other end to 0v rail, the current drops to 30%, making the circuit 300% more efficient.


    5 years ago on Introduction

    The efficiency of this circuit can be increased considerably by simply adding a capacitor.
    See Talking Electronics website:

    Joule Thief

    for full details and HOW THE CIRCUIT WORKS - which is very complex.


    Reply 5 years ago on Introduction

    Efficiency is always an important thing. I red that basic JTs are 40-60% efficient, so improving that is a must :)

    With the basic ~50% efficiency starting with a brand new battery, we use it down to 0.5V, but wasting half of its energy.


    5 years ago on Introduction

    The thing I love about Joule Thief is that it is such a simple circuit while still demonstrating some very complex electrical behavior.

    I've got a little more permanent example based on the same circuit if you want something the kids can play with: