by sajjad Haidar @ edn.com:
A simple blocking oscillator circuit can be used to step up voltage using properties of coil inductance (V = L di/dt). Such a circuit is shown in Figure 1, which is more commonly called a Joule thief.
The output will be pulses of voltage that can be filtered using a diode and capacitor. As there is no regulation, the output voltage will vary with the input voltage or load. As this circuit uses a BJT, the supply voltage needs to be at least 0.7V to work, and with enhancement-mode MOSFETs, the supply voltage must usually be even higher.
MOSFET-based Joule Thief steps up voltage - [Link]
by DIY Hacks and How Tos @ instructables.com:
A “Joule Thief” is a simple voltage booster circuit. It can increase the voltage of power source by changing the constant low voltage signal into a series of rapid pulses at a higher voltage. You most commonly see this kind of circuit used to power LEDs with a “dead” battery. But there are many more potential applications for this kind of circuit.
In this project, I am going to show you how you can use a Joule Thief to charge batteries with low voltage power sources. Because the Joule Thief is able to boost the voltage of a signal, you are able to charge a battery with a power source whose output voltage is actually lower than the battery itself.
This lets you take advantage of low voltage power sources such as thermoelectric generators, small turbines and individual solar cells.
Joule Thief Low Voltage Battery Charger - [Link]
By Sean Michael Ragan:
The Joule Thief (Wikipedia) is a well-known “instant gratification” hobby circuit that uses just a handful of components to pull off a pretty impressive parlor trick — using a single 1.5V battery, the Joule Thief can light a high-voltage blue or white LED that normally requires 3.5V or greater to turn on. Even more impressive, it can do so using a battery that is so drained of energy as to be counted “dead” for almost all other purposes. I have not measured this value myself, but it is commonly claimed that a Joule Thief can light a white LED from a battery with an open-circuit voltage as low as 0.6.
Bring “dead” batteries back to life with a toroid and the Joule Thief circuit. - [Link]
The Joule thief is a really fascinating circuit, simple yet very intricate. Basically, it’s a step-up converted in its most elementary expression. I will spare you the theory since there is plenty of information on it on the web; rustybolt.info is a good place to start.
Joule thieves in all sorts of forms have been featured countless time on DIY websites and I felt it was time I build one. However, I did not want to leave the circuit at the breadboard stage because as it stands, the joule thief has characteristics that make it very attractive for all sorts of low power applications and I figured a flash light would be a very good home for a joule thief, where having the option of using dead batteries is certainly a big plus not to mention using less cells because the circuit steps the voltage up. Why dead batteries? Because a battery is never really dead, its voltage just falls down logarithmically until it hits a point where the device it was powering up stops functioning, which does not mean the battery is totally drained but rather that its voltage has fallen below a usable level. Since joule thieves are step-up converters, they can take that “dead” battery, and give it a new life by stepping up its output voltage to usable levels again.
Maglite Joule thief - [Link]
Milen @ instructables.com writes:
Normally the Joule thief produces output voltage, which value is difficult to predict. Without load (the LED) I have measured voltages over 30 V. I wanted to create a Joule thief, which can be used to supply some small electronic devices, but having well defined and stable output voltage. There are known some solutions in which instead the LED load, a one-diode rectifier is used, and the output voltage is stabilized by the use of Zenner diode. I did not like this solution, because through the Zenner diode flows always a constant DC current, what reduces drastically the efficiency of the device and empties fast the supply battery. I was looking for other, better solution of the output voltage stabilization (limitation).
High efficiency regulated Joule thief - [Link]
Dave Kruschke writes:
Yep, no transformer and no hard to get IC. But, … two transistors and other parts are required. I actually found this circuit by accident while roaming Colin Mitchell’s Talking Electronics website (talkingelectronics.com). This website is very rich with examples and explanations of a huge variety of circuits. In fact, this website is so abundant in circuits that later on, I couldn’t find the circuit revealed here. Anyhow, I believe that the TE circuit can be considered “robust” as it works even if different parts are used.
“Joule Thief” – no IC and no Transformer - [Link]
Making LED devices portable can be a little bulky due to the batteries. The Joule Thief solves that, by boosting a single AA battery’s voltage to a high enough level to light a LED. Check it out.
Joule Thief – use LEDs with only one AA battery! - [Link]
This is a 1.2-volt single transistor flyback (Joule Thief) circuit that features a third coil. With it, flash duration and brightness is much enhanced, even with just a 10uF capacitor, as can be seen in the following schematic
1.2v LED Flasher – Joule thief - [Link]