In this Youtube video comment, he asks the question
[A] Joule Thief is a concept or it defines a device that uses a toroid?
About your Q: The original Joule Thief was a simple one transistor device that used the inductive kick or Counter EMF of a coil to boost a low voltage up to the voltage needed to light the LED. The coil originally was a toroid, but it can be a bobbin, bar or rod of Ferrite, or it can be a coil of wire with an air core, or any other non-magnetic core such as a piece of wood or plastic. This original “Conventional Joule Thief” used a 1.5V battery, a single transistor, a single resistor, and a coil and LED. A bypass capacitor of 10 or more uF across the battery wasn’t in the original but is optional and recommended as it helps stabilize the circuit when the battery is weak. Anything more complex than this is not a true Conventional joule Thief. Others may call it a Joule Thief but it is not the original circuit that was given the name Joule Thief.
Because the Joule Thief needs to be kept oscillating, which requires positive feedback, the coil must have a second ‘feedback winding’ to invert the feedback so it’s positive. This can be wound along with the primary winding – what’s called bifilar winding, or it can be wound before or after the primary. This feedback winding is typically the same number of turns as the primary but can be less or more turns. And it can be very thin wire because it doesn’t have to carry high current.
I will admit that my Supercharged Joule Thief is not a true conventional Joule Thief, but it’s closer to the Real Thing than the two transistor V boost circuit that is often called a Joule Thief. The original was not the best, but it was simple and did the job. Experimenters have tried so many variations of the V boost circuit while giving them the name Joule Thief, apparently to ride on the coattails of the popularity of the original circuit.
