Have you read the Joule Thief topic in Wikipedia? I think it’s poorly written and lacking in comprehensive information,. It reminds me of the blind men and the elephant: one blind man felt the elephant’s trunk and said the elephant is like a snake; another blind man felt the elephant’s tail and said the elephant is like a rope; another blind man felt the elephant’s ear and said the elephant is like a big leaf, etc. Everyone has a different idea of what should be there. No one has left a good description of the Joule Thief there.
I learned my lesson, though. Someone posted a picture of his version of a Joule Thief: for the coil it used two chokes next to each other, with one choke being the primary winding, the second choke being the feedback winding. The magnetic field from the primary passes through the feedback choke and develops the feedback current to turn on the transistor.
Yes, it works, but it’s mickey mouse because the feedback depends on the location and orientation of the chokes, and how close they are to each other. This is not a typical Joule Thief; I have never seen such a jury-rigged setup previous to this one. I removed the picture and explained my reasons in the discussion tab area.
The person put it back in, and left a note there, too. He apparently thought that everyone should be building Joule Thiefs like this. But I said that it was not a typical Joule Thief and I replaced it with a picture of a conventional Joule Thief. He complained about the blue background color. Picky, picky, picky, so I replaced the picture with a white background.
The lesson I learned is that if you decide to try to improve this Joule Thief topic in Wikipedia with comprehensive information, you may be contradicted by someone else who has decided he is the authority with the god given right to say what is right and wrong and changes what you’ve done.
The only solution I can think of is to start another topic in Wikipedia, with a different title, but dealing with the same subject, which is a low voltage LED driver.
Low Voltage LED Driver
This Low Voltage LED Driver is about a simple one transistor LED driver circuit that allows an experimenter to power a LED from a voltage that is lower than the LED’s forward voltage. Thus a white or blue LED, which requires about 3 volts to light, can be powered from a single 1.5V AA cell or even lower voltages.
History
In the November 1999 issue of Everyday Practical Electronics a circuit was published by Z. Kaparnik that boosted the 1.5V from an AA cell to drive a LED. He gave it the name Joule Thief because it will steal the last remaining Joules of energy from the battery to power the LED. The circuit is not new; it was derived from the circuit known as a blocking oscillator, which had been used since before World War II, in its vacuum tube or thermionic valve form.
Function
The circuit is shown in the pictorial.
The two winding coil that is used is popularly called a transformer, but technically it is a coil, because only the primary winding is needed to do the voltage boosting (there is no transformation); the feedback winding is only used to invert the signal to the transistor’s base so the transistor will continue to oscillate.
Understanding the Principle
In order to understand the principle that is used in the Low Voltage LED Driver, we strip the circuit down to four parts needed to light the LED: a AA cell battery, a white LED, a coil and a push button switch.
We connect the cathode or flat spot of the LED to the negative of the AA cell. and the other lead of the LED to one of the coil leads. We connect the other lead of the coil to the positive of the AA cell. We have a circuit, but because the AA cell is 1.5 volts and the white LED needs 3 volts to conduct current and light, nothing happens and the LED remains dark.
We now connect the pushbutton switch across the LED. Now every time the switch is pressed and released, the LED lights up briefly. When the switch is pressed, the current flows through the coil, and a magnetic field is built up in the coil. When the switch is released, the current stops, the magnetic field collapses and generates a voltage across the switch. But the switch is open, so the voltage rises, and when it gets up to 3 volts, the LED, which is connected across the switch, starts to conduct, and emits light. The magnetic field releases its energy as a brief flash of light from the LED.
The Transistor
If you can press and release the switch fast enough, the flashes of light will be faster than the eye can see, and they will look like a continuous light. So we remove the switch and replace it with the transistor, its emitter connected to the negative of the AA cell and LED, and its collector connected to the coil and LED. The transistor can act as a super fast switch, turning on and off tens of thousands of times each second. But the remaining base lead of the transistor needs a small amount of current to switch the transistor on and off.