The Joule Thief circuit is a simple, self-oscillating voltage booster that allows low-voltage sources, like a nearly depleted 1.5V battery, to power devices requiring higher voltages. It uses a single transistor, a resistor, and a toroidal transformer with a feedback winding. When the circuit is energized, the transistor initially conducts, allowing current to flow through the primary winding of the transformer. This builds a magnetic field. As the current increases, the voltage across the resistor also increases, eventually turning the transistor off. The collapsing magnetic field in the transformer induces a voltage in the secondary winding, which, combined with the remaining battery voltage, creates a high voltage pulse suitable for driving an LED or other small load. The feedback winding further reinforces this process, ensuring oscillation and efficient energy extraction from the battery.
The Stack Exchange post elucidates the operational principles of a Joule Thief circuit, a minimalist voltage booster capable of extracting useful power from nearly depleted batteries. This circuit, built around a single transistor and a toroidal ferrite core with a bifilar winding, leverages the principles of electromagnetic induction and transistor switching to achieve this energy harvesting.
At its core, the Joule Thief utilizes a positive feedback loop. When the circuit is initially powered, a small current flows through the primary winding of the transformer and through the base of the transistor. This current induces a magnetic field within the ferrite core. Due to the bifilar nature of the winding, where the two coils are wound together, this magnetic field also induces a voltage in the secondary winding. This induced voltage, initially small, further drives current into the transistor base, amplifying the transistor's conductivity.
This amplification leads to a rapid increase in the current flowing through the primary winding, intensifying the magnetic field within the core. This intensified field, in turn, induces a higher voltage in the secondary winding. This positive feedback loop continues until the transistor reaches saturation, meaning it is fully conducting.
At saturation, the magnetic field buildup ceases as the primary current no longer changes significantly. With no changing magnetic field, the induced voltage in the secondary winding collapses. This collapse removes the drive current from the transistor's base, causing it to switch off abruptly. The collapsing magnetic field in the core now induces a high voltage spike in the secondary winding due to the rapid change in magnetic flux. This high voltage spike, potentially many times greater than the input voltage from the depleted battery, can be used to power a load, such as an LED.
The cycle then repeats. As the transistor switches off, the magnetic field collapses, inducing the high voltage spike. Once the magnetic field has fully dissipated, the transistor's base current, sourced directly from the battery through the primary winding, starts to rise again, initiating the next cycle of the oscillation.
The toroidal ferrite core is crucial to the circuit's operation due to its high magnetic permeability and low core losses. The bifilar winding, with its tight coupling between the primary and secondary coils, ensures efficient energy transfer and facilitates the positive feedback mechanism. The resistor connected to the base of the transistor limits the base current and prevents damage to the transistor. The load, connected across the secondary winding, utilizes the high voltage pulses generated by the collapsing magnetic field.
In essence, the Joule Thief cleverly exploits the properties of inductive coupling and transistor switching to convert the low voltage of a nearly depleted battery into a higher voltage suitable for powering small loads, effectively scavenging energy that would otherwise be unusable.
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https://news.ycombinator.com/item?id=43235671
Hacker News users discuss the Joule Thief circuit's simplicity and cleverness, highlighting its ability to extract power from nearly depleted batteries. Some debate the origin of the name, suggesting it's not about stealing energy but efficiently using what's available. Several commenters note the circuit's educational value for understanding inductors, transformers, and oscillators. Practical applications are also mentioned, including using Joule Thieves to power LEDs and as voltage boosters. There's a cautionary note about potential hazards like high-voltage spikes and flickering LEDs, depending on the implementation. Finally, some commenters offer variations on the circuit, such as using MOSFETs instead of bipolar transistors, and discuss its limitations with different battery chemistries.
The Hacker News post titled "Understand the Joule Thief Circuit" linking to an Electronics Stack Exchange question about the same topic has several comments discussing various aspects of the circuit and its functionality.
Several commenters focus on correcting or clarifying details about the Joule Thief's operation. One commenter points out that the circuit doesn't actually "steal" joules but rather makes use of energy otherwise wasted in a nearly depleted battery. They emphasize that the voltage is boosted, not the current, allowing the LED to operate at a higher voltage than the battery can directly provide. Another commenter builds upon this by explaining how the circuit functions as a self-oscillating boost converter, using the transformer's feedback to regulate the switching.
Another thread of discussion revolves around the efficiency and practicality of Joule Thief circuits. One commenter questions the circuit's actual efficiency, suggesting that the rapid switching might lead to significant losses in the components. Another commenter responds, agreeing about potential inefficiencies, but acknowledges that the simplicity of the design makes it useful for extracting the last bit of energy from a battery in low-power applications. This commenter further suggests a potential improvement using a CMOS 555 timer for potentially higher efficiency.
A few comments delve into more technical aspects of the circuit. One explains how the circuit exploits the transformer's behavior during the "flyback" period, where the collapsing magnetic field induces a higher voltage. Another discusses the role of the feedback winding in controlling the transistor's switching, clarifying why it is wound in the opposite direction to the primary winding.
Other comments offer practical advice, such as selecting appropriate components, like the transistor and the ferrite core for the transformer. One comment specifically cautions against using higher voltages, emphasizing the circuit's design for single-cell batteries, and highlighting safety concerns.
Finally, some comments discuss alternative circuits and applications. One user mentions using a similar circuit to power a white LED from a single AA battery and discusses component selection based on desired brightness.
Overall, the comments provide a wide range of perspectives, from basic explanations of the circuit's function to deeper discussions about its efficiency and limitations, as well as practical tips and alternative approaches.