Joule Thief Circuit

The voltage output from a joule thief circuit. X-axis is time in seconds and Y-axis is potential difference in volts Source: VandeWettering 2011.
The voltage output from a joule thief circuit. X-axis is time in seconds and Y-axis is potential difference in volts Source: VandeWettering 2011.

The Joule Thief Circuit is a voltage booster circuit which converts a constant low voltage input into a periodic output of a higher voltage. This circuit can be most often seen lighting an LED with an almost dead AA battery. The peaks in voltage occur rapidly, causing the LED to flash at a very fast rate. However, the LED appears to be constantly lit to the human eye due to the persistence effect.


  • How does the Joule Thief circuit work?
    Video explaining how Joule Thief circuit works. Source: RimstarOrg 2012.

    The circuit is an arrangement of a power source, a resistor, a transistor and a ferrite toroid core wrapped with two wires coming from the positive terminal of the power source, one through a resistor.

    A magnetic field is created around the ferrite toroid because of the current that passes through the wires. The extra current causes the transistor to switch off and power to the ferrite toroid is cut off. As a result, the magnetic field is converted into electrical energy which is given as output. Once the magnetic field no longer exists(the pulse ends), the transistor is switched on again and conducts electricity to create the magnetic field again. This process occurs rapidly enough to provide a somewhat constant power output. The frequency of voltage spikes generated by the Joule Thief circuit is over 5KHz . The video explains the working of the Joule Thief circuit very well.

  • How can a Joule Thief circuit be made?
    Video explaining how to build a Joule Thief circuit. Source: RimstarOrg 2012b.

    The components required are a NPN transistor, a 1kΩ resistor, a ferrite toroid (that can be salvaged from an old CFL bulb), wiring, a 3Volt LED and an AA battery which doesn't light up a LED by itself. Use the AA battery and connect two wires from the positive terminal and take one through the 1kΩ resistor and wrap the wires around the ferrite toroid. The wire that goes through the resistor, after it comes out from the ferrite toroid, is connected to the base terminal of the transistor. The other wire from the ferrite toroid goes to the collector terminal of the transistor which is also connected to the positive terminal of the LED. The negative terminal of the LED is connected to the emitter terminal of the transistor.

    More information is available at an Instructables tutorial.

  • How can the Joule Thief circuit be optimized or modified?
    Chart of relation between variables in the Joule Thief circuit. Source: Karras 2014.
    Chart of relation between variables in the Joule Thief circuit. Source: Karras 2014.

    The inductance in the Joule Thief circuit is determined by the loops around the ferrite toroid. The more the number of loops, the greater the inductance. An increase in inductance generally decreases the current flowing through the circuit and an increase in the duty cycle(on time%). This suggests that an increase in inductance increases the efficiency of the circuit. However, too much inductance is not good either. Trial and error can help find the sweet spot of the circuit where the least current is drawn. Increasing or decreasing one loop at a time will help find the number of loops a certain circuit requires for maximum efficiency.

  • What are the applications of this circuit?
    Circuit diagram of a Joule Thief circuit. Source: Fulvio314 2018.
    Circuit diagram of a Joule Thief circuit. Source: Fulvio314 2018.

    Lighting an LED with a dead battery is not the only application of the Joule Thief circuit. The circuit helps utilize almost all the energy that is stored in a battery. For example, a battery that has come out from a toy can easily light up a torch that makes use of the Joule thief circuit for hours or even days. The circuit can also be used in battery chargers, wall clocks, solar cell chargers where a low voltage input has to be increased for its intended application.

    The principle behind the joule thief can be used on any low voltage source, even if it's not a "dead" battery. For example, aluminium cans filled with water and wood ash, and simple electrodes can be used with a joule thief circuit for a battery charging application.

  • What are the disadvantages of using a Joule Thief circuit?

    A major disadvantage is a drop in output current because the current required to carry a given power decrease when voltage is increased because power is the product of current and voltage.

    Another disadvantage is Without significant improvements to the circuit, it is hard for the circuit to power more than simple led because heavy demand is put on the transistor for high current at very low voltage

  • What are some alternatives to the Joule Thief circuit?
    A buck-boost converter circuit. Source: Arrow 2017.
    A buck-boost converter circuit. Source: Arrow 2017.
    • Supercharged Joule Thief circuit: Has an efficiency of above 80% while conventional Joule Thief circuits have efficiencies between 40% and 60%
    • Buck-boost converters: Can be used for applications which require more power. The output voltage is always reversed in polarity with respect to the input
    • Voltage multiplier: Converts AC electrical power of a lower voltage to DC electrical power of a higher voltage
    • Split-pi topology: DC-DC converter that uses MOSFETs making it bidirectional and good for applications revolving around regenerative braking
  • How does the supercharged Joule Thief circuit have such a high efficiency?
    Circuit diagram for Supercharged Joule Thief circuit. Source: Rustybolt 2012.
    Circuit diagram for Supercharged Joule Thief circuit. Source: Rustybolt 2012.

    All that is required in addition to the components for a conventional Joule Thief circuit is a 680 pF capacitor. Apart from this, the feedback wire which is connected between ground and the wire from the ferris toroid which is also connected to a 1.5kΩ resistor and a diode, according to the circuit diagram which shows a circuit that can be used to switch between the conventional Joule Thief circuit and the Supercharged Joule Thief circuit.


Mechanism through which electronic devices can control an electron stream Source: Weber 1930.

Harold C Weber files a patent for a mechanism to control electron streams in electronic devices.

First-page clipping of patent Source: Bohan 1988.

Honeywell Inc files a patent for a self energized circuit that steps up ultra low voltages (as low as 0.1 volts) to a stepped-up alternating current.

Three drive circuits featured in Everyday Practical Electronics, Nov 1999 issue. Source: Kaparnik 1999, fig. 1.

An article titled One-Volt L.E.D — A Bright Light by Z. Kaparnik, and published in Everyday Practical Electronics, features a circuit that uses the same principles as the initial Joule Thief circuit in order to create high voltage pulses.

A small bulb lit by a button cell using the Joule Thief circuit Source: Mitchell 2002.

The name Joule Thief is coined by Clive Mitchell and a tutorial is published on his website. Clive's variation includes a smaller resistance than the original circuit published in the EPE magazine.


  1. ArtSee. 2020. "How to make a joule thief charger." Blog, ArtSee. Accessed 2020-07-31.
  2. Bohan, Jr, John. 1988. "Low voltage driven oscillator circuit." Espacenet, March 29. Accessed 2019-05-30
  3. Cockfield, Bryan. 2014. "Joule Thief Steals Power For A Clock." Hackaday, October 3. Accessed 2020-07-31.
  4. Electronics-tutorials. 2013. "Voltage Multiplier." Electronics-tutorials, October 15. Accessed 2019-05-29
  5. Fulvio314. 2018. "File:Joule thief.svg." Wikimedia, June 9. Accessed 2019-05-29
  6. Gatto, Katie. 2011. "The Joule Thief uses cans as a battery power.", June 21. Accessed 2020-07-31.
  7. Gudino, Miguel. 2017. "Types of Switching DC to DC Converters." Arrow, July 12. Accessed 2019-05-29.
  8. Kaparnik, Z. 1999. "One Volt L.E.D. — A Bright Light." Everyday Practical Electronics, November, pp. 804. Accessed 2020-07-31.
  9. Karras, Phil. 2014. "Joule Thief Information Page." Circle Science Consulting Site, January 20. Accessed 31-05-19
  10. Mitchell, Clive. 2002. "Make a Joule Thief." Bigclive, August 29. Accessed 2019-05-30.
  11. RimstarOrg. 2012. "How a Joule Thief Works." Youtube, July 5. Accessed 2019-05-29
  12. RimstarOrg. 2012b. "Make a Joule Thief for Zombie Batteries." Youtube, June 7. Accessed 2019-05-29
  13. Rustybolt. 2011. "Joule Thief Conventional & Supercharged." Rustybolt, December 4. Accessed 2019-05-29
  14. Rustybolt. 2012. "Supercharged High Efficiency Joule Thief." Rustybolt, September 20. Accessed 2019-06-01
  15. Rustybolt. 2012b. "JT-Conv&Superchgd" Rustybolt, February 17. Accessed 2019-06-01
  16. Rustybolt. 2013. "When And When Not To Use A Joule Thief." Rustybolt, June 27. Accessed 2019-05-29
  17. Smith, Jason Poel. 2015. "Joule Thief Battery Charger: Bring Back the “Dead”." Makezine, February 3. Accessed 2019-05-29
  18. Sounddezign. 2008. "Split-pi topology." Wikipedia, Sept 15. Accessed 2019-05-29
  19. TZDZ. 2014. "Why the current decreases with increase in voltage in transmission lines? But according to Ohm's law current should increase with Voltage." physics.stackexchange, October 18. Accessed 2019-05-31
  20. VandeWettering, Mark. 2011. "Simulating the Joule Thief with LTSpice." Brainwagon, October 5. Accessed 2019-05-29
  21. Watson. 2011. "The Joule Thief!" r3uk, April 28. Accessed 2019-05-29
  22. Weber, C, Harold. 1930. "Electronic device US1949383A." Google Patents, February 13. Accessed 31-05-19

Further Reading

  1. A quantitative analysis of the Joule Thief circuit
  2. How the Joule Thief circuit can be used for power saving emergency lamps
  3. Joule Thief circuit powering a clock
  4. An improved Joule Thief circuit
  5. An authentic Joule Thief circuit

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Devopedia. 2020. "Joule Thief Circuit." Version 11, July 31. Accessed 2020-11-24.
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Last updated on
2020-07-31 13:32:51
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