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Joule thief

Joule thief is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Joule thief rather than just read about it. In short: A joule thief is a minimalist self-oscillating voltage booster that is small, low-cost, and easy to build, typically used for driving small loads, such as driving an LED using a 1.5 volt battery. It can use nearly all of the energy in a single-cell electric battery, even far below the voltage where other circuits consider the battery fully discharged (or "dead"); hence the name, which suggests the notion that the ci…

Joule thief — main illustration
Joule thief — illustration

Key takeaways

  • Joule thief belongs to science; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect Joule thief to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of Joule thief from memory before moving on to harder problems.

Reference excerpt

A joule thief is a minimalist self-oscillating voltage booster that is small, low-cost, and easy to build, typically used for driving small loads, such as driving an LED using a 1.5 volt battery. It can use nearly all of the energy in a single-cell electric battery, even far below the voltage where other circuits consider the battery fully discharged (or "dead"); hence the name, which suggests the notion that the circuit is "stealing" energy or "joules" from the source – the term is a pun on "jewel thief". The circuit is a variant of the blocking oscillator that forms an unregulated voltage boost converter.

History The joule thief is not a new concept. It adds an LED to the output of a self-oscillating voltage booster, which was patented many decades ago.

US patent 1949383, filed in 1930, "Electronic device", describes a vacuum tube based oscillator circuit to convert a low voltage into a high voltage. US patent 2211852, filed in 1937, "Blocking oscillator apparatus", describes a vacuum tube based blocking oscillator. US patent 2745012, filed in 1951, "Transistor blocking oscillators", describes three versions of a transistor based blocking oscillator. US patent 2780767, filed in 1955, "Circuit arrangement for converting a low voltage into a high direct voltage". US patent 2881380, filed in 1956, "Voltage converter". US patent 4734658, filed in 1987, "Low voltage driven oscillator circuit", describes a very low voltage driven oscillator circuit, capable of operating from as little as 0.1 volts (lower voltage than a joule thief will operate). This is achieved by using a JFET, which does not require the forward biasing of a PN junction for its operation, because it is used in the depletion mode. In other words, the drain–source already conducts, even when no bias voltage is applied. This patent was intended for use with thermoelectric power sources. In November 1999 issue of Everyday Practical Electronics (EPE) magazine, the "Ingenuity Unlimited" (reader ideas) section had a novel circuit idea entitled "One Volt LED - A Bright Light" by Z. Kaparnik from Swindon, Wiltshire, UK. Three example circuits were shown for operating LEDs from supply voltages below 1.5 volts. The basic circuits consisted of a transformer-feedback NPN transistor voltage converter based on the blocking oscillator. After testing three transistors (ZTX450 at 73% efficiency, ZTX650 at 79%, and BC550 at 57%), it was determined that a transistor with lower Vce(sat) yielded better efficiency results. Also, a resistor with lower resistance would yield a high current.

Operation

… excerpt ends here. Continue reading the full article.

Illustrations

Joule thief: A conventional joule thief, showing components and how they are connected. This example uses a red LED. A ferrite toroid is wound to form a coil with primary (white) and feedback (green) windings. A PN2222A transistor and 1000 ohm resistor are used
A conventional joule thief, showing components and how they are connected. This example uses a red LED. A ferrite toroid is wound to form a coil with primary (white) and feedback (green) windings. A PN2222A transistor and 1000 ohm resistor are used
Joule thief: A joule thief with two axial inductors replacing the ferrite toroid, shown on a solderless breadboard
A joule thief with two axial inductors replacing the ferrite toroid, shown on a solderless breadboard
Joule thief: Joule thief with regulated output voltage
Joule thief with regulated output voltage
Joule thief: Example of a joule thief circuit driving an LED. The coil consists of a standard ferrite toroid core with two windings of 20 turns each using 0.15 mm (0.006 inch) diameter wire (38 swg) (34-35 AWG). The circuit can utilize an input voltage down to about 0.35 V and can run for weeks using a 1.5 V LR6/AA. The battery voltage is usually 1.5 V. The resistor is ~1 kΩ, 1/4 W. The transistor could be a 2N3904, BC547B, 2SC2500, BC337, 2N2222, 2N4401 or other NPN. Vceo= 30 V, P= 0.625 W.
Example of a joule thief circuit driving an LED. The coil consists of a standard ferrite toroid core with two windings of 20 turns each using 0.15 mm (0.006 inch) diameter wire (38 swg) (34-35 AWG). The circuit can utilize an input voltage down to about 0.35 V and can run for weeks using a 1.5 V LR6/AA. The battery voltage is usually 1.5 V. The resistor is ~1 kΩ, 1/4 W. The transistor could be a 2N3904, BC547B, 2SC2500, BC337, 2N2222, 2N4401 or other NPN. Vceo= 30 V, P= 0.625 W.
Joule thief: A closed-loop regulated joule thief
A closed-loop regulated joule thief

Worked examples

Example 1 — a first encounter with Joule thief

Start with the simplest possible case. Write down what Joule thief claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Joule thief before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Joule thief ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Joule thief

In research
Joule thief appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses Joule thief in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
Joule thief is common in secondary-school and first-year university syllabi. It links to neighbouring topics Electric power conversion, Electronic oscillators, Power electronics, so understanding it makes those chapters shorter.
In everyday life
Look for Joule thief outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.
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How to study Joule thief in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what Joule thief means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain Joule thief out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Joule thief in simple terms?

A joule thief is a minimalist self-oscillating voltage booster that is small, low-cost, and easy to build, typically used for driving small loads, such as driving an LED using a 1.5 volt battery. It can use nearly all of the energy in a single-cell electric battery, even far below the voltage where…

Why does Joule thief matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Joule thief?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Joule thief.

Tags

  • Electric power conversion
  • Electronic oscillators
  • Power electronics
  • Power supplies

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