How a Joule Thief Works

Step-by-step run through of how a Joule Thief circuit works. Includes how all the parts, the 1.5 volt AA battery, the resistor, the transistor and the ferrite core with its two coils of wire work together to build up energy in a magnetic field which then collapses to produce enough voltage and current to light an LED (Light Emitting Diode.) This includes explanations of the feedback that rapidly opens the transistor between base and emitter to open up the emitter to collector too. And also the feedback that slams the transistor shut again.

See also this video on How to Make a Joule Thief (called Make a Joule Thief for Zombie Batteries):

For how to make a joule thief power a compact fluorescent light (CFL) watch "How to Make Joule Thief Light a CFL – Jeanna's Light":

For all sorts of measurements and demonstrations of my joule thief powering a CFL watch "Fun with Joule Thief Powering a CFL":

And also this webpage about Joule Thiefs:

Follow behind-the-scenes on:

33 thoughts on “How a Joule Thief Works

  1. Jay H

    The Transistor in a joule thief, just acts as an automatic on/off switch,
    while the 2 coils act sort of like a rechargeable battery. So the magic
    isn’t really the Transistor, the MAGIC actually happens with those 2 coiled
    wires. So, it starts out with a (street Traffic) Red Light. The wire coils,
    act like traffic getting backed up at a red light (storing the extra energy
    in a magnetic field). The Transistor is the traffic light, which waits for
    enough cars waiting in line, before turning to a green light, which
    releases the cars, while also blocking traffic from other directions. The
    Transistor does not really amplify energy, instead, the wire coils amplify
    the energy and the transistors only job is to be forced open from excessive
    electricity built up, which quickly drains, which forces it back closed.
    Without the transistor, the Gate/Switch/Traffic-Light would have to be
    MANUALLY toggled by a human (like jiggling the second wire on/off the
    negative). I made one without a transistor, and successfully lit an LED
    with 1 AAA battery. This is a horrible picture, but I didn’t expect it to
    work, so, forgive me for the bad image quality and having nothing labeled.

  2. RimstarOrg

    +Vignobles Lac Saint-Jean I’ve used it for a number of things, which I’ve
    shown in other videos. The most frequent use is to power an LED using a
    battery whose voltage would otherwise be too low to meet the LED’s minimum
    required voltage. I show that one in my “Make a Joule Thief for Zombie
    Batteries” video Then I found
    you can power a CFL using 2 AA batteries by modifying the coils And then I learned how to
    modify the coils again to use this to transmit electricity wirelessly Don’t be misled by the “thief”
    in the name. This is just what’s also called a blocking oscillator circuit.
    It provides higher voltage and current by repeatedly building up energy in
    a magnetic field and then releasing it in a short burst. The higher voltage
    and current exist only during the brief burst. The total energy out is
    smaller than the energy in.
    PS There’s no Reply button under your comment because of your Google+
    – go to your Google+ page,
    – in the top, right corner click on your thumbnail icon,
    – in the popup that appears, click on “Settings”.
    – for the 2nd question down “Who can comment on your public posts?” set it
    to “Anyone”.

    1. RimstarOrg

      +Rohan Zener If you’re referring to just the coils and the toroid core then
      it’s one-to-one, no transforming would take place since both coils have the
      same number of turns.
      If you’re talking about he whole joule thief circuit being x2, then no,
      it’s much more than that. Also, the output is neither DC nor AC. You can
      see the output waveform in my video about how to make it at 0:38 Here a 1.5 volt battery is on
      the input and the output is the waveform shown with a peak-to-peak voltage
      of around 24 volts and a frequency of around 22 kilohertz.
      Both the voltage and frequency are affected by a number of things,
      including the resistance of the resistor used, as I demonstrate in this
      other video here
      I don’t have any way of calculating what you should get as output.

    1. RimstarOrg

      +Shaheer Syed Since you asked for 60Hz I assume you’re looking for an
      inverter or something that can put out a useful sine wave, like for
      household appliances. This isn’t that. The output is very spikey, and even
      with a smoothing capacitor you still won’t get a sine wave. But to answer
      anyway, a smoothing capacitor would go in parallel with the LED, i.e. one
      capacitor leg on one leg of the LED and the other capacitor leg on the
      other leg of the LED. I don’t know what value capacitance you’d need so
      you’d have to experiment. It should not be an electrolytic capacitor. This
      works at a much higher frequency than 60Hz so I don’t think you can get

  3. Saarang Kelkar

    Sir, you explained the concept beautifully, I had no idea how it worked, it
    seemed very confusing to understand when I read it, but now it feels so
    easy, thank you.

    1. Mark Wright

      +Hilmi Albums You should be able to, they function the same as p channels,
      the base still controls the current, but the current direction is reversed.
      There’s no difference between N type or P type besides the direction of
      current. In fact, if you reverse-bias the P type FET and install in this
      same circuit, the circuit would function the same exact way.

    2. RimstarOrg

      +Hilmi Albums I think I once saw someone somewhere using a MOSFET for a
      joule thief type circuit but that was a long time ago and I don’t have a
      link. So I really don’t know for sure if you can.

  4. Kennynva T.

    Nice video..I have just found out, that if you add a electrolytic capacitor
    across the resistor with positive towards the plus + side of the battery
    the led will get twice as bright…I wonder what it is helping the current
    or the voltage…?

  5. Ron Evans

    Thanks for the video. You are speaking very quickly, even for an EE.
    However viewers can slow down the video by clicking on the Settings icon at
    the lower right, the gear shape, then changing the speed. It’s a little
    weird-sounding but it works.

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